The tropical woody and grassy savannahs.
Rene Fernando Devia
Tropel de cimarronera (1989)
1 From open forest to grassy savannah
1. Shrinking rains, growing drought
1.1 The impreciseness of the concept of savannah
The Spanish term sabana was one of the first words to be incorporated into a European language from an Amerindian language. It initially appeared in the Relacion sobre las antiguedades de los indios, written by the Catalan monk Brother Ramon Pane in Hispaniola in 1498. (This manuscript is now lost.) And in a famous account of the Indies titled Historia general y natural de las Indias, writer and historian Gonzalo Fernandez de Oviedo (1478-1557) defines the savannah as land without trees but with lots of grass, both short and tall. The Spanish language of Castile, an area with poor pastures and unproductive terrain, lacked a term to describe these grasslands, so the name savannah was borrowed from Taino, an Arawak language that died with its last speakers in the seventeenth century.
The savannah biome is the tropical space that shows with great clarity the triumph of the life strategy of the grasses (Poaceae). Savannahs occur wherever insufficient rain, excessively nutrient-poor soils, rocky outcrops, or other adverse conditions prevent the growth of dense forest but where water is not so scarce as to cause desertization. Woody plants are not excluded from the savannah biome, though. Above the dominant grass layer there may be clumps, shrubs, and even trees, but their cover is never so great as that of the herbaceous layer that defines the biome.
The forest-savannah border
The separations between contiguous segments of the Earth's surface are not always clear because the zone of contact between two communities may be gradual (an ecocline) or relatively abrupt (an ecotone). For example, American ecologist Robert H. Whittaker's (1924-1980) continuum principle, a widely accepted description of plant communities, states that whenever there are environmental gradients along which the intensity of the variables varies gradually in space, the populations of plant species of a region also vary gradually, without forming sharp discontinuities. In reality, true ecotones only occur in cases in which a given environmental variable changes relatively suddenly, while ecoclines correspond to situations in which the changes in environmental variables are relatively gradual. Yet both situations can be considered as borders or frontiers between two communities in which the conditions for plants and animals differ greatly.
Savannahs, which occupy about 10% of Earth's surface and account for 14% of its primary production, are a typical case of a continuum, including all the tropical formations with a sparse or continuous grass layer--both those with one or several layers of woody plant (woody savannahs) and those lacking a woody layer (grassy savannahs). Woody savannahs include all the tropical ecosystems in which biomass and production are dominated by woody species but where the crowns or tops of the trees do not form a completely closed canopy.
As a result, an abundant herbaceous layer is allowed to grow, at least in the wet season. Woody savannahs include open forests; forested savannahs such as cerrado, miombo, and other similar formations; and shrub savannahs. Grassy savannahs include the typical savannah clearly dominated by the grass layer. The presence of woody plants on grassy savannahs is restricted to a few isolated individuals (tree savannahs) or small patches of denser vegetation (park savannahs). One convenient, but arbitrary, division between the two is a canopy cover of 40%, the point at which the mean distance between crowns is the same as the mean canopy radius.
Ecotone communities and tree growth in the savannah
An ecotone is a narrow transition zone between adjacent ecological systems. This zone has a set of unique spatial and temporal characteristics and is defined by the force of its interactions with the ecological systems it separates and their interactions with each other (see volume 1, p. 163). Ecotone communities may contain species from the two neighboring communities, but very often there is a group of typical species exclusive to the ecotone.
In tropical regions, one of the most important ecotones is the forest-savannah limit. The tropical savannah biome is characterized by the presence of very diverse plant communities, all sharing a continuous herbaceous cover of grasses and sedges--most of them plants with C4 photosynthetic metabolism (see volume 1, p. 185)--and showing clear seasonality related to the water deficit. Trees, shrubs, and palms may be present in variable densities depending on soil depth and water availability, and they determine the difference between grassy savannahs and woody ones.
The savannah biome occupies large areas of South America, Africa, and India, and smaller areas of Southeast Asia and Australia. It shows a continuum ranging from 1) very dry savannahs on unleached eutrophic soils (nutrient-rich soils with soluble components that have not been separated out) situated on the edge of semidesert areas to 2) wet savannahs, generally on dystrophic, or nutrient-deficient, soils. Yet this continuum appears to be interrupted at the forest-savannah border. This highly dynamic border may remain stable or favor one of the two communities. As the number of dry months decreases and annual rainfall increases, the forest-savannah border becomes unstable and the forest tends to advance, unless limited by soil factors such as very sandy soils or outcrops with ferruginous (iron-containing) crusts.
In high rainfall regions with large areas of forest in contact with the savannah, the absence of fire or grazing may cause the forest to advance and the cover of the woody layers of the savannah to spread across the transitional ecotone. This advance is sparked by the growth of seedlings of woody species from both the forest and the savannah. A lack of fires in a savannah region results in the gradual increase of woody elements. The increase in tree cover is very slow for the first six years, then it increases exponentially for 20 years until it reaches a period of saturation. In some cases, especially in relatively moist regions, this process may be reinforced by positive feedback favoring increasing tree cover, as long as there are no fires in the first 20 years or so.
The types of savannah
Many different criteria have been used to classify the savannahs, including climatic ones. In climatic terms, perhaps the most suitable of all possible classifications is Guillermo Sarmiento's 1975 proposal for the South American savannahs. This author divides the savannahs into three types: hyperseasonal, seasonal, and nonseasonal, emphasizing the drastic changes in the water presence in the two contrasting seasons of the year.
Hyperseasonal savannahs form in areas where rainwater accumulates or rivers overflow. These slow-draining clay soils are flooded for nearly the entire rainy season but may be dry at a depth of 2-3 ft (0.5-1.0 m), due to the presence of an impermeable clay layer near the surface. When the dry season begins, these soils dry out rapidly, crack, harden, and eventually suffer total water shortage. In South America, apart from grasses and sedges, only palms of the genus Copernicia from the llanos (open plains) of Venezuela and Colombia grow on these sites. If the soil is flooded almost the entire year, a permanent wetland develops.
Seasonal savannahs are the most common, occurring in regions where the soil is well drained and where no water accumulation or water table occurs near the surface. These areas may also support large areas of deciduous forest. The vegetation type is basically determined by the soil characteristics: savannahs grow on thick well-drained soils with little ability to retain water and a low cation exchange capacity; forests occur on more fertile soils with more loam and a greater ability to retain water.
Nonseasonal savannahs are found in zones with an Af tropical climate (a wet equatorial climate), without a dry period, where the dominant vegetation is tropical rainforest. On very poor sandy soils with a low nutrient content and an inability to retain water, the vegetation consists of low sclerophyllous and open forests (known as caatinga in Brazil and also as savannah woods or savannah forests) and nonseasonal savannahs. When conditions are insufficient to support rainforest, the tropics are replaced by savannahs and caatinga.
Monica M. Cole (1986) established a similar, but more complex, classification for African and Australian savannahs. But basing the classification of savannahs on climatic criteria alone is not enough; soil standards are of even greater importance.
1.2 Seasonal drought as the determining factor: climatic savannahs
The savannah biome includes very large areas surrounding the rainforest areas and occasionally within it. No single precise climatic limit can be used as a border between the forest and the savannah, nor is it possible to define climatic conditions restricted to the savannah. Seasonality--the succession over the course of the year of distinct climatic situations that occur repeatedly at the same time year after year--is the feature that distinguishes the savannah biome's climate from that of the rainforests. But seasonality varies greatly from one place to another. Open tropical formations dominated by a grass layer are supported by a seasonal climate; a seasonal climate, however, is not the most important factor determining the spread or local characteristics of the savannah.
The tropical climatic regimes
Between roughly the tropic of Cancer the tropic of Capricorn (or slightly higher latitudes) exists a zone dominated by heat, without a cold winter. Areas within this zone are said to have tropical climates. Within these climates, a whole range of variants can be distinguished on the basis of rainfall and atmospheric humidity, both in absolute terms and in relation to their distribution over the course of the year. Precise limits cannot be drawn between these variants.
Thus, in terms of the rainfall regime, the rainfall near the equator is abundant, but it gradually decreases as the distance from the equator increases. North of the equator, the dry season occurs between December and March, April, or May; south of the equator, it occurs between May and September. Temperature changes also increase with increasing distance from the equator. The savannahs are located at the midpoint of the series: they grow when the precipitation declines and rainfall becomes seasonal (with a sharp dry season but without reaching subdesert or desert conditions).
The tropical regions are divided into four continental blocks (South and Central America, Africa, southern and Southeast Asia, and Australia) that are separated by three large oceans (the Atlantic, the Indian, and the Pacific). Above these oceans, between the equator and each tropic, anticyclonic centers regularly form, and between them the trade winds blow constantly (from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere). Near the equator, the trade winds converge in a zone of permanent low pressures known as the Intertropical Convergence Zone (ITCZ). The location of the ITCZ moves between the two hemispheres, depending on the respective force of the corresponding trade winds, but it never moves far from the equator. The intense evaporation and consequent rainfall occurring around this low pressure zone establish the conditions that allow the development of the tropical forest biome (see volume 2, pp. 22-27).
However, in the areas farthest from the equator or where the amplitude of the oscillation in the ITCZ is greatest, rainfall becomes more variable over the course of the year, and the rainfall regime becomes seasonal, setting the stage for the growth of monsoon forests (see volume 2, pp. 417-420), open tropical woody formations, and savannahs.
Thus, over the equator, between about 5[degrees]N and 5[degrees]S, the dominant climate is the wet equatorial climate, known as Af in Koppen's climatic system (1936). Temperatures are high and constant throughout the year, and rainfall often exceeds 79 in (2,000 mm) per year. (There is no dry season and no month with rainfall below 2 in [60 mm].) A more seasonal variety of moist tropical climate--that of the monsoonal zones--is known as the Am climate. And in extreme contrast to these rainy climates are the excessively dry strips of land centered on the tropics of Capricorn and Cancer. The driest deserts on the planet occur here, and their climates are called subdesert (or steppe) and tropical deserts (BSh and BWh in the Koppen system).
Between the wet equatorial zones and the desert zones lies a strip of land with intermediate climate characteristics. Rainfall is lower than at the equator but is greater than typical values for subdesert and desert. Temperatures are almost uniform throughout the year. This climate--called Aw in the Koppen system--is also known as a wet-dry tropical, or savannah, climate. At first sight, it appears similar to the Am or monsoon climate. Both, for example, are seasonal, and in the dry season average monthly rainfall is less than 2 in (60 mm). In fact, many savannahs in India and Thailand form under Am and not Aw climates, but they are secondary systems provoked by human actions. A climate is considered Am when monthly rainfall in the dry months is less than 2 in (60 mm) but the total annual rainfall is above a certain threshold--the drier the dry season, the higher the threshold. A climate below this value is considered Aw. For example, Cochin, in Kerala State (southwest India), has two months with rainfall of only 1 in (20 mm), but because the average annual rainfall is 115 in (2,920 mm), it is clearly considered to be within the zone with Am climates. However, Calabozo (Guarico State, central Venezuela) experiences almost three months without any rainfall and has a total annual rainfall of 51 in (1,300 mm), making it a classic Aw system. Of course, these limits are general and imprecise, and there may also be savannahs in areas with Am climates because of other factors.
Seasonality and the savannah
One of the main variables controlling the presence of savannahs is the seasonality of the rainfall. The seasonal conditions expose the roots of the grasses to a prolonged period of water stress that causes the senescence of the aerial biomass and a decline in the community's rate of primary production. But as the latitude decreases in the more tropical regions, the total annual rainfall increases and the number of dry months gradually decreases. When annual rainfall tops 79 in (2,000 mm) and the dry season is less than three months long, tropical deciduous, tropical semideciduous, and then tropical rainforest appear. In these climatic conditions, the presence or absence of savannahs and their contact with the tropical rainforest depends basically on the soil conditions (whether ferruginous crusts are near the surface; whether soils have a high sand content) and the frequency of fires (often due to human action). In wet tropical climates in which the dry season is relatively short, the forest-savannah border is unstable and supports the expansion of the forest, unless the periodic action of fire together with soil characteristics greatly limit the establishment of forest tree species.
Seasonality, especially the regular occurrence of a dry season, is such a strong determining factor when separating the savannah biome from that of the tropical rainforests that there are sites and even relatively large regions--particularly in Africa--occupied by savannahs or open dry forest despite receiving greater rainfall than neighboring regions of rainforest (more than 118 in [3,000 mm] annual rainfall at several points in the coastal plains of the Republic of Guinea or in some regions of eastern Democratic Republic of Congo and India). By limiting the plant-growing season to the months when soil water is available to plants, the dry season prevents the growth of many rainforest species unable to survive this prolonged period of water stress.
In the savannah biome, the length and timing of the dry and rainy seasons may also vary considerably within the annual cycle, thus affecting the growth of vegetation at any given point. With sufficient rainfall and a relatively short dry period--three to seven months-monsoon forest may still grow; when less than 24 in (600 mm) of rain falls in the six rainiest months (or if the rainy season is shorter), the dominant vegetation type is woody or grassy savannahs.
In the tropical regions, both annual rainfall and the length of the rainy period generally decrease with increasing distance from the equator. However, local variations are numerous and differ from one continent to another; their only common feature is that the rainy season coincides with the summer solstice, when sunshine is strongest (although, precisely because of the greater cloud cover, it does not coincide with the highest temperatures of the year, which often occur at the end of the dry season). Thus, for example, in Africa, the open miombo forests receive 28-39 in (700-1,000 mm) rainfall per year, concentrated in a six-month-long summer rainy period. In the campo cerrado of central Brazil, annual rainfall is between 43-79 in (1,100-2,000 mm) a year, with a sharp dry season three to five months long. The tree savannahs of northern Australia receive less rainfall--between 20-59 in (500-1,500 mm)--but it is highly concentrated in the period between December and March. In India and Myanmar, countries subject to the characteristic monsoon regime of southern Asia, the savannah areas receive 32-49 in (800-1,250 mm) of rain, distributed in five to seven months with very sharp dry seasons between November-December and March-April.
The heat regime for areas closest to the equator is less sharply seasonal. Most savannah areas receive annual solar radiation of between 140 and 190 kcal/[cm.sup.2], with peaks of 0.50.6 kcal/[cm.sup.2] per day in the dry season and a minimum of 0.3-0.4 kcal/[cm.sup.2] per day in the wet season. Mean annual temperatures are never less than 75[degrees]F (24[degrees]C), while the mean temperature in the coldest month is between 55[degrees]F (13[degrees]C) and 64[degrees]F (18[degrees]C).
Paleoclimatic and radiometric effects
It is known that the glaciations that occurred in the temperate zones during the Quaternary led to drier climates than now present in the tropical lowlands. This caused sharp changes in the regional vegetation patterns. The equatorial rainforests of the Amazon Basin, for example, retreated to Pleis-tocene refugia, whose proximity to the sea, location in the path of moisture-laden winds, or siting in regions at higher elevations meant they enjoyed a rainfall regime less seasonal than in the rest of the basin. The rainforest was presumably replaced by savannah. Climatic changes did not only affect the proportion and spatial distribution of rainforests in relation to the savannah but also left their mark on the geomorphological development of the different types of landscape found in the tropics. In the basins of the Amazon and the Orinoco, small areas of savannah still interrupt the continuity of the equatorial rainforests; these savannahs in a nonseasonal climate are considered relicts and are generally associated with soil conditions that greatly limit the presence of forests (see p. 20).
In a mountainous region of the central Rift Valley in Kenya, researchers have been looking into variations in the forest-savannah border associated with the most recent climatic changes in the late Holocene. These variations are based on the measurement of the ratio [sup.13]C/[sup.12]C in soil organic matter. Changes in this ratio are thought to show spatial changes in the forest-savannah limit because the typical C4 savannah grasses show less discrimination between the different isotopic forms of atmospheric carbon dioxide than the C3 forest trees. The [sup.13]C/[sup.12]C ratio in the organic matter in a series of soil profiles, along an altitudinal gradient, showed that in the last dry period (in the mid-Holocene [3,400-3,000 years ago]), the border was 984 ft (300 m) higher than the current location at 2 mi (2,600 m) above sea level. This influenced the location of pre-Neolithic human settlements, as they apparently preferred to live in the ecotone zone of contact between the montane forest and the savannah.
The availability of light for seedlings and young plants in the ecotone zone has been identified as one of the critical factors controlling the dynamics of the savannah. Light is one of the factors that varies most sharply over the ecotone, varying in intensity over a few meters from that reaching the ground in open savannahs to that found under the closed canopy of the rainforest. The pyrophytic savannah species, even those of the densest woody savannahs, are unable to establish and grow under a closed canopy, while the young individuals of the rainforest species are generally sensitive to fire and grow slowly in the more open savannah conditions. In northwestern Queensland, the dynamic nature of the limit between the rainforest and the dense woody savannah dominated by Eucalyptus intermedia gives rise to a relatively narrow ecotone between the dense forest (with a closed canopy) and a more or less open woody savannah (with a canopy cover of 30-70%). The transition between the two communities is accompanied by sharp changes in the microclimate, with sharp gradients in temperature, humidity, and photon flux density. In a Kirrama-based (based in northwest Queens-land) study of the changes in the structure, floristic composition, and light regime between the Eucalyptus savannah and the neighboring rainforest, it was observed that during the summer direct photon flux density declines semiexponentially over the ecotone, while in winter the reduction is more linear. The reduction of diffuse light, however, is linear throughout the year. The average percentage of diffuse light at ground level varies from 10% in mature forest to 66% in the savannah, while the direct radiation varies in the same sites from 3% to 38%. A positive correlation exists between the average grass cover and the average annual total photon flux density over the ecotone, showing the interrelationship between the increasing light levels, increasing grass biomass, and increasing vulnerability to fire as the grass cover and biomass increase in the brightest areas of the ecotone.
The climatic savannah
Savannahs are generally found where there is a tropical seasonal climate with medium to low rainfall but not in areas as arid as subdeserts or deserts (Aw climates). It should be noted that savannahs may also occur in areas with equatorial wet climates, like the savannahs on the Atlantic Coast of Honduras and Nicaragua, in Trinidad, on the northern coast of Brazil, within the Amazonian forest, or on Borneo--all of them in areas with Af and Am climates. Likewise, within savannah areas with an Aw climate, other plant formations may occur, deciduous forests and semievergreen forests among them. The savannah-climate relation varies from continent to continent. In the Americas there are no savannahs in areas with annual rainfall of less than 32-39 in (800-1,000 mm), while in Africa and Australia there are very dry savannahs with annual rainfall of 20-28 in (500-700 mm). In South America, when rainfall is this low, a dry deciduous forest or spiny scrub may develop.
It is hard to explain why under a tropical climate the natural and stable vegetation would consist of a continuous or almost continuous layer of xeromorphic grasslands dominated by grasses and sedges. In general, if trees can establish in a given area, they tend to cause the competitive elimination of the herbaceous plants by shading them; the climax vegetation, then, is usually some kind of forest. Thus, the presence of large areas of savannah can only be explained by the existence of local conditions that do not encourage the establishment of trees. One of these conditions is the Aw seasonal climate, but it is not enough or even necessary in some cases. Zones with a savannah climate contain deciduous and other types of forest that grow well. The mechanism is thus more complex: the answer apparently lies in the anatomical and physiological differences between grasses and woody plants.
Areas with an Aw climate in a single geographical zone may often contain different types of savannah. One classic example is the series campo limpo, campo sujo, campo cerrado, and cerradao in Brazil; another example is the series de banco, bajio, and estero (riverbank, lowland, and shallows, respectively) in the low-lying Venezuelan plains. There are many cases throughout the world showing that what is called savannah is nothing more than a group of tropical ecosystems with hardly anything else in common than the presence of a more or less continuous grass cover. These differences between the climate and the biocenosis have led many authors to state that the savannah is not the result of a given type of climate but the result of the combination of climate with other factors, such as soil type, drainage, the frequency of fires, and grazing activity. This is known as the holocoenotic theory of savannah origin.
Climate, then, is not the only factor influencing the dynamics of the forest-savannah border. Other variables include light, the action of herbivores, mycorrhiza, soil texture and depth, and the related substrate water storage capacity. The borderline is also influenced by human activities, which in most cases favor the advance of the savannah into closed forest formations. Some factors, such as fire and soil conditions, may have a greater influence than others in controlling the forest-savannah dynamics, but they all interact and have to be considered at the same time; what varies is the weight and importance of each factor in local conditions.
1.3 Geological and soil factors: soil savannahs
Factors relating to the soil substrate of a given area are as important as--or even more important than--climatic factors in the development and distribution of different types of savannah. For this reason, the classification of grasslands as climatic savannahs or soil savannahs is sometimes used. Soil savannahs are said to be present in places where, for climatic reasons, a savannah might not be expected to flourish.
The effect of the soil substrate
Soil substrate is a key factor in the control of the forest-savannah border. In Venezuela, for example, the climatic regime found in the llanos (open grassy plains) also supports deciduous and evergreen forest formations. The deciduous forests always occur on less developed soils, originally Tertiary lutitic sediments of marine or lake origin that have a much higher nutrient potential, especially of calcium. This contrasts with the soils derived from the Mesa formation, a terrestrial sedimentary deposit that accumulated over these Tertiary lutitic sediments about two million years ago. The Mesa soils were formed from the weathering of rocks with a high quartz content (sandstones and granites from the Guiana Shield).
Over time, soils in the upper horizons have become dystrophic, with a high concentration of sand. This formation is thicker in the eastern llanos, while in the central and central-eastern llanos it has been largely removed by the erosive action of water related to the raising of the Interior Range. The total erosion of the Mesa formation (all the way down to outcrops of soil substrates derived from Tertiary sediments) determines the presence of deciduous forests. Yet, in many cases, dystrophic grassy savannahs dominated by Trachypogon are not in direct contact with deciduous forests; instead, semievergreen forests form islands in the middle of the dominant grassy savannah vegetation. The change from savannah to the ecotone and from the ecotone to semievergreen forest is accompanied by changes in the soil; it becomes deeper and better drained, lacks a surface ferruginous crust, and, because of a decrease in sand content and an increase in fine loam and clay particles, has greater soil water availability.
According to studies performed in different sites in the Coastal Range and Interior Range in Venezuela, the forest-savannah border may be gradual or abrupt, depending on the greater or lesser frequency of fires and the time elapsed since the abandonment of activities related to shifting agriculture and extensive stockraising. In most of the sites studied, the grassy savannahs dominated by Trachypogon spp. are in the highest areas of the mountains--the peaks and the upper or middle layers of the slopes. As the lines of drainage converge on the lower parts of the slopes, the density of woody elements in the savannah gradually increases until a border with an ecotone community is established. In the case of the central part of the Coastal Range, the ecotone consists of two woody strata: an open tree layer dominated by Adenanthera peregrina (Leguminosae) and to a lesser extent by Xylopia aromatica (Annonaceae), and a shrub layer that often contains Eupatorium odoratum and E. amigdalinum (Asteraceae). Generally, in the better-lit frontal zone, a grass layer consisting of Andropogon bicornis and, to a lesser extent, Panicum maximum is in contact with the Trachypogon savannah.
The width of the ecotone community is variable, but it is usually narrow, not exceeding 16 ft (5 m). In most cases, it coincides with a decrease in the slope's gradient and a greater density of drainage lines or intermittent drainage axes. The results obtained indicate that this limit is controlled by a combination of the action of fire, soil depth, the availability of soil water, and surface stoniness. Stoniness decreases along the savannah-ecotone-forest gradient, while soil depth, loam content, and soil humidity all increase. There is also more organic carbon and available phosphorus, possibly related to the greater input of leaf litter from the forest. Both forest and savannah substrates are dystrophic in nature, and soil fertility does not appear to influence the change in vegetation.
Similar cases have been studied in other sites in Venezuela, Brazil, and Ghana. Texture, water status, and soil depth are also among the primary factors controlling the separation between savannahs and forests.
The effect of fertility
Forest soils are often clearly more fertile than savannah soils, but most experts consider this a secondary feature, not a determining factor, in controlling the forest-savannah border. A higher degree of fertility may well be related to the forest's nutrient circulation mechanisms and greater production of leaf litter than the savannah.
The results of certain studies indicate that the control of the forest-savannah border is regulated by changes in soil fertility. Some of these results, contradicting those of other researchers in similar zones, show that in Brazil, along the gradient grass savannah (campo limpo), shrub savannah (campo sujo), scattered tree savannah (campo cerrado), dense tree savannah (cerradao), and semievergreen forest, there is a gradual and parallel gradient in soil fertility. These conflicting results may be explained by the existence of two types of cerradao, one on mesotrophic soils with a higher calcium content than the more common type, found on dystrophic soil substrates.
The effect of geomorphology
The climatic changes in the Quaternary together with changes related to tectonic movements have led to sharp changes in the landscape of the tropical regions. They have modified the original landforms and removed the surface soil horizons. During the Quaternary pluvial periods, soils were dominated by ferralization processes (see p. 46), with the loss of silicon and bases, and the differential concentration of iron oxides in the B horizon (biostasis).
In the dry interpluvial periods, the reduction of the plant cover favored accelerated soil erosion (rexistasis) and was generally accompanied by crust formation linked to the destruction of the upper soil horizons. The gradual in situ accumulation of iron oxides, due to annual oscillations in the level of the water table over considerable periods of time and the later disappearance of the poor seasonal drainage, forms ferruginous crusts known as laterites or plinthites (also see p. 46). Once the descending water table has disappeared, the iron oxides in this subhorizon slowly and gradually dehydrate, causing laterite to form in the soil profile at a given depth. This may be caused by changes related to tectonic activity (faults) and the geomorphological development of the landscape.
Ferruginous crusts at the soil surface or a few centimeters below it greatly impede tree root growth by forming a substrate that is hard for forest species to colonize. In the Los Llanos Biological Station in the Calabozo region of Venezuela lies a grassy Trachypogon savannah with trees, broken by small patches of semievergreen forest (thickets). The most common soil type in the area is relatively deep, with open sandy-textured upper horizons.
A second soil type, characterized by outcrops of laterites and ferruginous crusts on the surface and a few centimeters below it, is less frequent and occurs only in scattered patches. The research station's vegetation has been protected from fires and grazing for 25 years; this has led to increasing tree cover of the grassy savannah by savannah and forest woody species. The presence of laterite seems to slow colonization by woody species. In the areas where there are surface outcrops of laterite, or where laterite lies a few centimeters below the soil surface, gradual colonization by the woody liana (rainforest vine) Serjania atrolineata (Sapindaceae, which has a shrubby growth form when it is a juvenile) occurs.
In western Africa, the outcrops of laterite crusts in the plains mark a sharp border between the forest and the savannah. Prolonged human intervention in these areas over thousands of years hastened the erosion of the surface soil horizons. The erosion process, in turn, brought on major changes in the vegetation: as soil depth decreased, the forest was replaced by a woody sa-vannah, which was then replaced by a grassy savannah (called bowal in Guinea). The grassy savannah consists of xeromorphic grasses with surface laterite outcrops.
The ferruginous crust is not indestructible. Over time it becomes weathered, favoring the infiltration of the water and its resurgence on the slope at the grassy savannah-forest border. This leads to regressive erosion--the progressive breakdown of the laterite and the progressive colonization of the erosion front by the forest at the expense of the savannah--completing the plant-soil cycle.
In general, the transformation in the landscape caused by geomorphological agents takes thousands of years to occur. But sometimes, as in the central-eastern region of Guarico State (Venezuela), regressive erosion can lead to the rapid formation of gullies in the plains covered by grassy savannahs. Advancing at an estimated rate of 24-33 ft (8-10 m) per year, these erosion gulleys form in the lowest areas of the landscape, acting as collectors of the surface and subsurface runoff. Their advancing fronts are colonized by woody savannah species such as the curatella (Curatella americana, Dilleniaceae), alcornoco (Bowdichia virgilioides, Leguminosae), and the peralejo (Byrsonima crassifolia, Malpighiaceae). With time, water conditions improve due to the widening of the gully, and species arrive that are typical of the local gallery forests, including Andira scandia (Leguminosae), Xylopia aromatica (Annonaceae), Cassia moschata (Legumi-nosae), and Genipa caruto (Rubiaceae). Thus, in less than a hundred years, a new patch of forest interrupts the continuity of the original grassy savannah vegetation.
1.4 The decisive role of fire
Fire, together with dystrophic soils, seasonal climates, and the action of herbivores, is thought to be one of the main factors leading to the presence of savannahs. Unlike the woody species of the savannah, species associated with the semievergreen forests bordering savannahs generally lack fire-tolerance mechanisms. Thus, the thickness of the bark in forest species is measured in millimeters, while woody savannah species may have the bark several centimeters thick with a corky outer layer.
The effect of fire varies with the nature of the combustible material, its moisture content, the biomass, and the presence or absence of substances like terpenes, which may increase the temperatures of the flames. It is also necessary to consider the frequency of fires and the season in which they occur (at the beginning or end of the dry season). In general, if the savannah is burned every year, the accumulated biomass at the end of the dry season is not very great. When there is nothing limiting the depth of the soil, the annual production of the herbaceous layer will depend on the length of the rainy period. The temperatures measured in savannah fires vary between 158[degrees]F (70[degrees]C) and 1,472[degrees]F (800[degrees]C) at soil level and between 392[degrees]F (200[degrees]C) and 1,472[degrees]F (800[degrees]C) at 3.3 ft (1 m) above ground level. These very high temperatures may occur in herbaceous communities when the quantity of fine fuel exceeds 7.80 tons/ha (1 hectare=2.47 acres), a value very close to the annual estimated rate of primary production of Hyparrhenia rufa in the Los Llanos Biological Research Station (Calabozo, Venezuela).
Fire and the African savannah
In the central region of the Ivory Coast is a large area of grassy savannah with patches of semievergreen forest. This savannah penetrates far south into the tropical rainforest region. It appears to have its origin in the Quaternary climatic changes (dry interpluvial periods), the last of which, 12,000 years ago, led to the spread of the savannah at the cost of the forest. These savannahs are still present despite the improvement in the rain regime. Some observers feel that the spread of fire by humans has prevented the reforestation of the region. The ecotone zone consists of a type of woody savannah known locally as bodga. It consists of a 23 ft (7 m) tall layer of typical savannah trees such as Terminalia glaucescens (Combre-taceae). The shrub layer is 10-23 ft (3-7 m) high and consists of savannah species such as Bridelia ferruginea (Euphorbiaceae) and Bauhinia thonningii (Leguminosae). A subshrub layer about 3-10 ft (1-3 m) tall consists of a mixture of savannah and forest species whose cover in the ecotone zone depends on the level of shade and the degree of exposure to fire. Forest species such as Uvaria ovata (Annonaceae) and Mallotus oppositifolius (Euphorbiaceae) are concentrated on the shadier inner side of the ecotone, which is less exposed to fire. The well-developed grass layer is dominated by Pennisetum uniseta and Andropogon macrophyllus, both especially abundant in the shade of the aforementioned shrubs. On the edge of this community are large grasses such as cogon grass (Imperata cylindrica) and other assorted plants, including Aframomum latifolium (Zingiberaceae).
In the Lamto Experimental Station at the southern tip of this savannah (6[degrees]13'N; 5[degrees]02'W), the development of the tree layer has been studied in several parcels protected from fire for 18 years. For the first six years the increase in the number of woody species was minimal, but growth then accelerated rapidly. After 15 years there were more than five times as many species as at the beginning. The first stage (the first six years) of this secondary succession is characterized by the rapid increase in height and spread of the crowns of the woody savannah species, with the consequent reduction of the grass layer under these thickets due to shading. The second stage (the following six years, i.e., between six and twelve years after the last fire) is distinguished by a large increase in species diversity, especially of forest species, whose implantation and colonization of the thickets is favored by the closing of the canopy. The closure of the canopy attracts animals, mainly birds, bats, and monkeys, to the thickets. These animals help to disperse the seeds of the plant species arriving in this second phase. The third stage (between 12 and 18 years after the last fire) is characterized by the saturation of the space potentially available for the entry of new species, which may result in a halt in the increase in biodiversity even though the formation continues to become denser. After 18 years, a change in the species hierarchy occurs, and the abundance of savannah tree species declines. (Terminalia glaucescens, Cochlospermum planchonii [Bixaceae], and the fast-growing forest pioneers such as Allophylus spicatus [Sapindaceae] and African mahogany Afzelia africana [Leguminosae] are among the declining species.) There is a parallel increase, though, in species related to the presence of a forest canopy, which implies greater shade and higher humidity. (The rising species include Psychotria obscura [Rubia-ceae] and Erythroxylum emarginatum [Erythroxy-laceae].)
On the basis of 1:5,000 scale aerial photographs of the region, a 1990 study into the changes in the areas of the forest and savannah communities over 25 years (from 1963-1988) revealed major differences in the landscape. The forest cover increased at the expense of the savannah. The tree communities expanded in two ways: 1) from the forests growing at the top of hills and on plateaus and 2) from the progressive increase in woody species in the different savannah communities. It should be pointed out that in the savannah regions of western Africa there are two different fire regimes, both related to human activity. The early fire season occurs at the beginning of the dry season (between December and January), and the late fire season occurs at the end of the dry period (March). Their effects on the vegetation structure are different. (At the end of the dry season the accumulated biomass is greater, so the temperatures reached in the later fires are higher.) In this region, the recurrent action of early fires has not prevented the gradual advance of the forest into the savannah or the progressive enrichment of the different ecotone communities with woody species of forest origin and typical savannah species.
Fire in the Venezuelan savannahs
In the Los Llanos Biological Station (Calabozo), fire and grazing have been excluded from a 390 ha plot, and changes over time in the composition of both the grass and the woody layers have been studied. During the first nine years of protection, the zone's physiognomy remained basically the same as the burned savannahs. After 16 years of protection, however, the density of woody elements began to increase exponentially. After 25 years, the number of woody specimens in the savannah was 131 times greater than at the beginning, while in the forests it was only 20 times greater. Protection of the herbaceous layer from fire and grazing led to gradual changes in the savannah's structure and floristic composition. In a savannah initially dominated by Trachypogon plumosus, Axonopus canescens became increasingly important, and, after 1977, areas with deeper soils were invaded by the taller-growing African grass Hyparrhenia rufa, leading to a change in the grass layer's physiognomy.
Increased density of woody elements in the savannahs of the Los Llanos Biological Station is related to the reduction of the grasses present on the periphery of the small groups of woody plants in the region (Curatella americana, Bowdichia virgilioides, and Byrsonima crassifolia among them). In the absence of fire, the abundance and growth of C. americana increased greatly. This increase led to changes in the microclimate that, together with the production and accumulation of leaf litter, favored the establishment on the edges of these groups of heliophilic species typ-ical of the ecotone zone with the semievergreen forests, such as Goldmania macrocarpa (Bignonia-ceae), Cochlospermum vitifolium (Bixaceae) and Allophylus occidentalis (Sapindaceae). These species may establish as solitary specimens in the grass layer, or they may form a forest front advancing into the savannah together with other ecotone woody species such as Erythroxy-lum orinocensis (Erythroxylaceae), Arrabidaea coralina (Bignoniaceae), Xylopia aromatica (Annonaceae), and Genipa caruto (Rubiaceae). Furthermore, in the absence of fire, it has been shown that the greater height and ground cover of the groupings of woody savannah plants may increase soil fertility while locally reducing ground level temperatures, light levels, and the rates of evapotranspiration. As a result, recruitment becomes easier for the forest shade (sciaphilic) species that cannot establish in open savannah.
The woody groupings in the savannah vegetation of the Los Llanos Biological Station have been mapped using aerial photographs at the same scale taken 17 years apart. The data show that in this period when fires and grazing were excluded, there was a considerable increase in the number of woody groups and in their cover, which increased from 11-59%. Unfortunately, the station's vegetation was totally burned in 1991 and partially burned in 1993. Even so, Hyparrhenia rufa recovered rapidly from the soil seed bank and from the unburned patches of savannah, leaving no changes in the savannah's physiognomy or its floristic composition. The consecutive effect of the two fires, together with the accumulation of dead woody matter and the high biomass production of the introduced African grass (8 tons/ha/year), led in 1992 to the total destruction of a small forest dominated by Cassia moschata.
The results obtained in the Venezuelan savannahs coincide with those obtained from other fire exclusion experiments in western Africa (Nigeria and Ghana), northern Australia, and Brazil. All these cases show that protection from fire favors the entry of tree species. If the climate is relatively moist and the savannah is next to the forests, the exclusion of fire--and, to a lesser extent, of grazing--gives rise to a successional sequence from savannah to forest, as long as there are no limitations related to soil depth. When the climate is drier and there are no forests close to the protected savannah, the increase in tree density is due only to the recruitment of individuals of the same species as in the woody savannah.
Fire and the Australian savannah
In the north of Australia, where the climate is not clearly seasonal, the monsoon forest is in contact with the Eucalyptus savannahs that cover large sections of the area. An archipelago of small islands of monsoon forest, less than 13 acres (5 ha), interrupts the continuity of the savannahs. An Australian research group has studied the factors regulating the establishment of seedlings of monsoon forest species in Eucalyptus savannahs protected from the action of fire for a long time. A dense shrub understory develops, providing shade and reducing the soil's moisture deficit. It has been shown that this change in microclimate leads to an increase in the survival and recruitment of forest plants in the ecotone zone.
Certain experts question whether soil fertility and humidity play exclusive roles in the advancement of forest under controlled conditions (namely, the absence of fire). They suggest that the action of mycorrhizal fungi is necessary for the establishment and growth of the seedlings of forest species. Some studies performed in northern Australia show that a time interval of several decades without fire is required for the monsoon forest to advance; the slowness of this succession is attributed to the inefficient spore dispersal systems of these fungi and their need for host trees. Now, the high frequency of uncontrolled late fires makes the advance of the monsoon forest difficult, as this fire regime affects the monsoon forest's border with the savannah.
1.5 The influence of the fauna and the influence of humans
Animal life is central to any biome. The fauna on the savannah, then, actually forms part of the savannah biome. Human action has interfered with many savannah systems by imposing the presence of certain herbivores that act as disturbing agents.
The action of the large herbivores
There is a great contrast between herbivory in the savannahs in Africa, Australia, and tropical South America. The eastern and southeastern African savannahs are home to a wide range of wild large herbivorous mammals (and also of predatory carnivores). In Australia, there are kangaroos species of the genera Macropus and Thylogale. In the savannahs of tropical America, the white-tailed deer (Odocoileus virginianus) can be found in the well-drained areas, while the large rodent known as the capybara (Hydrochaeris hydrochaeris, also called chiguiro and carpincho) lives in the periodically flooded savannahs. Overall, though, the most important primary consumers are ants.
In addition to the wild fauna, the savannahs also support many domesticated herbivores, which in Australia, South America, and in some regions of Africa are now more common than wild herbivores. Herbivorous mammals, whether wild or domesticated, may alter the dynamics of the forest-savannah border, depending on the number of individuals per unit area, their food preferences (grazers, browsers, or mixed), and their social behavior. The role of herbivores in the dynamics has been studied in detail in eastern Africa, where, for example, it has been observed that the African elephant (Loxodonta africana), which lives in both forests and savannahs, may affect the dynamics of the border between low forests, semievergreen scrub, and savannahs. They inflict major damage on tree species, often knocking over an entire tree to get at the leaves in the crown. Elephants also dig up roots from the soil and strip the bark from the trees, especially those with yellow or green bark, such as several species of Acacia and Sterculia. Alteration of the forest-savannah border by elephants and other browsers such as the giraffe (Giraffa camelopardalis) is apparently related to increases in their density in certain areas where animals are protected from hunting. In the last few years there have been rapid increases in the elephant populations in the Tsavo National Park in Kenya and the Serengeti National Park in Tanzania.
In the interaction between fire and grazing by giraffes and elephants, fire has the greatest effect on the dynamics of the ecotone. The removal of the trees and increased light levels undeniably favor the establishment of heliophilic grasses. During the dry season these grasses encourage the spread of fires, thereby preventing seedling regeneration and inhibiting the growth of young trees. The system is interactive and cyclic; the accelerated destruction of forests and scrub reduces one of the elephants' food sources, resulting in a decrease in the elephant population. This marks the start of a new cycle of gradual increase in the tree populations. Grazing ungulates may influence the dynamics of the border due to their negative effects on the different stages of the regeneration of woody plants.
Episodes of Acacia species establishment have been observed in Tanzanian savannahs as a result of anthrax epidemics that devastated the populations of impalas (Aepyceros melampus), an antelope whose mixed feeding habits include eating acacia seedlings.
Browsing ruminants such as impalas and goats apparently disperse the seeds of various tree species throughout the eastern African savannah. A similar effect has been observed in the savannahs near Venezuela's Los Llanos Biological Station. These savannahs are burned each year and grazed by introduced herbivores such as cows (Bos taurus). The number of adult and subadult individuals of Cassia moschata (Leguminosae) per unit area is significantly higher within the biological station--where the vegetation has been protected from fire and grazing--than in the adjacent area. In the grazed area where fires also occur, the individuals of C. moschata tend to grow in the most sheltered sites within the small thickets where the ruminants live. In this case, the herbivores are not only dispersing the seeds but also promoting their germination by chemically scarifying the seeds as they pass through their digestive system.
Human activities may also greatly influence the dynamics of the forest-savannah border, mainly by provoking fires and increasing their frequency, by destroying the woody cover to increase the area of grazing, or by establishing shifting agriculture. In the case of shifting agriculture, the effect is greatest where human population growth has forced a shortening of crop rotations and fallow periods. In fact, many current savannahs are clearly of anthropogenic origin.
Except for accidents, humans use fire in the savannah as a working tool. Their activities alter the dynamics of the forest-savannah boundary and generally favor its progress. Only a single case has been observed that contradicts this trend: the agricultural activities of the Baule inhabitants of the savannahs in central-southern Ivory Coast actually favor the advance of the forest. The Baule practice itinerant subsistence agriculture, carrying out their agricultural activities in the ecotone community (bodga), which they then abandon.
Apparently, the transition community between the semievergreen forest and the dense tree savannah contains more fertile soil. The Baule clear this section of the land and plant several species of yam (Dioscorea) and other secondary crops-maize (Zea mays), black pepper (Piper nigrum), and banana (Musa)--and for three years in a row the farmer removes the weeds from the crop, especially the large grasses such as cogon grass. This helps prevent fires.
After a while, the Baule farmer abandons the site, and a successional sequence starts, initially dominated by highly ruderal species, including Mikania cordata (Asteraceae), members of the Malvaceae such as Urena lobata and Sida spp., and members of the Rubiaceae such as Borreria spp. This first phase is replaced within four years by a dense secondary thicket that remains less than 13 ft (4 m) high. It is formed by forest species from the forest-savannah border, such as Paullinia pinnata (Sapindaceae), and Mallotus oppositifolius (Euphorbiaceae). The third stage is characterized by the development of a secondary shrub layer formed by young specimens of typical forest tree species, including Chlorophora excelsa (Moraceae) and Cola gigantea (Sterculi-aceae). The final phase is the formation of a low forest characterized by a tree layer with these two species and Spondias mombin (Anacardiaceae).
The Gran Sabana area of southeastern Venezuela receives annual rainfall of more than 79 in (2,000 mm), but its vegetation is now dominated by savannah interrupted by small patches of thickets and low and medium evergreen forest. This is largely a result of the actions of the region's indigenous peoples, who have formed permanent settlements in small stable population centers. The indigenous peoples use fire in their agricultural activities and also in some hunting practices. Their practice of shortening crop rotations has led to an imbalance in the nutrient flow of the forest communities, a result of the destruction by fire of the nutrient conservation mechanisms associated with the action of the fine roots and the mycorrhiza. The death of the fine roots in fires provokes the rapid loss of the nutrients (calcium, potassium, magnesium) released by the fire. The oxidation of the organic matter by the fire causes a major loss of nitrogen, both as nitrogen gas and nitrates, which acidify the water when they dissolve and increase its washing capacity. This process, together with the continuous action of fire, gradually degrades the vegetation from primary evergreen forest to secondary forest, then to scrub, and finally to savannah.
Studies performed in the forest-savannah border in the central part of Venezuela's Coastal Range reveal the effects of extensive stockraising and periodic fires on the landscape. When parent geological materials rich in quartz, a landscape with sharp ups and downs, and shallow dystrophic soils are combined with an intense seasonal drought, the forest-savannah equilibrium shifts abruptly. The synergistic, or enhanced, effect of ranching and regular fires results in rapid soil erosion and a change in the quality of the upper soil horizon. The soil is replaced by a pavement of relatively fine siliceous gravel formed by the weathering of quartzite outcrops and the rapid breakdown of aluminium-rich schists that contain many quartz fragments. The forests are restricted to the flattest sites where the confluence of drainage lines and the accumulation of finer eroded soil materials improve the soil's water characteristics.
2. Mud in summer, dust in winter
2.1 Soils and savannahs, savannahs and soils
Climate, soils, fire (whether linked to the long dry season or to human activities), and grazing are the main factors determining the presence of savannah vegetation. Yet there is still no well-established and generally accepted pattern correlating the structure and floristic composition of savannahs and open woody formations with the various known soil types. This may result from the fact that a sort of no-man's land exists between the different disciplines of soil science and plant ecology.
The imprecise idea of the climax savannah
Ecological thought has been profoundly influenced by the climax theory of Frederick E. Clements (1874-1945). According to his theory, the climatic climax community--the community that a site in a given climate develops into in the absence of disturbances-tends to be a closed woody formation if the climate permits it. Furthermore, the theory suggests that areas with sufficient rainfall, but open vegetation, are the result of the destructive effects of post-Neolithic human forestation, grazing, and fires. The original forest's potential to regenerate is dependent on both climatic and soil factors; the effect of disturbances would be more permanent if the fires occurred repeatedly or if soil conditions were unfavorable, as these would prevent regeneration, especially in the driest climates. The least favorable soils would thus be the ones with the least water storage capacity (the shallowest, stoniest, and sandiest soils).
From this perspective, woody and grassy savannahs only represent the climax vegetation in the most arid climates, and the mixture of grasslands and scrub in the other areas now occupied by savannahs should be considered as an anomaly, a subclimax due to fire. Biotic (and especially human) factors would play a more important role than climatic and soil factors. The presence of open vegetation next to forest would almost never result from soil factors. This bias in relation to the origin of the biome's vegetation is understandable: forests, scrubs, and grasslands form part of an inseparable mosaic, and fires have left their mark everywhere. The question is whether the fire is the decisive factor or if it only makes a partial contribution. It is only generally accepted that savannah formation is a result of soil factors when the savannah occurs in areas subject to alternating periods of waterlogging and drought. Examples of this type of savannah include the African savannahs of the Pleistocene clay plains of Sudan and around Lake Chad; the savannahs on the plains of volcanic ash in Serengeti in Tanzania; and the dambo or mbuga grasslands of eastern Africa.
Water, nutrients, and plant growth-forms
Most authors agree that the structure of the vegetation, that is to say the competition between woody and herbaceous species, is partly conditioned by the physical properties of the soil and by the climate, which together determine the availability of water and oxygen. The closed woody vegetation is benefited by soils that allow deep root growth and are well oxygenated, while soils whose physical limitations only permit shallow rooting, or that experience occasional but regular waterlogging, favor the development of grasslands. This may explain, for example, the vegetation pattern in central and southern Sudan.
Plants undeniably compete for water, but when water is available competition for nutrients is even greater. This is why some authors suggest that vegetation structure depends on the water regime, but floristic composition depends on the soil's nutrient content. Savannah types can be arranged along a gradient going from moist dystrophic soils to arid eutrophic ones; woody savannahs occupy one end of the scale, and grassy savannahs with Acacia and members of the Combretaceae (Combretum, Terminalia) are at the other end. But terms like dystrophic and eutrophic do not quantitatively define the chemical properties of soils, nor have they been related to the soil classification systems. They refer to the soil's capacity to supply nutrients and micronutrients during the growing season--something that is not easy to quantify. This value can be calculated indirectly using parameters such as the cation exchange capacity (CEC), measured in milliequivalents in centimoles per kilo ([cmoles.sup.(+)]/kg soil) or 100 g (meq/100 g), the relative proportions of the cations, the levels of nitrogen and phosphorus, or indicators like the salinity, sodicity, and soil pH (as an index of the toxicity of some soil components, such as aluminum).
In general, it can be stated that in a given moisture regime, forest development requires a minimum quantity of nutrients; if this is lacking, grasslands develop with ferns and sedges or shrubby members of the Ericaceae. The minimum quantity of nutrients depends on the length of the growing season. In humid climates with annual rainfall greater than 98 in (2,500 mm), the threshold is lower (CEC < 0.5 [cmoles.sup.(+)]/kg soil in Sri Lanka). When the growing season is shorter and annual rainfall is between 59-98 in (1,500-2,500 mm), the threshold is higher (CEC < 1 [cmoles.sup.(+)]/kg soil in the Rupununi savannah in Guyana). In the dry seasonal climate of Mozambique where annual rainfall is 39-51 in (1,000-1,300 mm), this threshold corresponds to a CEC between 1-2 [cmoles.sup.(+)]/kg soil.
Once the minimum nutrient requirements have been satisfied, the competition between woody and herbaceous species is governed by the distribution of nutrients in the soil profile; increasing nutrient content with depth favors deeply rooted woody species over herbaceous species. This would explain the different physiognomy of the vegetation on soils with argillic horizons; the relative impoverishment of the surface horizons in comparison with the underlying horizons favors the growth of woody vegetation. However, when the CEC value in the surface horizons exceeds values of 6 meq/100 g (in a subhumid climate), the woody vegetation is displaced by a vigorous herbaceous vegetation; greater increases are irrelevant to the vegetation.
Calcium plays a critical role in these ecosystems, where the balance between the migration of clay, the washing of silica, and podzolization (migration of the iron compounds and organic matter) determines the soil's physical and chemical properties. The calcium functions as a chemical buffer preventing podzolization, the dispersal of the clay, and the washing of nutrients. The ratio of calcium ions to magnesium ions in the cation exchange complex is, in this sense, an important indicator of the soil processes taking place. Calcium is also physiologically essential for many root functions and important as a buffer for toxic elements. However, when present in excess, calcium it may immobilize soil phosphorus.
There is no evidence at all that any given macronutrient or micronutrient preferentially benefits grasses or trees. Nor do aluminum toxicity in very acid soils or salinity in more arid regions seem to play any role in the competition between herbaceous and woody species. Nor is there any simple relationship between floristic composition and total amount of nutrients, although relations can be established between groups of species and indicator parameters, such as pH, the toxicity of some elements, or the relative proportions of cations in the cation exchange complex.
2.2 Soil-forming factors and edaphic processes in the savannah
On a global scale, climate is the main factor determining the distribution of the biomes (see volume 1, p. 350). Yet in areas with anomalous environmental conditions (for example, in mountains, riversides, areas with inadequate or excessive drainage), the climate may deviate from that corresponding to zonal conditions, or another factor may become dominant. In these cases, the zonality of the biomes is not maintained. Within the large intertropical zone with seasonal climates--the zone where savannahs occur-factors such as the site's relief, the characteristics of the regolith, and the soil profile are, together with the quantity and distribution of the rainfall, the factors determining the growth of different forms of vegetation. Land use, the impact of fires, and other human-generated impacts eventually cause major deviations from the savannah types that would develop as a result of purely abiotic factors.
Soil formation factors
Climatically determined soil zonality could only occur in a region subject to a defined climatic gradient, one in which all the other soil formation factors (parent materials, relief, hydrology, organisms, length of the soil formation period, human action) remained more or less constant. Regions with soils determined by climate alone are not common; they can be found on stable sites not subject to erosion or sedimentation, with good but not excessive drainage, and with intermediate textures. In most cases, the zonal model is disturbed by the variability of the other soil formation factors. This is what happens in the case of mountain soils or hydromorphic soils related to the relief and in soils developed from materials with very specific physical and chemical characteristics (volcanic ash, smectic clays, coarse sand).
Logically, the large tropical area occupied by savannahs shows great variability in soil formation factors. Without forgetting the limitations of the concept of zonality discussed above, when considering the soil formation processes that occur in tropical savannahs it is worth considering soils determined basically by the climate first. Perhaps the best example of zonality in savannah soils is in western Africa, where the isohyets, the basic vegetation zones (Guinea savannah, Sudan savannah, and Sahel savannah), and the main soil types follow the same latitudinal gradient from the rainforest in the south to the desert in the north.
This makes it possible to define the zonal savannah soils. On the drier side, in subdesert areas, the soil's water balance is negative, meaning the potential evapotranspiration (the total quantity of water lost to the atmosphere by evaporation from the soil and transpiration from plants) is greater than the water the soil receives from rainfall and runoff. In these conditions, soil water may rise for at least part of the year, and the soluble and semisoluble soil components (gypsum, carbonates, and soluble salts) may accumulate on the surface or in the surface horizon. On the wetter side, though, the soils are highly washed and have lost all their soluble components, most of the cations from the cation exchange complex, and the silica from the original rocks. There is a relative, sometimes absolute, accumulation of compounds of iron and aluminum.
Grassy and woody savannah soils lie between these two extremes; they show accumulation of soluble compounds in the driest areas but in the wetter areas show silica loss and accumulations of residual oxides of iron and aluminum. The weathering of rocks is greater than in desert conditions, where it is restricted to physical breakdown, but less than in the wet conditions of the rainforest, where the more soluble compounds have been washed from the regolith. The weathering process is accompanied by the neoformation of clay minerals and of oxides, hydrated oxides, and hydroxides of iron and aluminum. The clay minerals determine the soil's main features, and, in the savannah biome, they show a gradient from smectite in drier areas to kandite in the wetter areas. In most of the biome, the surface horizons (A horizons) contain less clay than the subsurface horizons (B horizons), often but not always as a result of the translocation of clay. The translocation of iron (as ferrous [[Fe.sup.2+]] iron) and its concentration (as ferric [[Fe.sup.3+]] iron) is clearer in the wetter parts of the savannah biome than anywhere else in the tropics; the ferrous iron appears in the forms of concretions, ferruginous crusts, hardened layers (hardpans), nodules of plinthite, etc. (see volume 2, pp. 36-38).
Soil processes in the savannah area
The soil formation processes (see volume 1, pp. 334-336) of greatest relevance in the savannah zonal area are: translocation of clay, ferruginization and rubefaction, the weathering of rocks, ferralization, the neoformation of clay minerals, the formation of plinthite, the production of organic matter, and the action of erosion.
The translocation of clay
The migration of the clay, also known as eluviation/illuviation, requires the downward movement of soil water for at least part of the year, preferably when the subsurface horizon is dry. Furthermore, the dispersion (peptization) of the clay in the surface horizon has to occur for the water to carry the clay in suspension downward through the empty soil spaces. Clays disperse easily when the pH of the soil is between 5.5 and 7; in these moderately acidic conditions, the soil water does not contain enough divalent cations or exchangeable aluminum to immobilize the clay. The dilution of the soil solution in the epipedon, for example by rainwater, also contributes to the dispersal of the clay, which provokes the coagulation and collapse of the soil aggregates. All these conditions occur frequently in the more rainy savannahs, where they give rise to soils with a subsurface argillic horizon (a clay accumulation horizon; [B.sub.t] horizon).
In soils with a high exchangeable sodium content (greater than 15%), a special type of clay migration occurs, leading to dispersal of the clay and organic matter in the epipedon, followed by their accumulation in the subsurface horizons ([B.sub.tna]). The resulting accumulation horizon is called a natric horizon, and soils that have undergone this process are known by their original Russian name, solonetz.
Ferruginization and rubefaction
Ferruginization is the result of moderate chemical breakdown or weathering that releases iron from the mineral components of the rock, where it is generally present in the ferrous ([Fe.sup.2+]) form. The ferrous iron is oxidized to ferric ([Fe.sup.3+]) hydroxides and oxides, which form minerals such as goethite (yellowish brown), hematites (reddish), or noncrystalline components. These iron oxides are deposited on soil particles or aggregates, turning the soil a reddish or brown color. The result of this process is known, precisely for this reason, as rubefaction.
The weathering of rocks and ferralization
In the tropics, the regolith shows a loss of cations and silica and a relative gain of oxides of iron and aluminum. It sometimes maintains the original rock structure as a ghost structure, as the quartz veins in the original rock, for example, may maintain their position in the regolith.
Weathering of highly degraded regolith and soil materials follows a process known as ferralization, typical of humid tropical environments and the wettest part of the savannah biome. The action consists of strong hydrolysis leading to the gradual release and washing of the cations of potassium, sodium, calcium, and magnesium, thus causing the loss of all the weatherable minerals from the soil. The ferrous ([Fe.sup.2+]) iron is oxidized and retained in the soil as goethite or as hematites, and the aluminum released forms gibbsite or combines with the silica to form kaolinite.
The process of ferralization is favored by a relatively low pH, basic parent materials (mafic rocks, which contain relatively more iron and aluminum and less silicon than felsic and intermediate rocks), and, given that ferralization is a very slow process, the presence of a stable geomorphological position.
The neoformation of clay minerals
The weathering of rocks is accompanied by the neoformation of clay minerals. Clays form when some of the weathering products combine to form new silicate minerals, but they may also be the result of the direct transformation of the minerals in the rock.
In tropical soils there are two main groups of clay minerals: smectites and kandites. Smectites form when weathering is not very great and the parent material is mafic. They have a more complex crystalline structure and a higher proportion of silica than kandites, and they also contain basic cations in their crystalline structure. In addition, smectites have a much greater cation exchange capacity than kandites and expand and contract with changes in the soil's water content. (For this reason they are known as swelling clays.) Montmorillonite and kaolinite are respectively the most common smectite and kandite. On a gradient going from desert to rainforest, the clay fractions would show a decreasing montmorillonite content and increasing kaolinite content.
The formation of plinthite and ferruginous crusts
One of the characteristic features of the savannah biome is the presence of horizons with different forms of iron concretion. Ferruginous crusts, also known as laterites (from the Latin word later, a sun-dried brick), are hard cemented horizons rich in secondary iron oxides, reddish brown in color, and poor in humus.
The most common mechanism of formation involves the precipitation of iron oxides at the level where the water table fluctuates, whether in the underlying material, in the transition horizon between it and the soil, or in the regolith that covers the surface of the bedrock. The iron oxides accumulate as flat polygonal or reticulate dark red patches. This iron-rich material, called plinthite, hardens when exposed to air or subjected to cycles of wetting and drying. The second step in the formation of ferruginous crusts is the hardening of the plinthite, although crusts may sometimes be due to the ferruginization of the weathered rock. The two processes (the precipitation of iron and its hardening) may take place within a relatively short period of time or may be separated in time--plinthite formation occurring first, followed by its hardening, for example, as a result of a fall in the water table.
Although plinthite and ferruginous crust formation processes still occur, the ones that cover large areas (such as those in western Africa) are thought to be relics: they are no longer progressing and are in fact undergoing physical disintegration and chemical breakdown. Ferruginous crusts are most likely to form in the savannah biome, where there is a regular seasonal oscillation in the level of the water table, although they are also widely distributed in the equatorial rainforest biome.
The production of organic matter
The savannah biome's plant formations are highly variable, and thus the production of organic matter varies greatly as well. In the less rainy areas, production of organic matter is lower and the organic compounds decompose rapidly in the hot, dry, climate. In the areas with more rain, enough organic matter is produced for a considerable portion to be retained in the soil.
2.3 The main soil types in the savannah
The cartographic units of the soil map produced by the Commission de Cooperation Technique en Afrique (CCTA), drawn up by J.L. d'Hoore and published in 1964, correlate better with the climate and vegetation zones than the units of the United Nations' Food and Agriculture Organization (FAO) World Soil Map (1990) on the same scale, the classification adopted in other volumes of this work. D'Hoore followed the French classification system (although this was not published until 1967) based on soil formation processes, which is more suitable for establishing correlations with climatically determined vegetation zones than the Soil Taxonomy or the units of the FAO system. Therefore, bearing in mind the great relevance of the African continent in the savannah biome, we have used the terminology of this map as a reference in the description of the savannah soils, although the nearest equivalents in the FAO system will also be given.
The brown soils of arid and semiarid tropical regions
Brown soils, the zonal soils of the semiarid zone, typically develop under conditions of annual rainfall of 12-20 in (300-500 mm), generally on acid igneous rocks and sedimentary rocks. They have a well-developed soil profile with A, B, and C horizons, and a calcium carbonate accumulation ([B.sub.k]) horizon. Most of the soils mapped as brown soils in the arid and semiarid tropical regions in the CCTA map appear in FAO World Soils Map (1974) as luvic and cambic arenosols, and some small areas as regosols. Arenosols are soils that have developed on unconsolidated sands, but they have been grouped together with the brown soils of the arid and semiarid tropical regions in the CCTA map because of the similarity of the processes leading to their formation and because they occupy large areas in the same climatic zone. Cambic arenosols have a structural subsurface (Bw) horizon, or endopedon, while in luvic arenosols there has also been translocation of clay. In both units, reddish layers of goethite may form on grains of quartz, the result of a process of ferruginization.
Ferruginous soils are the zonal soils found in woody savannahs in zones with an annual rainfall of 20-39 in (500-1,000 mm), mainly on felsic or intermediate rocks. They correspond to the ferric lixisols in the FAO system. They also form over sedimentary rocks (limestones, siliceous sandstones) and unconsolidated sands. Sandy ferruginous soils are widely distributed in this rainy area of Africa and, in the FAO classification, probably correspond to luvic arenosols. In these soils, weathering is moderately intense because of the greater availability of water, and free iron oxides are released, but alumina is not. Soluble salts, such as carbonates and gypsums, have been washed from the soil, and there is also some washing of cations, differential elimination of silica, and a low cation exchange capacity (CEC) (see volume 1, p. 338). There is generally an argillic Bt horizon where iron and clay complexes have accumulated, giving the soil an intense red color.
Ferruginous soils are widely distributed in the African continent's northern and southern savannahs, but they are also the main soil type in the subhumid lowlands of India and the dry zone of Sri Lanka. They often develop in association with weathered ferralitic soils, on steeper slopes or on less acid parent materials. Soils with ferruginous crusts or with horizons with reddish patches or nodules of iron oxide are widespread; they have ferric properties, a diagnostic criterion for the ferric soil units in the FAO Classification.
Ferralitic soils include both the highly washed soils of the rainforests and the moderately washed soils of the wet savannahs. They all have a low content of primary minerals and a low CEC in the clay fraction, but they show considerable differences in their morphology and chemical properties, mainly related to the differing intensity of weathering. The classification (proposed by Young in 1976) separating the weathered ferralitic soils of the wet savannah from the washed soils of the tropical rainforest is of great practical use.
The weathered ferralitic soils of the wet savannah, the ones under discussion, would correspond to the ferrasols, ferric acrisols, and ferric lixisols of the FAO Classification. They occur on flat or slightly sloping sites on highly altered felsic rocks. They are present throughout the woody savannah where annual rainfall is 20-47 in (500-1,200 mm). In Africa and in other sites they have been described as plateau soils, and they are typical of plateau areas in savannah climates, as in the Guiana Highlands and the Mato Grosso Plateau, areas of southern India, and large areas at elevations of 3,281-4,921 ft (1,0001,500 m) in Tanzania, Zambia, and Malawi.
Nitisols and vertisols
In large areas of the savannah biome it is not possible to apply the concept of zonality due to the dominant effect of the parent material as a factor in soil formation. Arenosols on unconsolidated sands have already been mentioned; other less common examples include soils derived from mafic rocks (basalt, limestone), called ferrisols in the CCTA map, and eutrophic brown soils. On sandy plains (and also on mafic rocks), vertisols also occur. Hydromorphic soils (mainly gleysols), young soils with a poorly developed soil profile (regosols), are less typical of savannahs but are present locally over large areas.
Nitisols have a well-developed structure, with angular blocks with shiny faces due to their covering of clay or pressure faces. This clear structure can be attributed to the presence of significant quantities of active iron oxides.
The soils figuring in the CCTA map as eutrophic brown soils cover some of the nitisols and andosols (those formed on volcanic ashes) in the FAO classification. Brown eutrophic soils are moderately deep soils, varying in color from dark brown to dark reddish, that have developed over basic rocks in a dry savannah climate with an annual rainfall of 27-47 in (700-1,200 mm); when rainfall increases to 47-59 in (1,200-1,500 mm), there is a gradual transition towards ferrisols. The clay fraction is dominated by montmorillonite. The soil formation processes involved include the translocation of clays and ferruginization. Cyclic wetting and drying of the montmorillonitic clay partly explains the block structure of the [B.sub.t] horizon.
The ferrisols on the CCTA map are equivalent to another group of nitisols in the FAO terminology. Ferrisols develop on basic rocks, both in the rainforest and on the forestsavannah border. Like brown eutrophic soils, ferrisols have a strong block structure in the [B.sub.t] horizon, with patches of clay reddened by free iron, but with a lower base saturation (less than 50%) and a lower CEC in the clay fraction. The formation processes include clay translocation, ferruginization, ferralization, and soil erosion. Like brown eutrophic soils, ferrisols are often present on steep or gentle slopes. The elimination of the surface material by erosion is a factor that prevents the formation of a highly altered regolith.
In the savannah biome, vertisols are typical of clay plains that crack during the dry season; their morphology is so characteristic that they appear in all the classifications, described in more or less the same terms. Three conditions must be fulfilled for them to form: materials with a high clay content dominated by smectites, a climate with alternating wet and dry seasons, and deficient drainage. The parent materials are alluvial deposits or alluvial-colluvial pediment deposits or rocks that produce a lot of smectic clay when they weather (limestone and basalts, for example). Internal drainage is always deficient because of the presence of the clay; deficient external drainage is an additional factor contributing to the formation of vertisols.
These soils crack deeply during the dry season but swell in the wet season, closing the cracks and decreasing the hydraulic conductivity. Water only penetrates to a depth of a few centimeters and along the widest cracks in the dry season. Vertisols occupy large areas in Australia, India, and Sudan, but they are also present in South America, in several African countries, and in Southeast Asia.
2.4 The correlation between soil types and savannah types
Broadly speaking, the succession of brown soils, ferruginous soils, and ferralitic soils corresponds to the parallel succession of eutrophic, mesotrophic, and dystrophic savannahs. To some extent, these successions are conceptually related to climatic zonality. For some experts, eutrophic savannah has become synonymous with arid grassy savannah, and dystrophic savannah has become synonymous with wet woody savannah. This model is especially well represented in Africa and Australia, where there is a gradual change from the woody savannahs in wet areas to grassy savannahs with Acacia and members of the Combretaceae. The woody savannahs occupy the soils between acidic and neutral (at least in their upper layers). The grassy savannahs with acacias occupy soils ranging from neutral to alkaline. Yet this zonation is not found in the savannahs of other continents. Eutrophic or dystrophic formations, woody or herbaceous, can be found side by side under a wide range of climatic conditions.
Despite this complexity, some generalizations can be made. On highly ferralitic soils, at the most dystric end of the scale, sites whose climate would permit thickets or forest are occupied by savannahs with small trees. This is related to the extreme nutrient poverty of sandy senile ferralitic or washed soils (ferrasols, ferric acrisols, ferralic and albic arenosols). The cerrado in Brazil belongs to this category. Its ecological equivalent in Africa is the shrub savannah with Parinari and Uapaca, in Asia the Vaccinium scrub and grasslands of citronella grass (Cymbopogon nardus), and in Australia the savannahs of Melaleuca and proteas. Semiarid dystrophic savannahs with woody species occur in Africa and Australia on soils derived from preweathered and impoverished eolian sands (formed by the wind; ferralic arenosols).
The most typical dystrophic or mesotrophic woody savannahs are in Africa, Indochina, and Australia. Their soils, derived from felsic rocks, belong to the ferruginous group; they show a clear difference in texture between the surface horizons and the subsurface horizons (ferric lixisols and ferric acrisols). The upper layer of these woody savannahs is dominated by species that share the formation of ectomycorrhiza: species of the genera Brachystegia and Julbernardia (both Leguminosae) in the African miombo, dipterocarps in Indochina, and eucalyptus in Australia.
Not all ferruginous soils are occupied by dystrophic woody savannahs. Savannahs with members of the Combretaceae (Terminalia, Combretum, Anogeissus) grow on mesotrophic soils, where eluviation and illuviation are not so strong. Acacias dominate those dark soils in the semiarid tropical regions where the pH varies from neutral to alkaline.
Nitisols, or eutrophic brown soils, form a genuinely eutrophic medium covered by a grassy savannah with tall grasses. Base-rich vertisols may also be conceived as eutrophic, although nutrient availability in most cases is not optimal; nitrogen and sometimes phosphorus may become limiting factors. In the best-drained sites, woody savannahs and even small patches of forest develop, while grassy savannahs grow in temporarily flooded sites. Examples include the open forests of teak (Tectona grandis) in India and the woody savannahs of mopane (Colophospermum mopane) in southern Africa or of brigalow (Acacia harpophylla) in Australia, both of them salt-tolerant.
The soils of the American savannahs
In the humid and subhumid zone of tropical America with annual rainfall of 39-98 in (1,000-2,500 mm), where one might expect to find semideciduous or deciduous forests similar to the monsoon forests (see volume 2, p. 424), there are large areas of grassy savannah, while the semiarid areas are covered with woody savannahs. Nowhere else in the world is there such a clear inverse relationship between tree density and rainfall gradient, and nowhere else are savannahs so clearly controlled by soils, whether due to poor drainage or nutrient deficiency.
Badly drained soils: llanos and other wet savannahs
The large depressions that border the Andes are covered with Tertiary and Pleistocene sediments. Their deficient drainage means that large areas in these depressions are periodically waterlogged by seasonal rains. The sediments in the llanos of the Orinoco, the llanos de Mojos, and the llanos of the northern part of the Pantanal area are intensely weathered and washed, giving rise to nutrient poor acid soils. Dystric gleysols (nutrient-poor hydromorphic soils) and plinthic acrisols (impoverished soils with accumulations of iron) often contain plinthites.
These poorly drained soils hardly support any trees, except for some open palm groves. The shrub savannah, floristically similar to the vegetation of the cerrado, occupies the highest sites in the landscape. Deciduous or semideciduous forests are restricted to the richest soils, which occupy the highest parts of table reliefs and the bases of the mountain ranges surrounding them. These soils are classified as ferric luvisols, haplic acrisols, or haplic ferrasols.
The sediments of the basin of the Parana River are rich in nutrients, so in dry climates the soils that dominate are young and rich in nutrients (eutric): eutric fluvisols (in alluvial deposits) or eutric planosols (with an argillic horizon and low permeability). The salt input in the runoff water from the mountain ranges surrounding the basin leads to some salinity in the subsurface horizons. Vertisols have developed in the drier areas of the Venezuelan llanos and in the south of the Pantanal.
The old rocks and poor soils of the cerrado The ancient surfaces of the Guianan Highlands and the Mato Grosso Plateau have undergone a slow and gradual process of drying out and experienced prolonged tropical weathering. The soils that have developed on them, possibly the oldest soils in the world, are extremely nutrient-poor (geric ferrasols and ferralic arenosols), and their extreme aridity has mobilized aluminum over half the area. Aluminum, although it is present in these soils in small quantities, plays a dominant role in the ecology of the vegetation because of its relative importance with respect to the other cations. These soils vary in texture: the availability of phosphorus for plants depends on the soil's clay content, as its high levels of iron and aluminum oxides mean it tends to fix the phosphorus and immobilize it.
The result is the soil-related savannah known in Brazil as cerrado. It is found in a wide range of climatic conditions, forming "islands" in the middle of the Amazonian rainforest and in the arid regions of northeastern Brazil, but has an impressively uniform floristic composition. The soils of the cerrado do not contain enough nutrients to support tree vegetation, although the climate would permit it. The vegetation typically shows a relatively scattered cover of small trees and many shrubs above a herbaceous layer. The tree cover may vary from almost nonexistent (campo limpo) to woody formations or open woodland (cerradao). Correlations have even been established between tree density and nutrient levels (nitrogen, phosphorus, and potassium) in the surface soil horizons.
The domination by evergreen sclerophyllous species should be seen as a strategy to conserve nutrients. The cerrado, like the African woody savannahs of Brachystegia, awakens before the wet season begins, producing new leaves and shoots so that the released nutrients in the first rains are used more effectively. It is not known if the species of the cerrado form ectomycorrhiza.
When cerrado occurs in the middle of the rainforest, it develops on soils that are even poorer in nutrients than the surrounding areas. In the Rupununi savannahs in Guyana and the Sipaliwini savannahs in Surinam, where annual rainfall is 79-98 in (2,000-2,500 mm), campo limpo occurs on soils that have developed from siliceous sandstones, granites, or aluminous lavas, with partly blocked drainage. The soils are ferralic arenosols, ferrasols or haplic acrisols, or plinthic acrisols. There are very few exchangeable cations, the pH is low (4.5-5), and the aluminum is probably mobile. In the dry region of northeastern Brazil, the cerrado and the campo limpo occur on highly altered ancient siliceous sandstones with annual rainfall of about 28 in (700 mm).
Aridity and soil texture problems: caatinga and chaco
The cerrados of central Brazil are bordered to the northeast by caatinga, spiny thicket (a formation typical of the subdesert, not of the savannah) and to the southeast by the dry woody formations known as chaco. In both regions, the soils show great differences in texture with depth, from a sandy surface to a highly clayey deeper layer. Caatinga soils contain very few nutrients and are slightly acid, while chaco soils are alkaline and rich in salts. Even so, the vegetation of both regions is similar in that it is xeromorphic.
The spiny caatinga region is a denudation plain that has developed on siliceous sedimentary rocks and acid metamorphic rocks. The soils show a gradient from west to east, from haplic ferrasols to ferric lixisols and chromic luvisols. This reflects the increasing denudation and renewal of the surface and the consequent higher nutrient content. The soils are very stony, often sealed with a surface crust when they dry out. Their most important ecological property is their high cation saturation; aluminum is not mobile, so woody legumes can grow on these soils. Annual rainfall is low and highly variable at 12-30 in (300-750 mm). Strong winds and high evapo-transpiration add to the climatic adversity. The spiny caatinga shows similarities in both soil conditions and floristic composition with some woody savannahs in Africa.
The chaco is a large flat area formed by sediments proceeding from the eastern slopes of the Andes. The soils contain considerable quantities of primary minerals and are often saline or alkaline. Most of them show highly impermeable clay accumulation horizons, making them susceptible to waterlogging in the wet season. The dominant soils are kastanozems--often saline--planosols and solonetz. The critical factors governing the distribution of the vegetation types are the soil moisture regime, the periodic flooding, salinity, and alkalinity. As these properties vary depending on the relief, small differences in the relief determine the development of woody or herbaceous vegetation and shape the region's mosaic of forests, thickets, and grasslands. The woody vegetation, the chaco, tends to occupy the best drained and least saline zones, which are often associated with termite mounds. The campo palmar (palm grove), savannahs with carnauba palm (Copernicia), are associated with badly drained depressions and sites subject to flooding (eutric planosols and eutric fluvisols).
The soils of the African savannahs
Africa is the continent where is it possible to draw the clearest parallels between soils and vegetation; the grassy savannahs gradually turn into woody savannahs as water stress diminishes and nutrient stress increases. This is most clearly shown in western Africa, where the well-known succession of Sahel, Sudan, and Guinea savannah reflects climatic zonality. Yet soil conditions, often inherited from past climatic periods that were very different, are superimposed on the climatic zonality. (For example, large areas of ancient dune systems are covered by arenosols, and vertisols have formed on Pleistocene clay plains.) In subhumid zones, prolonged weathering has given rise to ferruginous crusts on senile ferralitic soils. There is no continuum between the rainforests of the wetter area and the woody savannahs; the transition zone is a mosaic of thickets, grasslands, and wetlands. When the structure of the vegetation deviates from what might be expected, it can often be attributed to periodic fires, grazing, or cultivation.
Wet African savannahs
The dystrophic woody savannahs known as miombo (communities of Isoberlinia in western Africa and communities of Brachystegia and Julbernardia south of the equator) occupy ferric lixisols and, to a lesser extent, the ferric acrisols that can be considered as the zonal soil types. There are dystrophic herbaceous savannahs on highly nutrient-deficient weathered ferralitic soils (haplic ferrasols). The woody elements are sclerophyllous species of genera like Uapaca (Euphorbiaceae), Parinari (Chrysobalanaceae), or shrubby members of the Ericaceae or Proteaceae. In western Africa, similar communities occur on ferruginous crusts, with species of Uapaca and dipterocarps (tall hardwood trees) of the genus Monotes. The Quaternary sands of the geological formation known as Kalahari (a much larger area than the Kalahari desert) cover the entire upper basin of the River Zambezi in Zambia, Angola, and Democratic Republic of Congo. There, the ferralic arenosols are associated with albic arenosols, gleysols, and podzols. This medium determines the previously mentioned mosaic of rainforest and savannah, with rainforest found mainly on deep and well-drained ferralic arenosols. In western Africa, similar formations occur in northern Sierra Leone.
Tall grassy savannahs with acacias and other scattered trees develop on soils derived from calcium-rich gneiss and on some recent terraces with a medium texture. Some of these eutrophic savannahs have been brought into cultivation.
Arid and semiarid African savannahs
Some large areas of arid and semiarid savannah in Africa grow on Pleistocene clays, while others grow on soils formed by layers of eolian sand deposited during the driest phase of the Pleistocene. On clay plains, vertisols have developed under a cover of soil-related grassy savannah. On layers of sand, the soils are luvic or cambic arenosols, with a savannah of finer grass. Acacias are often the dominant woody elements in both types of savannah. It is generally accepted that in a climate with an annual rainfall of less than 20 in (500 mm), competition between grasses and trees is mainly governed by the availability of moisture and the rooting conditions. The mineral composition of the sands is, however, also important and greatly influences the floristic composition. On white calcareous sands the dominant plants are acacias and species of the genus Commiphora (Burseraceae), while on red sands with iron the dominant plants are species of Anogeissus, Combretum, and Terminalia (all members of the Combretaceae). (They also dominate the vegetation on laterite crusts and on ferric lixisols that have developed from acidic gneiss.) Whether the soil can be ploughed for agricultural purposes is also conditioned by the sand's mineral composition. The eutrophic savannahs with acacias are found on haplic and chromic luvisols and sometimes on vertic luvisols that are derived from more basic rocks such as basalts. The South African savannah of knoppiesdoring (Acacia nigrescens) provides the best lands for extensive grazing. Between the miombo and the South African savannahs of acacias and members of the Combretaceae, there are woody savannahs with mopane (Colophospermum mopane, Leguminosae), typically occupying the areas of shallow vertisols associated with calcic luvisols and calcareous cambisols, derived from the Triassic calcareous siliceous sandstones and schists of the Karoo. These soils have a high content of magnesium and are saline and sodic, unlike any others in western Africa.
The soils of the Asiatic savannahs
India and Indochina do not have the large areas of open savannahs so characteristic of tropical Africa and America. The hydromorphic soils of many floodplains and low terraces, mainly eutric gleysols and fluvisols (and not dystric ones, as in tropical America), have in some cases been cultivated for many centuries. Ecologists in general think that the climax vegetation in the highest areas should be a forest, but over the centuries human occupation has transformed it into the current mosaic of savannahs and other secondary anthropogenic formations. Without denying the effects of agricultural activity, there are correlations between soil and vegetation similar to those in other continents. Dystrophic woody savannahs have a cover of citronella grass (Cymbopogon nardus), and eutrophic ones have a cover of acacias. The mesotrophic woody savannah communities contain members of the Combretaceae (Anogeissus, Terminalia).
Nutrients and aluminum toxicity
In large areas of India and Indochina, the parent materials are mostly aluminous quartzitic rocks with scattered more basic rocks, such as limestones, basalts and charnockites. The aluminous quartzitic rocks are associated with orthic and ferric acrisols and dystric cambisols, while the basic rocks are associated with nitisols, haplic lixisols, and eutric cambisols. These groups correspond to dystrophic and eutrophic environments, respectively. Most of these soils belong to the ferruginous group.
The effect of aluminum toxicity on the floristic composition of the vegetation is greater in Asia than in other continents. In areas where annual rainfall exceeds 98 in (2,500 mm), all the soils are washed and the aluminum is mobile, but this is uncommon in areas where levels are less than 59 in (1,500 mm). In areas with annual rainfall between these two limits, the development of aluminum toxicity depends on the composition of the parent material; aluminum toxicity will be greater in soils formed on porous, siliceous substrates than in those formed on substrates rich in alkaline earth elements. There may be very clear differences between them in sites separated by very short distances.
The dystrophic grassy and woody savannahs are linked to highly weathered and washed soils. In Indochina, on the border region between Vietnam, Cambodia, and Laos, on senile soils derived from basalts (rodic and haplic ferrasols), a mosaic arises of a sort of bilberry (Vaccinium) thicket, with grasslands of citronella grass and other species of the genus Cymbopogon. The mosaic of grass meadows and forests in the Southern Ghats and the mountains of Sri Lanka, where in some cases they are called patanas, might be related to soil nutrient deficiency, as they are humic and haplic acrisols. In Indochina, woody savannahs with dipterocarps develop on siliceous sediments derived from Mesozoic siliceous sandstones, lutites, and quartzite and on skeletal soils derived from schists. Haplic acrisols dominate dissected surfaces, while ferric and gleyic soils dominate the ancient terraces of the River Mekong and the Khorat Plateau in Thailand. The vegetation structure resembles the African woody savannahs of Brachystegia and Julbernardia, and the development of the soil profile in these acrisols has much in common with that of the ferric lixisols of the African woody savannahs with a similar structure. These Asian savannahs dominated by dipterocarps, however, are more likely to show aluminum toxicity than the African savannahs dominated by members of the Leguminosae. In some places there is a tendency to form humic podzols, and in both Indochina and Sumatra the dipterocarps are replaced mostly by gymnosperms (Pinus merkusii, P. khays), forming pinewoods that closely resemble those in Central America. No equivalent formations exist in India or Malaysia, although the monsoon forest of the subhumid lowlands of India, dominated by sal (Shorea robusta, Dipterocarpaceae) (see volume 2, p. 426), might be their ecological equivalent. They grow on ferric lixisols, derived from acid or intermediate pre-Cambrian gneiss. The forests of teak (Tectona grandis, Verbenaceae) reach their greatest development in India on soils rich in alkaline earth cations (eutric nitisols, eutric vertisols, and chromic lixisols). High soil fertility and low permeability favors vigorous grass growth, while teak, a sun-loving plant that needs open formations, forms a mosaic of scattered thickets within the grassland. In southern India, teak is replaced by acacias on haplic lixisols derived from gneiss.
Diminishing rains: dry savannahs
The dry zone of India, with an annual rainfall of 20-28 in (500-700 mm), is covered by savannah dominated by acacias such as khair (Acacia catechu) or A. arabica, or members of the Combretaceae such as dhao (Anogeissus pendula) or bakla (A. latifolia). Savannahs with acacias form mainly on the vertic cambisols and thin vertisols on the Deccan Plateau, with a neutral to alkaline pH. The savannahs with members of the Combretaceae are typical of infertile and slightly acid eutric cambisols and haplic lixisols, derived from the gneiss and quartzitic and aluminitic schists of the Aravalli Range.
The soils of the Australian savannahs
The zonation from north to south shown on the Australian continent, from tall thickets to grasslands with scattered shrubs and small trees (that is, from woody savannahs to grassy savannahs) is similar to that of Africa north and south of the equator. The woody species dominating the Australian savannahs are members of the Myrtaceae (Eucalyptus, Melaleuca), Proteaceae (Banksia, Grevillea), and Leguminosae (Acacia, among others). These families are also important in Africa's grassy and woody savannahs. Some authors consider that the woody eucalyptus savannahs are equivalent to the African savannahs of Brachystegia and Julbernardia and compare the grassy savannahs of the interior of Australia with the African Sahel savannah.
The Australian vegetation is evergreen and highly sclerophyllous, whatever the climate. Even in the hummock grasslands of the arid zone, the species of genera like Triodia or Plechtrachne are evergreen; they are highly lignified and form the hummock grasslands called "porcupine" or spinifex grasslands. These features can be considered adaptations to the generalized nutrient deficiency, mainly of nitrogen and phosphorus, aggravated by the high sodium and magnesium content of the cation exchange complex and by the lack of micronutrients such as zinc, copper and molybdenum (a deficiency also found in the arid and semiarid areas). Because of its low agricultural potential, land use has for centuries been restricted to hunting and gathering, in sharp contrast to the shifting agriculture practiced in Africa. Grazing, introduced barely two centuries ago, stayed at very low levels until a hundred years ago. As a result, in Australia the biotic factors are considered subordinate to soil factors in determining the presence of savannah, and the great influence of the soil on the vegetation is more widely accepted than in Africa or Asia.
In Australia's landscape, two extreme cases may arise: poor siliceous soils and vertisols affected by periodic waterlogging. Regardless of the climate, these two extremes support grasslands. In the poor siliceous soils, because of the lack of nutrients, a perennial hummock grassland develops, while in regularly water-logged vertisols an annual tussock grassland develops. Under more favorable conditions woody formations may develop whose floristic composition and structure are controlled respectively by the soil's chemical characteristics and the availability of water and oxygen.
Rich in magnesium and sodium, but poor in nutrients: poor savannahs
One general feature of Australian soils is their high proportion of magnesium and sodium in the cation exchange complex; in these conditions, the clay disperses and accumulates in argillic horizons that are often impermeable, and the epipedons show a clear tendency to seal the surface as a result of the action of rain. They are gleyic acrisols, ferric lixisols, and albic lixisols with a sodic phase. In conditions of intense illuviation, true eluviated E horizons form, and the subsurface horizons solidify and form natric [B.sub.tna] horizons, with a columnar structure unfavorable for root growth. They are planosols, orthic solonetz, and solonchaks.
In northwest Australia, the highly fragmented pre-Cambrian quartzites and siliceous sandstones form shallow sandy soils (regosols) and leptosols. Toward the interior, the soils of the highly weathered mantle are massive and unstructured, but with coherent sandy or loose surface horizons; there is a gradual increase in clay at depth and moderate acidity ("red earths" and "yellow earths"). Most woody eucalyptus savannahs form on these soils.
In drier climates with annual rainfall below 20 in (500 mm,) the soils show neutral epipedons and alkaline endopedons, or are completely alkaline. There may also be layers cemented by silica, called silcrete, which contain iron and are impermeable and resistant to weathering. The soils are regosols and ferralic arenosols. The characteristic vegetation is the mulga, a woody savannah with acacias (mainly Acacia aneura, the mulga). On slopes on dissected surfaces, calcic luvisols develop (petrocalcic and sodic phases) that may contain carbonates or gypsum. The dominant trees are often gidgee (Acacia cambagei) and A. georginae.
Fertile vertisols: rich savannahs
In Australia, vertisols mainly form in two different types of situations: on recently exposed sedimentary rocks or on deep sediments in the colluvial plains at the base of the western slopes of the northeastern mountain ranges. In both cases, the dominant cation in the endopedon is magnesium.
The vertisols formed on sedimentary rocks are shallow (less than 3 ft [1 m] deep) and have a high phosphorus content. They develop tussock grassy savannahs dominated by species of Astrebla (Mitchell grasses), with a few scattered acacias, and are considered the best grazing land in Australia. The vertisols formed on colluvial sediments are deep and are characterized by a rugged microrelief (known as gilgai) featuring raised areas and depressions with a diameter varying between 10-66 ft (3-20 m). The higher parts, which are also the best drained, may develop woody savannahs with a dense tree cover of brigalow (Acacia harpophylla), whose nitrogen-fixing root nodules maintain the soil's high nitrogen levels.
3. The world's savannahs
3.1 The American savannahs
In the American tropics, as in the other tropics, nutrient availability is surely as important a limiting factor for the plants as water availability. These factors combine, together with fire, to shape an environmental setting that favors the persistence of open formations. The largest areas of woody savannahs and dry forests are in the tropics, but in South America they extend far beyond the tropical latitudes, crossing the subtropical regions and reaching the temperate areas of central and southern Argentina and Chile.
Savannahs exist under a wide range of environmental conditions and are dispersed throughout Central and South America. There are many very different types of savannahs, some related to the equatorial rainforests, others (at the very edge of the savannahs) clearly related to the subtropical xerophytic forest and scrub and the steppes of cold climates. The discussion begins with the woody savannahs of wetter environments with affinities to forests. Separating them from the different types of grassy savannah often appears arbitrary, as there are transition forms such as those discussed in the section on the Brazilian cerrados. Following an examination of the grassy savannahs, the final section of this chapter will focus on the forests and woodlands (parklands) of the Chaco and the transition formations (with the steppes and temperate meadows in Argentina and with the forests and scrubs of central Chile). The one characteristic they all share--one that distinguishes them from the African and Asian savannahs--is the relative poverty of the large mammalian herbivore fauna, whose role in many American savannahs is partially replaced by invertebrates, especially by leaf-cutting ants of the Attini tribe.
The Amazonian caatinga and the tepuys of Guyana
The immense Amazonian region, covered mostly by rainforest, contains many patches of nonforest ecosystems, especially wetlands, grassy savannahs, and woody savannahs. Something similar happens with the Guyanan jungles that run from southeastern Venezuela to near the mouth of the giant River Amazon. The woody savannahs occupy very unusual habitats within this immense landscape of continuous rainforest.
In fact, on the unconsolidated sandy materials consisting mainly of quartz, known as areias brancas (white sands), the forest is replaced by low open woody formations that were given different names when they were discovered and analyzed in different areas of Amazonia. These areas of white sands--although they never occupy large continuous areas--are particularly frequent in the upper basin of the Negro River, where there are outcrops of the old geological structure of the Guianan Shield. The sands also characterize a strip of the coastal plain in the Guyanas. In Brazil the open woody vegetation is called Amazonian caatinga or campinarana (see volume 2, p. 50); in Venezuela it is known as bana; in Colombia it is called varial; and in Guyana and Surinam it is considered a special type of woody savannah or a tall sclerophyllous scrub (not the same as the spiny caatinga of the arid northeast of Brazil). The extreme poverty of the almost pure quartz sandy soils (Amazonian podzols) on which it grows gives rise to a structurally and floristically poor vegetation, showing clear physiognomic similarities to the woody savannahs of the Brazilian cerrado.
The dominant species are trees or palms that reach heights of 33-49 ft (10-15 m) in the areas where the vegetation is most developed, while in the lower and more open formations they do not exceed 20-26 ft (6-8 m). The lower layer, often very dense, is dominated by shrubs and herbaceous bromeliads, aroids, and other broad-leaved monocotyledons, but with very few grasses. Mosses, lichens, and algae are very abundant, covering the soil and the base of the trunks. The fauna of these environments, like their flora, is very poor.
The formations typical of the Guyanan tepuys resemble the Amazonian caatingas to some extent. Strange relics of a world that has disappeared, tepuys are large, eroded, tabular blocks of pre-Cambrian siliceous sandstones. Their biotas have evolved in isolation and are exceptionally rich in endemic species, despite their structural poverty, as these formations can at best (if the composition of the rocky substrate is favorable) grow into a low open forest, despite normally being restricted to thicket (but a thicket linked to the rainforest; see volume 2, pp. 32 and 359).
This thicket shows an extraordinary richness and diversity of herbaceous and subshrub elements, dominated by members of the Bromeliaceae, Rapateaceae, and other related families. The characteristic vegetation of the tepuys of the Guiana Highlands covers its largest area in Venezuelan Guiana, in the southeast of the state of Bolivar.
These two types of formations--the Amazonian caatinga and the tepuys of the Guiana Highlands--share the extreme ecological adversity of a quartzitic sand substrate almost totally lacking nutrients, together with a highly acidic organic soil in which organic matter decomposes and recycles very slowly.
Water, however, because of the very high annual rainfall (from 138-197 in [3,500-5,000 mm] or more), is not a limiting factor, although the soil is very permeable. These tropical formations on siliceous sandstones can be compared, on the basis of their trophic characteristics, to another sort of ecosystem--the peat bogs of the high mountain or the tundra (see volume 9, p. 43).
The Brazilian cerradao and cerrado and similar formations
A different sort of woody savannah, highly characteristic of the South American tropics, is the cerradao, low open sclerophyllous forests typical of the plateaus of the interior of Brazil known as cerrados. The Portuguese name cerrado is applied to a very large natural region of the center of Brazil. The same term is applied to: 1) a biogeographical province with its own characteristic flora and fauna and 2) a set of plant formations with differing proportions of woody and herbaceous elements. As a natural region, the cerrados cover almost 772,200 [mi.sup.2] (two million [km.sup.2]) of the ancient Brazilian Shield, mainly in the states of Minas Gerais, Bahia, Goias, and Mato Grosso.
The relief of the cerrado zone is characterized by deeply dissected plateaus forming vast stepped landscapes at elevations of 1,083-6,562 ft (330-2,000 m). In some cases, the dissection of these ancient surfaces has turned the peneplains into mountain ranges (Espinhaco Mountains, Canastra Mountains). As a natural region on a continental scale, the cerrado is a recognizable unit lying between the rainiest Amazonian plains, the dry formations of the spiny caatinga in the northeast and the subtropical forests to the south, where the winter is much more discernible. The factor unifying this landscape, in addition to its relief, is the plant cover consisting of grassy savannah formations or cerrado in a broad sense, ranging from pure grasslands to open forests.
Even in the densest form--the cerradao--the tree canopy never closes completely, allowing in enough light for the growth of a significant herbaceous cover rich in grasses. Thus, it can be considered either a forest or the densest form of tree savannah with the most developed trees. In any case, the floristic and faunistic similarities with the surrounding savannahs are so great that it is difficult to understand the cerradao as anything other than the most forestlike member of a sequence of savannah formations. That sequence goes from almost pure grasslands (campo limpo), through other grasslands with relatively more shrubs and low trees (campo sujo), to the typical woody savannahs (campo cerrado or cerrado "sensu stricto"), and finally reaches the greatest richness in tree elements and the highest biomass in the cerradao.
The Serra do Roncador, the division between the watersheds of the River Xingu and the River Araguaia in the Brazilian state of Mato Grosso, is one of the areas where the cerradao has been most thoroughly studied. The tree layer, consisting of trees that rarely exceed a height of 39-49 ft (12-15 m), covers around 50% of the ground surface, allowing the development of a large grass layer that encourages the periodic spread of fire. The existence of discontinuous patches of cerradao in the central Brazilian planalto has been interpreted in two distinct ways. The first hypothesis holds that these patches are relics of a forest cover that was continuous until humans arrived; human action, especially the frequent fires set for hunting purposes, has since reduced these to "islands" of open degraded forest within a "sea" of savannahs. The second explanation holds that the savannah is the stable formation in equilibrium with the environmental conditions of central Brazil (climate, geology, soils, and fires), while its richest form, the cerradao, only prospers in the few environments more suitable for tree growth due to their more favorable soil moisture and nutrient conditions.
The campo cerrado consists of woody savannahs that are more open than the cerradao and have tree and shrub cover of 10-30%. The great diversity of woody species, together with the rich herbaceous flora, makes the campo cerrado the world's most floristically diverse tropical savannah formation. For example, 300 species of plants were recorded in a 1 hectare (1 hectare=2.47 acres) protected plot of campo cerrado near Brasilia. Apart from the central core area of cerrado, this open formation spreads northeast and southeast in a series of islands. To the northeast, they stand out from the shrub and subdesert formations of the northeastern caatinga, while to the southeast, they interrupt the original continuity of the southern Brazilian subtropical forests (see volume 6). There are relatively large islands of cerrado in both northeastern Brazil (in the states of Bahia, Pernambuco, and even Rio Grande do Norte) and southeastern Brazil (in the states of Sao Paulo and Parana). These woody formations always occupy tables--known as chapadas or tabuleiros--rising above the surrounding plains. There is also a large area of grassy and woody savannahs, equivalent to the cerrados of the planalto, but on the other side of Amazonia, in the state of Roraima, in the lowest areas of the Guiana Shield. These savannahs in the basin of the Rio Branco continue in Guyana (Rupununi) and in the Gran Sabana (the Great Savannah) area in Bolivar State (Venezuela).
The llanos of the Orinoco and the Mojos region
Venezuela and Colombia contain the second largest savannah region in South America, the llanos of the Orinoco, covering a total of almost 193,050 [mi.sup.2] (500,000 [km.sup.2]). Like the cerrado, the llanos show a varied mosaic of ecosystems, from tropical deciduous forests to evergreen gallery forests along rivers and seasonal rivers. But the landscape is dominated by different grassy and woody savannah ecosystems, as well as by esteros and other types of wetland.
There is also the same gradient of tree density and diversity (from pure grasslands to cerrado type woody savannahs) as discussed in the Brazilian planalto, although the diversity of tree species is much lower in the llanos. Some authors use the term chaparral for the savannahs with a significant cover of low trees of different species (and even families) called chaparro or peralejo, mainly Curatella americana (Dilleniaceae), but it is preferable to reserve the term chaparral for the Mediterranean chaparral formations of California, which are ecologically very different from the savannahs (see volume 5, p. 59). The largest area of these tree savannahs is in the central llanos of Venezuela, in the states of Guarico and Anzoategui, as well as in the north of the state of Bolivar to the south of the Orinoco. In the Colombian llanos a very special type of open woody formation appears on seasonally flooded soils. It is known as saladillales and is dominated by saladillo (Caraipa llanorum, Guttiferae), whose appearance and physiognomy resembles some evergreen oak woodlands of the Mediter-ranean Basin. In the Mojos region of Bolivia are savannahs similar to the Venezuelan and Colombian llanos. These flood-prone formations are often dominated by palms. (A more detailed discussion appears in the section on the Beni Biosphere Reserve; see p. 414.)
The subtropical chaco
Moving from the tropics to the subtropics in South America, the clearly tropical formations of rainforest, grassy savannahs, and woody savannahs are gradually replaced by the completely different formations of the Chaco region, the increasingly tree-covered savannah or chaco (a Quechua word that means hunting place). The Chaco covers an area of almost 386,100 [mi.sup.2] (1 million [km.sup.2]) and dominates the subtropical plains of Bolivia, Paraguay, and Argentina.
The Chaco is a diverse and fascinating biome, often dominated by spiny plants (see p. 53). It is characterized by forests, varying in growth-form with the climatic conditions, which become drier and more continental moving across the region from the River Paraguay and the River Parana toward central-western Argentina (where temperatures may reach 118[degrees]F [48[degrees]C]). On the banks of these two great rivers, the chaco forest shows the growth-form and diversity of a subtropical forest (the wet eastern Chaco, popularly and revealingly known as "impenetrable" forest); as the winter becomes longer, colder, and drier, the forest becomes poorer until eventually--at the southwestern tip--it turns into a xeromorphic thicket or dry chaco. The dry chaco gradually merges into the subdesert of the Argentinean monte scrub bordering it on the mountainous foothills and western inland valleys at the base of the Andes.
This gradient of increasingly low, open, and sclerophyllous forest formations is interrupted where the mountainous foothills (for example, the Pampean Sierras in northwest Argentina) retain the humidity and thus favor the presence of a richer chaco forest (chaco serrano [mountain chaco]), or when annual rainfall is higher, leading to the appearance of even wetter subtropical savannahs and forests. There used to be an open forest with majestic trees such as the tipa (Tipuana tipu, Leguminosae) or the timbo (Enterolobium contortisiliquum, Leguminosae), as well as what are called transition forests, with many valuable wood species such as cebil (Piptadenia macrocarpa, Leguminosae), cedrella (Cedrela mexicana, Meliaceae), and the southern walnut (Juglans australis, Juglandaceae); however, the reckless advance of farming and stockraising along the slopes and lowlands of the mountains of northern Argentina and Bolivia threatens the last remnants of these original formations.
In addition to the diversity of its forest systems, the chaco contains rich and varied communities dominated by a herbaceous layer: floodable savannahs, grasslands, an extraordinary variety of aquatic and semiaquatic environments in the eastern part, broad saline or alkaline grasslands on its limit with the pampa, and--in both the many eastern valleys and in some depressions in the western Chaco--large palm groves of the carandai palm (Copernicia australis). In a broad transition strip between the moist formations of the eastern Chaco and the subhumid and dry formations of the western Chaco, a special "parkland" physiognomy develops that strongly recalls the seasonal grassy savannahs of the cerrado and the llanos, but with a completely different flora and fauna.
Until a few decades ago, the northwestern part of the Chaco was a mosaic of subtropical grasslands and savannahs. The introduction of cattle ranching exceeded the carrying capacity of these herbaceous systems and rapidly led to the secondary introduction of spiny shrub species, of which the vinal (Prosopis ruscifolia) is the clearest and saddest example. These almost monospecific spiny thickets, known as vinalares, are of no economic interest at all, either as forest or as grazing, and the increase in their area is a clear example of the negative consequences arising from the nonsustainable use of natural resources.
Towards their southern limits, the chaco formations gradually become poorer in woody elements, so the importance of the herbaceous layer increases. These grassland formations with small, often spiny trees are known in Argentina as mesopotamic parkland or espinal, and form a large semicircular strip around the pampas grasslands. The most characteristic trees of the espinal include the calden (Prosopis caldenia), found on sandy soils in the Argentinean provinces of Cordoba and San Luis. The calden has long been exploited for its high-quality wood.
The savannah formations of the Southern Cone
In the temperate zone of central Chile under a Mediterranean climate with winter rains, the typical original sclerophyllous forest of this climatic region (see volume 5, p. 60) has been replaced by savannahs as a result of human activity. These savannahs are very similar to the Argentinean espinal, but the dominant grasses are mainly annual species and the typical espino is Acacia caven. Excessive grazing and frequent fires have stabilized some secondary formations of scrub, which retain some species from the original forest, but within a much lower and more open plant structure--one similar to the primary vegetation that grows in drier climates (see volume 4).
The temperate-cold region of the southern Andes in Chile and Argentina contains other open woody formations that form broad ecotones between the dry steppes and the moist forests of southern beech (Nothofagus). The most interesting open savannah-type formations in these areas include the forests of southern conifers, such as the forests of monkey puzzle tree or Chilean pine (Araucaria araucana) and the more open forests of incense cedar (Libocedrus chilensis). Both formations occur as outposts of tree vegetation on the steppes of Patagonia.
The savannahs of Central America and the Caribbean
On the Isthmus of Panama and the Caribbean islands, woody savannahs are not as important as in South America, as this region lacks the types of environments where these ecosystems typically flourish. Rather, the main open formations are grassy savannahs together with pinewoods, which may be considered as a very special type of savannah. The savannahs of Mexico, Cuba, and Costa Rica form many patches of open vegetation, although they cover only a limited area. They are similar in composition, structure, and ecology to the seasonal savannahs of South America and are dominated by species like the curatella (Curatella americana) and the nance (Byrsonima crassifolia; another of the many species commonly known as chaparro or peralejo, both of them widely distributed throughout the New World tropical savannahs). In addition, in the dominant layer, there are some species of trees of boreal origin, such as oaks (Quercus) and pines (Pinus). Pine forests are very characteristic formations in some types of habitat, covering relatively large areas in Belize, Honduras, Nicaragua, Cuba, and the Dominican Republic. These open formations are generally associated with sandy substrates, as in the Mosquito Coast (on the Nicaragua-Honduras border on the Atlantic coastline), or on deposits of white sands, very similar to the Amazonian caatinga described above. This also occurs in Cuba. In Belize, pine forests of Honduras pine (Pinus caribaea var. hondurensis) grow on sandy soils with an impermeable subsoil on both the coastline and inland hills. In reality, these formations consist of mosaics of open formations such as pine forests, oak forests of Quercus oleoides, tree savannahs, and wetlands with palms.
The Sabanas de Misquitos are pine forests of Cuban pine (Pinus caribaea) running in a strip 31 mi (50 km) wide along the Caribbean coastline on both sides of the Nicaragua-Honduras border. They grow only on coarse materials, the lower Pleistocene gravels and sands present between sea level and an elevation of 164 ft (50 m). The pine forests occupy the higher well-drained soils, and in the lowland soils prone to flooding they are replaced by different savannahs and wetlands. The border with the tall forests that spread into the interior is very clear, because it corresponds to a clear difference in geology and soil; as soon as the sandy substrate gives way to finer-textured materials, the rainforest displaces the pines. These conifer-dominated tropical formations are related to the pine forests of Louisiana and Florida in the southeastern United States (see volume 6), and, like them, they are swept periodically by understory fires. (These do not greatly affect the adults but have an impact on regrowth). The pine has been heavily exploited commercially, but its natural regeneration ensures the survival of the system--although with lower densities than in the original system--in a phenomenon of savannization of the original ecosystem.
In Cuba and on the Isle of Pines (Isla de la Juventud), there are also pine forests of Cuban pine (Pinus caribaea var. caribaea) on sandy substrates. Thus, for example, in the province of Pinar del Rio in the west of the island of Cuba, an open woody formation characterizes the deposits of quartzitic sands. Cuban pines (Pinus caribaea and Pinus tropicalis), together with several species of palm and a relatively diverse shrub understory, rise above the low vegetation. These formations show a surprising scarcity of grasses, and almost all the species show highly sclerophyllous features, a notable case of scleromorphism caused by nutrient limitations. Other open woody formations occupying very specific habitats in Cuba include those growing on outcrops of serpentine, where the soils contain high levels of heavy metals that are toxic to most plants, and the formations typically found on soils covered with more or less dismantled laterite crusts. Both are open formations of low trees with a great variety of palms.
In the Dominican Republic, on a relict relief of highly dissected plateaus, there are tree savannahs with pines (Pinus occidentalis) or with the typical small trees (chaparros and peralejos) of the New World tropical savannahs such as Byrsonima and Curatella. Similar communities exist on many of the islands of the Lesser Antilles, although no species of pine has reached any of them.
3.2 The African and Madagascar savannahs
Africa contains the largest area of the savannah biome and the greatest diversity of vegetation types and animal populations. As in South America, the large plateaus of the interior have woody savannahs, which can often be considered open forests, while in the most depressed and often badly drained areas there are usually grassy savannahs; however, this general model includes an extremely wide range of types, even in neighboring zones.
The Guinea savannahs
From the Atlantic coastline between the lower Casamance River and western Liberia to the Upper Nile, woody and grassy savannahs form a broad strip to the north of the equatorial rainforests of the Guinea region with which they show many floristic similarities. These savannahs sometimes even occur within the rainforest, in patches of nutrient-poor soil or those exploited in the past by shifting agriculturalists. Savannahs are also present to the south of the equatorial rainforest area, both in the lower basin of the Democratic Republic of Congo and in some areas of Gabon, and contain small patches of mainly herbaceous vegetation (esobe) growing within the rainforest.
Among the typical features of these savannahs are: 1) the frequent presence of small patches of rainforest and of gallery forest along the watercourses; and 2) the absence of some species typical of the Sudan savannah to the north such as the shea butter tree or karite (Vitellaria [=Butyrospermum] paradoxa, Sapotaceae). The most common trees are Anogeissus leiocarpus (Combretaceae), Lophira lanceolata (Ochnaceae), several species of the genus Isoberlinia (Legumi-nosae), and the African palmyra palm or ronier (Borassus aethiopum). The grass layer is usually tall, from 4.9-9.8 ft (1.5-3 m), and is dominated by species of the genera Andropogon, Hyparrhenia, Loudetia, Panicum (such as P. maximum, Guinea grass), and Pennisetum. The savannahs of the southern edge of the rainforest of the Zaire Basin, in contact with the miombo, contain many dwarf woody species of genera well represented among the trees of the African rainforest, including Parinari pumila (Chrysobalanaceae), Landolphia lanceolata (Apocynaceae), and Anisophyllea poggei (Anisophylleaceae).
The mammalian fauna show some tendency to melanism, generally linked to the forest habitat. The most important herbivores include Derby's large eland (Taurotragus derbianus), roan antelope (Hippotragus equinus), the kob (Kobus kob), and common waterbuck (K. ellipsiprymnus); most of the large carnivores of the African savannah fauna are present, as are many of the smaller ones such as mongooses and civets.
The Sudan savannahs
To the north of the Guinean savannahs, running from the Atlantic Ocean to Ethiopia, lies a zone of savannahs bordered to the north by the Sahel subdeserts. These savannahs, together with those in eastern Africa, are probably the most typical and best known of the African grassy savannahs (although the zone also contains both tree and shrub woody savannahs, which in some cases adopt the appearance of open woodlands dominated by some of the tree species also present sporadically in the grassy savannahs).
The southern limit of the shea butter tree or karite (Vitellaria [=Butyrospermum] paradoxa), as already mentioned, more or less forms the limit between the Sudan savannah and the Guinea savannah. To the north, the transition towards the Sahel zone of subdeserts with its thorny plants and discontinuous herbaceous cover follows an almost imperceptible gradient. At the western tip of the Guinea savannah area, most directly subject to the Atlantic influence (Senegal and Mali), this border is near the 13th parallel (13[degrees]N) and moves gradually south to about 10[degrees]N in the Central African Republic and Sudan, and even farther south in southern Sudan. The presence of sedentary populations linked to cereal-growing agriculture is probably the feature most closely marking the limit between the savannah and the subdesert, which the persistent droughts of the last decades have displaced southward. The most common species belong to the Mimosoideae subfamily of the Leguminosae (Acacia, Prosopis, Albizia, Parkia), the Caesalpinoideae subfamily of the Leguminosae (Isoberlinia, Cassia, Afzelia), the Combretaceae (Anogeissus, Combretum, Terminalia), and others belonging to different families such as Uapaca togoensis (Euphorbiaceae), Khaya senegalensis (Meliaceae), and Monotes kerstingi (Diptero-carpaceae). In other agriculturally exploited regions, some tree species are not cultivated but are preserved by farmers because they are useful. Among these species are the shea butter tree, the cad or gao (Acacia [=Faidherbia] albida), and the famous baobab Adansonia digitata (Bombacaceae).
The large herbivore fauna of the Sudan savannahs is even richer and more varied than that of the Guinea savannahs. It includes all the antelopes mentioned above, as well as the bushbuck (Tragelaphus scriptus), the red-fronted gazelle (Gazella rufifrons), the oribi (Ourebia ourebi), and several duikers (Sylvi-capra grimmia, Cephalophus rufilatus), as well as the African elephant (Loxodonta africana), Cape buffalo (Syncerus caffer), giraffe (Giraffa camelopardalis), warthog (Phacochoerus aethiopicus), and anubis baboon (Papio anubis) and other monkeys. Their predators--the lion (Panthera leo), leopard (P. pardus), cheetah (Acinonyx jubatus), African hunting dog (Lycaon pictus), as well as the wide range of hyenas (Crocuta, Hyaena) and jackals (Canis adustus, C. aureus, C. mesomelas)--are also present.
The savannahs of eastern Africa
Eastern Africa contains the definitive savannahs with a physiognomy similar to the ones mentioned above, an image spread by safari stories, films, and even the tourist promotion of the region's large parks (without a doubt the best-equipped parks in Africa). Broadly speaking, these savannahs run from the Great Lakes region (in eastern-central Africa) to the Indian Ocean (bordered by Lake Victoria in the north and the lower Zambezi in the south). They typically consist of a tall herbaceous layer dominated by the grasses Themeda triandra and Heteropogon contortus, with scattered trees of several families, mainly members of the Mimosoideae subfamily of the Leguminosae but also of the Caesalpinoideae subfamily of the Leguminosae (Brachystegia, Julbernardia) that form miombo woody savannahs and open woodlands in other areas of southern Africa. These miombos appear locally in the most raised parts of the plateaus bordering the Great Rift, even though they are more characteristic of the interior of the African continent. These African savannahs have a rich and varied large mammal fauna that includes many herbivores and carnivores. Some of the herbivores, such as the brindled gnu (Connochaetes taurinus), Burchell's zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni) live in large herds that migrate seasonally. Others, such as the impala (Aepyceros melampus), live in small family groups of harems with a dominant male. African elephants (Loxodonta africana), on the other hand, form small groups of females with their young, led by the oldest female; the males separate from the family groups on reaching puberty and live alone or in groups with other young males.
The typical woody savannah or miombo
To the south of the rainforest and the floristically related savannahs of the Zaire Basin, running almost to the banks of the River Okavango and the River Zambezi, lies a vast region of plateaus covered mainly in woody savannah, often considered open woodland. This savannah is known locally as miombo and is characterized by the dominance of members of the Caesalpinoideae subfamily of the Leguminosae belonging to the genera Brachystegia, Julbernardia, and to a lesser extent Isoberlinia. The most typical miombos occur in higher areas (3,281-6,070 ft [1,000-1,850 m]) and have a dense herbaceous layer of perennial grasses. At lower elevations, in the bottoms of the broad valleys of the region's large rivers (Okavango, Zambezi, Luangwa, Limpopo, Kunene), the woody savannahs are represented by open forests of mopane (Colophospermum mopane, Caesalpinoideae subfamily of the Leguminosae). To the north, on the southern edge of the Congo Basin, especially in the relatively depressed areas of the province of Shaba (Katanga) in Democratic Republic of Congo and northern Zambia, another variant appears dominated by Marquesia macroura (Diptero-carpaceae) and Berlinia giorgii (Caesalpinoideae subfamily of the Leguminosae, a variant often considered a secondary formation resulting from the destruction of a dense evergreen formation). The grassy savannahs that occur locally in badly drained areas are similar to those of the plains of Kenya and Tanzania. The most characteristic feature of the grand herbivore fauna is the presence of forest species with striped coats such as the bongo (Tragelaphus euryceros) and nyala (T. angasi).
The South African mosaics of woody and grassy savannahs
From south of the miombo region to the southern tip of the continent (except in the most arid regions of the Kalahari desert and Namib subdesert) is a zone whose more complex relief is occupied by an intricate and inseparable mosaic of woody and grassy savannahs, Afromontane forests, temperate prairies, Mediterranean fynbos, wetlands, and gallery forests. We shall consider the savannah biome components: the western Kalahari grassy savannah and the woody savannah known in South Africa as bushveld.
The Kalahari, despite its reputation as a desert, is (at least in the central and northern parts) the southwestern limit of the savannah biome. Some experts feel this limit coincides with the southern limit of the distribution of Terminalia sericea (Combretaceae). Only the southern and western parts, bordering the Karoo and the Namib, are true subdesert. The absence of surface watercourses due to the permeability of the sandy substrate contribute to the common image of the Kalahari as a desert. The dunes contain only sparse populations of grasses with long rhizomes and the occasional specimen of witgat (Boscia albitrunca, Capparidaceae), a small plant similar to the caper, but most of the space is occupied by very open grassy savannahs. The herbaceous layer is dominated by Themeda triandra, known in South Africa as rooi grass, several grasses of the genus Aristida, and several species of Pentzia (Asteraceae). The tree layer contains several species of Acacia, mainly Acacia giraffae, the camel thorn or kameeldoorn, as it is known in South Africa, as well as A. heteracantha, A. mellifera, cross-berries (Grewia flava, Tiliaceae), buffalo jujube (Ziziphus mucronata, Rhamnaceae), and gigantic baobabs (Adansonia digitata, Bombacaceae) either scattered or in small patches.
The bushveld is the southern extension of the miombo type woody savannahs well beyond the Tropic of Capricorn, running along the midaltitude slopes of the mountains facing the Indian Ocean in Natal and the eastern tip of the Cape Province. Higher altitudes in eastern and northern Zimbabwe, in neighboring Mozambique, and in the northern Transvaal (at 5,249-5,905.5 ft [1,600 -1,800 m]), in contact with the Afromontane forests of the mountain peaks, are dominated by open forest of Brachystegia spiciformis, with Julbernardia globiflora, Faurea speciosa (Pro-teaceae), Ekebergia capensis (Meliaceae), Parinari curatellifolia (Chrysobalanaceae), and several species of Combretum and Terminalia (both Combretaceae). Human intervention, however, has often transformed this vegetation into a grassy savannah with P. curatellifolia (frequently left untouched for its fruits) and members of the Combretaceae or into secondary forests of Uapaca kirkiana.
At lower altitudes, between 4,593-5,249 ft (1,400-1,600 m), there are almost pure populations of Brachystegia boehmii with members of the Pro-teaceae, Monotes glaber (Dipterocarpaceae), and sometimes Parinari curatellifolia, with a grassy layer. These woody savannahs formerly covered much larger areas but have been replaced by more open savannahs with shrubs, mainly proteas (Protea angolensis, P. gaguedi). Below this, at elevations of 3,937-4,593 ft (1,200-1,400 m) from the Bulaweyo region of Zimbabwe to the southern limits of the biome, sandy soils derived from the breakdown of granite support the populations that mark the southern limits of the savannah biome--the woody savannahs or open forests of African lilac (Burkea africana, Caesalpinoideae subfamily of the Leguminosae) and silver limba (Terminalia argentea). Heavier soils at the same altitude are dominated by grassy savannahs with acacias, while badly drained soils are dominated by woody savannahs of mopane (Colophospermum mopane; shown in picture 38), which also occupy large areas at lower elevations, as happens in the more northerly parts of the region. The large fauna of the southern African savannahs is roughly the same as that already described for other areas of the African savannah. The region's characteristic animals include the small slender springbok (Antidorcas marsupialis), the endangered bontebok (Damaliscus dorcas), Burchell's zebra (Equus burchelli), and the mountain zebra (E. zebra). Southern African savannahs boast some of the largest remaining populations of sable antelope (Hippotra-gus niger); elephant populations are increasing in most of the protected spaces in the region.
Most of the island of Madagascar used to be covered by forests and thickets, but more than 80% of its land surface is now covered by secondary vegetation. Destruction and degradation of Madagascar's forests and thickets have given rise to several types of secondary vegetation, from sparse grasslands through grassy savannahs and woody savannahs to secondary forests.
Human activity is the main factor responsible for the destruction of the primary forest, and as the island is only thought to have been settled by humans 1,500-2,500 years ago, some scientists regard the development of grassy and woody savannahs as a relatively recent phenomenon. Yet doubts have been raised by the discovery of fossils of now extinct animals adapted to grazing, while pollen analysis of ancient sediments has shown that many of Madagascar's contemporary savannah and grassland plants were present long before the first humans arrived. In any case, although some savannah vegetation types may have existed before the first humans arrived, human activity is now a major factor in the dynamics of the savannah habitats and their expansion. The woody species of the Madagascar savannahs include fire-resistant relics of the original forests occupying the area, together with the opportunistic species that have invaded the secondary vegetation. It has been proposed that the current boundaries between savannah areas and forest areas correspond to limits between different classes of substrate and relief. This is probably a result of the different susceptibility to destruction by fire of the different types of forest. For example, forests with a poorly developed understory, such as those growing on limestone outcrops on the west of the island, are relatively fire- resistant and are generally well conserved, while neighboring areas with deeper sandy soils are now occupied by savannahs. In this case, the savannahs are maintained by regular burning and grazing by livestock.
The eastern region of Madagascar--consisting of the steep eastern escarpment (slopes between levels), the central highland plateau, and the mountain ranges--receives higher rainfall and typically has a plant cover of evergreen forests. In contrast, the western region, which descends gently from the central plateau to the western and southern coastlines and has a long dry winter, is typically covered by deciduous forests and thickets. In both regions, there are spaces with vegetation that can be classified as grassy or woody savannah, but they cover a larger area and are more diverse in the western region. Madagascar contains a very high number of species, with an estimated 10,000 species of vascular plant. As a whole, the level of endemism is very high in the eastern region as well as the western region. (Approximately 80% of the species and 15-20% of the genera are endemic.) Yet the savannah habitats show much lower levels of endemism, as well as lower species diversity in comparison with the climax forests and thickets.
The savannahs of the eastern region
In the eastern region savannahs only occur on the drier westward-facing slopes in the central highland plateau at elevations of 2,625-5,249 ft (800-1,600 m). The distinctive tapia (Uapaca bojeri, Euphorbiaceae) forest grows in the least disturbed areas of these slopes. This gnarled evergreen tree reaches heights of 39 ft (12 m) and dominates the vegetation in these areas. The forest is never dense, and in most cases disturbance has reduced it to woody savannah or grassland where only a few scattered trees survive. Other plants commonly associated with U. bojeri include Leptolae-na pauciflora, L. bojerana, and Sarcolaena oblongifolia (all three members of the Sarcolaenaceae, a family of understory trees and shrubs endemic to Madagascar and thought to be a relic of Gondwana), as well as Agauria salicifolia (Ericaceae), Asteropeia densiflora (Theaceae), Dodonaea madagascariensis (Sapindaceae), Brachylaena microphylla and Dicoma incana (both Asteraceae), species of Alberta and Taranna (both Rubiaceae), Faurea forticuliflora (Proteaceae), Protorhus buxifolia and Rhus taratana (both Anacardiaceae), Schefflera bojeri (Araliaceae), and Weinmannia spp. (Cunoniaceae); the palm Chrysalidocarpus decipiens grows in the wetter areas.
A number of species of shrubs and herbs may grow in the slight shade of the trees, but more open areas are dominated by grasses. The higher areas are dominated by the endemic grasses typical of the central highlands, including Loudetia simplex subsp. stipoides and Aristida rufescens, while lower sites are dominated by widespread nonendemic species such as Heteropogon contortus, Hyparrhenia rufa, and Hyperthelia dissoluta. Tapia (U. bojeri) is a highly fire-resistant tree that persists in sites where fires have eliminated other woody species; even so, tapia has now disappeared from large areas in Madagascar that it formerly covered. The people of Madagascar greatly appreciate this tree for its edible fruit and medicinal properties.
The savannahs of the western region
Both the grassy and woody savannahs of western Madagascar cover larger areas and are more varied than those of the eastern region. Four basic groups of woody components are found in this area. The first group is the relict trees from the former forest cover, generally large trees that have resisted the repeated fires and are too tall to fell. (This is the reason why they have survived.) Among these relict trees are several species of baobab (Adansonia, especially A. grandidieri), the tamarind (Tamarindus indica), and the large palm Bismarckia nobilis. The second group consists of the endemic savannah species, restricted to three trees: Acridocarpus excelsus (Malpighi-aceae), Dicoma incana, and D. oleifolia (Aster-aceae). The third group consists of the indigenous invasive species, especially the ones that are adapted to poor soils and grow on rocky outcrops such as Erythroxylum platycladum (Erythroxy-laceae), Fernandoa madagascariensis (Bignoni-aceae), Stereospermum spp. (Bignoniaceae), Mascarenhasia lisianthiflora (Apocynaceae), and Terminalia seyrigii (Combretaceae). The fourth and last group is the exotic invasive species, introduced relatively recently to Madagascar from the African mainland or elsewhere. Among these are Acacia farnesiana (Leguminosae), Cassine aethiopica and Maytenus linearis (both Ce-lastraceae), the palm Hyphaene coriacea, Sclero-carya birrea subsp. caffra (Anacardiaceae), and species of the genus Ziziphus (Rhamnaceae). In each given area, the woody components of the savannah will depend on local environmental and historic factors. Note that some woody species dominate certain sites; for example, in the extensive plains near the Isalo National Park, Bismarckia nobilis forms a remarkable palm savannah.
The herbaceous plants are mainly grasses, including endemic and nonendemic taxa. The endemic forms include Loudetia simplex subsp. stipoides, L. filifolia subsp. humbertiana, and Aristida rufescens, while the widespread nonendemic grasses include Aristida congesta; Heteropogon contortus; Hyparrhenia cymbaria, H. rufa, and H. schimperi; cogon grass (Imperata cylindrica); Guinea grass (Panicum maximum); and Themeda quadrivalvis. Several annual grasses are also common.
3.3 The savannahs of southern Asia and Malaysia
The different climatic conditions of the Indian subcontinent, the Indochina Peninsula, and the Malaysian Islands have given rise to two areas of savannah that differ greatly in size and distribution.
The imprecise status of the Asian savannah
The Indian subcontinent (an area south of the Himalayas that includes India, Bangladesh, and Pakistan) lies mainly in the tropical and subtropical zones (between 8[degrees]N and 38[degrees]N) and has a monsoonal climate (see volume 2, pp. 417-420). The region's rainfall is derived mainly from the southwest monsoon and is concentrated in a relatively short period, from late May to September, although the retreating monsoon (northeast monsoon) also brings rain to some areas, mainly in southern India. In much of the subcontinent, the rainy season follows a long and hot dry season. The result is that the predominant vegetation is mostly dry deciduous forest, whose species composition and other biotic characteristics vary from one zone to another as a function of the quantity and duration of the rains, as well as other geological, soil, and biotic factors.
The Indian subcontinent also has a long history of human settlement. It was, in fact, the birthplace of some of the earliest civilizations. The region's forests have long suffered intensive clearing for habitation and cultivation and have been burnt and grazed since time immemorial. Most of the area potentially covered by dry deciduous forests is now covered by grassy savannah-type vegetation subject to intense biotic pressure.
Southeast Asia is a subcontinent that has lost its heart to the sea; it is one of the most humid regions in the world. The high humidity has blurred the distinction between forests, woody savannahs, and grassy savannahs, and these world's other savannah formations. Open woodlands are abundant in mainland Southeast Asia and are the most important formation in Myanmar, Thailand, Laos, Cambodia, and Vietnam (often up to 45% of the entire forest cover). They also occur on the more easterly islands of Indonesia and Papua-New Guinea. True grassy savannahs are uncommon in the region and occur only under very special ecological conditions. The once-popular idea that all these savannahs were formed recently (due mainly to forest clearance) is no longer tenable, as there is more and more evidence that Southeast Asia has contained at least small core areas of savannah for a long time. These areas are apparently associated with certain features of the soil or relief, but the area of open woody formations and savannahs has always clearly fluctuated around these core areas, under the influence of climatic change and human activities.
During the last glaciation, when the tropics were drier than they are now, open woodlands and savannahs probably occupied much larger areas of Southeast Asia. They even occurred in the Malay Peninsula (west Malaysia and the southwestern part of Thailand), which is now entirely part of the rainforest biome. The mainland appears to have been joined to the island regions in the distant past by a savannah corridor. When the monsoon was fully reestablished after the ice retreated to more northerly latitudes, woody and grassy savannahs contracted to their core areas on coarse sandy soils, steep rocky slopes, and poor soils with a layer of laterite (plinthite) near the surface. Later, when the first Neolithic peoples reached Southeast Asia about 12,000 years ago (see volume 2, pp. 449-450), open woodlands and savannahs grew once more as a result of human use of fire and the axe, but only in the areas where the seasonal climate allowed the spread of running fires during the dry season (see photograph 10, p. 30).
Fires also occur in the core areas of savannah, but they do not determine the savannah's existence. Fires are, however, an essential element in the creation of new savannah areas; if the flames are controlled or prevented, the savannah gradually becomes denser and reverts to a forest. Both woody and grassy savannahs continue to spread in sites near the forest-savannah ecotone where fires are regularly set. For example, in the Sakaerat Experimental Station in northeastern Thailand, there are many well-defined fire boundaries (where a fire stopped) showing the progress of savannah and open woodland into the monsoon forest--and even within the rainforest. It is also worth noting that many of the Neolithic sites in Southeast Asia are in areas now covered by savannah, although it is hard to say if these savannahs resulted from human settlement or if they were originally chosen for their light soils and easy access. The savannahs of Southeast Asia are, thus, climatic formations, soil formations, and anthropogenic formations (created by human influences). The same woody or grassy savannahs may have been produced in different places by different combinations of processes.
India's subhumid savannahs
In the subhumid areas of the Deccan Plateau, where the annual rainfall of 32-47 in (800-1,200 mm) is spread over nine months (from March-April to November-December), there are savannah formations with a relatively dense woody layer but with floristic composition not very different from the African savannahs with members of the Combre-taceae. The herbaceous layer is dominated by daabsuli (Heteropogon contortus), or by pavna or sedwa (Sehima nervosum) and several species of Dichanthium. Above it, there is a woody layer dominated by blaka or dawra (Anogeissus latifolia, Combretaceae) and Terminalia tomentosa (Combre-taceae), together with salai or luban (Boswellia serrata, Burseraceae), bijasar or pitasara (Pterocarpus marsupium, Leguminosae), and the tendu or coromandel ebony (Diospyros melanoxylon, Ebe-naceae). To the north, on sandy soils, these species start to be replaced by sal (Shorea robusta) in a transition toward the monsoon forests dominated by a woody layer.
The spiny forests of semiarid India
From east to west, the rains decrease in both quantity and duration, and semiarid to arid conditions prevail. In peninsular India, the leeward slopes of the Western Ghats receive very little rainfall and have a semiarid climate. Due to the large year-to-year variability in rainfall, it is difficult to fix a precise limit to the semiarid region; the zones with an annual average precipitation of 8-32 in (200-800 mm) can be considered semiarid. Thorn forests with a savannah-type grass layer are characteristic of the semiarid regions of India. They occur in parts of Punjab and Haryana, in most of Rajasthan (east of the Aravalli Range), parts of western Uttar Pradesh and Madhya Pradesh, in southern Gujarat, and in a narrow belt on the leeward side of the Western Ghats in Maharashtra, Karnataka, and Tamil Nadu.
The thorn forest vegetation is dominated by members of the Mimosoideae subfamily of the Leguminosae, mainly of the genera Acacia and Prosopis. Rocky areas at higher elevation are dominated by scrub vegetation with succulent euphorbias. During the dry season, the soil is almost bare or shows sparse shrub growth; then, as soon as the first rains fall, the grasses and forbs (herbs) start growing vigorously and the trees produce abundant foliage. Thus, during the rainy season, the thorn forests form a typical woody savannah. Despite the high number of species of grasses and forbs in these forests, species diversity is very low when compared to the deciduous forests. These Indian thorn forest formations do not contain a single endemic plant species.
The thorn forests are usually grouped into two categories: the northern and the southern thorn forests, the northern ones being relatively better known. The typical northern spiny forests, to the north of the Satpura Range, consist mainly of two members of the Mimosoideae subfamily of the Leguminosae--kumta (Acacia senegal) and bordi (Prosopis cinerarea [= P. spicigera]). Other common tree species include: ronj (Acacia leucophloea), babul (A. nilotica), khair (A. catechu), vurtuli or kunlai (Dichrostachys cinerea), all members of the Mimosoideae subfamily of the Leguminosae, hingota (Balanites aegyptiaca, Zygophyllaceae), kahatai or bilangra (Flacourtia indica, Flacourtiaceae), and Maytenus spinosa (Celastraceae). The pilu--such as barapilu or khanjar (Salvadora oleoides) and true pilu or chhota pilu (S. persica)--are typical of saline areas. There is a great variety of shrub species, both scattered among the trees and in the form of a continuous scrub, the most common ones being tainti, also known as kair or karel (Capparis decidua, Capparidaceae), aak or madar (Calotropis procera, Asclepiadaceae), and jaharberi (Ziziphus nummularia, Rhamnaceae). Other shrubs are also common in these forests, especially the most degraded forests with the greatest quantity of herbaceous plants. The shrubs include phog (Calligonum polygonoides, Polygonaceae), ghorak boonti or chaya (Aerva javanica, Amaranthaceae), jhamo or khip (Crotalaria burhia, Leguminosae), jangli karonda (Carissa opaca, Apocynaceae), and Leptadenia pyrotechnica (Asclepiadaceae). In hilly areas, thor (Euphorbia nivulea, Euphorbiaceae) is the most common arborescent species, although it is usually present in the form of a large shrub. Justicia vasica (Acanthaceae) is another common shrub in rocky and stony areas. The grass cover is dominated by anjan (Cenchrus ciliaris) and other species of the same genera such as kal anjan (C. setigerus), as well as sewan (Lasiurus hirsutus), pavna or sedwa (Sehima nervosum), dhaab (Desmostachya bipinnata), daabsuli (Heteropogon contortus), makra (Dicanthium annulatum), bhanjuri (Apluda mutica), Dactyloctenium scindicum, and species of Aristida, Cymbopogon, Eragrostis, Bothriochloa, Eleusine, and others. Fire-resistant species like kans or wild sugar cane (Saccharum spontaneum) and S. bengalense are quite common.
In saline areas, especially in the Saurashtra and Kutch regions of Gujarat, the dominant woody species are the species Salvadora oleoides and jhau (Tamarix ericoidea, Tamaricaceae). The American species known as mesquite (Prosopis chilensis [=P. juliflora], Leguminosae), known locally as kabuli kikar, was introduced more than a century ago. It has naturalized in the semiarid zone, spreading over large areas with relatively saline soils, and has also been planted in reforestation programs. Another species of Prosopis, bordi (P. cineraria [= P. spicigera]), grows spontaneously in the region and is an important source of fuel and fodder. Its green fruit is also edible. P. chilensis is not attractive to livestock and its leaf litter produces allelopathic (toxic) substances that inhibit the growth of other plants beneath it. Many local communities, such as the Bishnois of Rajasthan, actively protect the bordi.
Along temporary and seasonal watercourses, seasonally flooded areas are dominated by the grass khus (Vetiveria zizanoides), with occasional palms of the genus Phoenix. The southern thorn forests are almost the same, except for some differences in the minor species. Acacia catechu and Acacia nilotica are the dominant tree species. In hilly areas, euphorbias (Euphorbia tirucalli, E. antiquorum) may be common.
The standing biomass of the thorn forests is estimated to vary between 5 tons/ha (1 hectare=2.5 acres) and more than 20 tons/ha, depending on its level of protection. The woody layer represents 30-50% of the total biomass. The grass layer shows considerable differences in the ratio of aboveground to belowground biomass, depending on the species composition and other ecological factors, but in general the underground biomass is greater than that aboveground. The production of the grasses has received a great deal of attention, and several studies have shown that the semiarid grasslands in the thorn forest areas can, with adequate management, yield up to 20 t/ha/year of forage. Regardless of silvipastoral practice, the tree species exert great influence of the production of the grasses. For example, under Prosopis cineraria [=P. spicigera], grass yields are higher than under Acacia senegal. Likewise, grass growth is reduced under some other shrubs (species of Butea, Mimosa, Ziziphus, Cassia).
The savannahs of Southeast Asia
In Southeast Asia, as in the rest of the tropics, it is difficult to divide the continuum that runs from densely forested woody savannahs to grassy savannah meadows (even more so considering the percentage canopy cover may vary from less than 50% when in full leaf to as high as 77%.) The through-woodland percentage visibility also varies from between (-1/4D + 10)2 in the densest woody savannahs to (-1/7D + 10)2 in the most open savannahs (where D is the distance in meters).
The woody savannahs of Southeast Asia are very similar in general structure and physiognomy to the miombo of southern Africa, the Eucalyptus savannahs of northern Australia, and forests of sal (Shorea robusta) in India.
In mainland Southeast Asia, the primary formation is dry deciduous forest of dipterocarps, known in Burmese as indaing (wild land with Dipterocarpus tuberculatus) and in Thai as pa tengrang (wild forest with Shorea trees). Their highly closed tree cover led German botanist A.F.W. Schimper (1856-1901), one of the first to study their ecology, to classify this formation as savannah forest. It is widely distributed throughout the areas with Am (monsoonal) and Bw (savannah) seasonal climates in the Koppen classification, but it appears mainly in the areas subject to Bw climates. Annual rainfall is generally between 39-59 in (1,000-1,500 mm), with a dry season five to seven months long (October-November to March-April). In some sites, the evaporation may exceed rainfall for nine months each year. The annual value of the Angstrom humidity coefficient (H) is normally less than 240, classified as semiarid. The mean temperature of the coldest month rarely falls below 68[degrees]F (20[degrees]C).
The formation generally grows at elevations below 3,281 ft (1,000 m), although some types may appear in some dry mountain ridges as high as 4,265 ft (1,300 m). Low- to mid-intensity fires (250-450 kW/m) are common between November and April and may be natural or caused by humans. The fires are only severe when there has been an unusual accumulation of leaf litter and dry grass (mainly grasses and pygmy bamboos). This may occur when forestry officials misguidedly try to prevent fire in these formations, allowing fuel to accumulate for several years. When, as inevitably happens, they catch fire naturally or accidentally, the destruction is great, as the fires may be very intense (up to 2,000 kW/m) with temperatures exceeding 1,652[degrees]F (900[degrees]C).
The dry deciduous dipterocarp forest is dominated by several mixtures of six deciduous species: Dipterocarpus intricatus, D. obtusifolius, D. tuberculatus, Shorea obtusa, S. roxburghii [=S. talura], and S. siamensis (=Pentacme suavis and its variants). Other highly characteristic trees include some Dillenia spp. (Dilleniaceae), Irvingia malayana (Simaroubaceae), the Burma lacquer varnish tree (Gluta usitata, Anacar-diaceae), the pine Pinus merkusii, Pterocarpus macrocarpus and Sindora siamensis (both Legumi-nosae), some species of Terminalia (Combre-taceae), and Xylia kerrii (Leguminosae).
Leaf fall normally occurs between November and March but varies greatly from one species to another and from one year to another. Fruit is dropped just before the seasonal fires or just after the fires have swept through the woodland. The greatest growth in diameter takes place in September, toward the end of the wet season. In the tallest savannah forests, the canopy may reach heights of 98 ft (30 m), but it is normally between 49-66 ft (15-20 m), while in the most impoverished soils in very dry sites they may not even reach 33 ft (10 m). Most of the trees show a wide range of adaptations to both moderate fires and water stress, such as tomentose (woolly) leaves and terminal shoots, and thick, rough bark; the tap roots and lateral roots are also protected in this way.
The herbaceous layer includes a range of geophytes, hemicryptophytes, and annuals, also highly adapted to fire and drought. Cycas siamensis (Cycadaceae), a widespread cycad, and the dwarf palms of the genus Phoenix, for example, are protected by an armor of overlapping dead leaf bases, while many geophytes such as the terrestrial orchids of the genus Habenaria survive the fires as underground organs. The herbaceous layer is dominated by pygmy bamboos of the genus Arundinaria and other grasses such as Hyparrhenia, cogon grass (Imperata), and species of Arundinella, Dichanthi-um, Eulalia, Heteropogon, Polytoca, and Themeda. The accumulation of their dead remains tend to peak in February and fuels the fires in the middle and late dry season.
The grassy savannahs
Some more open variants of the woody savannahs described above--such as the lighter forests dominated by ingyin (Shorea siamensis) and S. obtusa--could easily be included as grassy savannahs. In fact, the range from savannah forests to true grassy savannahs is very broad, and in addition to the dipterocarp forests and all their savannah derivatives, it includes very diverse and highly localized formations, including the forests of Diospyros burmanica (Ebenaceae), known as te in Myanmar, thickets of khair (Acacia catechu), Oliver's terminalia (Terminalia oliveri), and Hamilton's teak (Tectona hamiltoniana). Note that the grassy savannah formations typical of Southeast Asia reach as far as the Kabaw Valley in Manipur State (India) on the border with Myanmar, although they appear in rather specialized forms. The grassy savannahs also reach the islands of Malaysia, especially in eastern Indonesia and New Guinea, where the dominant climate is more sharply seasonal (Am and Aw types in the Koppen classification). Most of the grassy savannahs, however, occupy small areas and are much less widespread than the woody savannahs. Grassy savannahs are often enclaves within the woody savannahs that dominate the region.
Yet true grassy savannahs do exist in Southeast Asia, as in the bald hills of the Phetchaburi Range and the Khao Yai National Park on the south- western scarp of the Khorat Plateau in eastern Thailand. These savannahs form small patches within a mainly forested area, and they are often bordered not only by other types of savannah with more trees but also by semideciduous monsoon forests. These patches are used heavily by the park's large mammal fauna, including the sambar deer (Cervus unicolor) and barking deer or muntjak (Muntiacus). Other open savannahs occur around Mondolkiri (Cambodia) and on the Ban Me Thuot Islands (Vietnam).
The origin of these small islands of open grasslands within larger areas of forests and woody savannahs appears to be the result of a set of factors causing ecological stress. For example, the splendid grassy savannah near Sakon Nakhon in northeast Thailand, now almost vanished, appears to have been the result of regular heavy flooding during the rainy season. The bald hills of the Phetchaburi Range have been created by a regime of felling, grazing, and repeated and continuous fires over many years. In the region of some these enclaves of grassy savannah, the annual rainfall is less than 32 in (800 mm) and sometimes less than 20 in (500 mm). Fires are frequent in all these savannahs, even those in protected spaces, such as national parks or wildlife sanctuaries, and severe fires may open clearings in a forest and prevent its regeneration. Frequent severe fires usually favor grasses over woody plants. For example, some areas of rainforest in Wasur National Park (Irian Jaya) have recently turned into savannahs because of fires.
Some particular areas of savannah--ones that are highly localized--may also be related to archeological finds and thus to former settlement patterns, as in some areas on the Thailand-Myanmar border and in some Neolithic sites in Thailand. The woody savannahs are not uniform in their structure either, usually showing great horizontal and vertical heterogeneity and forming a mosaic of more open or more closed sites that reflect their ecological history, general vegetation dynamics, local topography, the stoniness of the soil, human land-use history, and the local patterns of fire frequency and intensity. Not all of these factors and processes are thoroughly understood at this time.
In the Siamese language, open savannahs are known as paa yYa, which means wild grass or grass forest. These habitats are of great importance in some national parks and wildlife sanctuaries, including the famous open savannahs of Thung Yai Wildlife Sanctuary, recently saved from a major development project. The most characteristic plants include cogon grass (Imperata cylindrica), creeping (Panicum repens), wild sugar cane (Saccharum spontaneum), sorghum (Sorghum halepense), Vetiveria zizanioides, and Eupatorium odoratum. The common woody plants include Careya arborea (Barringtonaceae), khair (Acacia catechu), Siamese acacia (A. siamensis), and the beautiful Pterocarpus macrocarpus (Leguminosae). In any case, the floristic composition of each savannah usually reflects the composition of the original woody formation from which it was derived.
3.4 The savannahs of Australia and the Pacific
Savannah covers almost a quarter of the land area of Australia, about 772,200 [mi.sup.2] (2 million [km.sup.2]), in an arc that crosses the tropical north of Australia from the northwest of Western Australia through the north of Northern Territory and Queensland to southeastern Queensland. Although savannah is the dominant biome across this entire region, there are small areas of rainforest, grassland, and heath mixed in. The tropical north of Australia shows a very sharp rainfall gradient from north to south. In Northern Territory, for example, the annual rainfall in Darwin (12[degrees]S) is 59 in (1,491 mm); in Katherine (about 15[degrees]S) it is 39 in (980 mm), while in Tennant Creek (19[degrees]S) it is only 20 in (450 mm); the high degree of variability greatly conditions the different classes of savannah present. There are savannahs similar to the Australian ones on many islands in the southwest Pacific from New Guinea and Timor, which are very near Australia, to the Mariana Islands and Fiji, which are much farther away.
The consolidated presence of the savannah in Australia
The biota of the Australian savannahs is still relatively intact and maintains its high biodiversity among the major components (plants, invertebrates, reptiles, and mammals). No small or medium-sized mammals have become extinct, although they have been extremely vulnerable to European land use practices throughout central and southern Australia. The flora has a relatively low percentage of alien plants (generally less than 10%), and in northern Australia there are none of the rabbits and foxes that have had such an impact on the native biota in central and southern Australia.
The Australian savannah appears to be ancient rather than recent, the result of many cycles of climate change in the Tertiary and Quaternary. Although everything suggests that most savannah species are Australian endemics, the woody savannahs with eucalyptus in tropical Australia have a much higher proportion of pantropical genera than the eucalyptus forests of southern Australia. Thus, many of the genera of the savannah and the open woody formation of tropical Australia must have existed on the Australian landmass before the Australasian tectonic plate separated from the former continent of Gondwana-land about 50 million years ago. At least 37 genera of grasses (Aristida, Eragrostis, Panicum, Chloris, Sporobolus, and Bothriochloa, among them), and several genera of the Leguminosae also occur in Africa, as do ten species of Acacia subgenus Acacia, very different from most of the acacia species found in Australia (members of a different subgenus).
About 30 genera of grasses occur not only in Africa but also in the Indo-Malayan region, which they must have reached on the Gondwanan Indian Tectonic Plate. This suggests that all these grass species evolved their distinctive ecomorphological attributes in the savannah herbaceous layer before the evolution of the marsupials in Australia or the ungulates in Africa. Only a few genera of grasses are endemic to the Australian continent (meaning they evolved after the Australasian Tectonic Plate separated from the rest of the ancient continent of Gondwana). These grasses include Astrebla, which forms extensive tussock grasslands on cracking clay soils in the most arid tropical regions, as well as Triodia and Plectrachne, hummock grasses that cover vast areas with sandy and nutrient-poor soils in central Australia.
The understory mainly consists of annual and perennial grasses with a C4 metabolism and annual and perennial forbs and clumps, all with a C3 metabolism. Of the ten main life forms, the most common are annuals and perennials with annual foliage. Grass biomass is usually 2-6 t/ha but may reach 8-10 t/ha in specialized habitats. The abundance of annual grasses decreases, and that of perennial grasses increases, along a gradient of decreasing rainfall and increasing soil clay content. The woody layers are dominated by many species of the genus Eucalyptus. Evergreen trees are the most abundant life-form in the upper layer, followed by deciduous trees and large shrubs, which account for a quarter of the species present.
Phenological patterns vary greatly between different layers, life forms, and species. In the lower layer, most growth and flowering takes place during the wet season and is normally complete by April. The annual species, especially the grasses--for example, the sorghums (Sorghum)--may start to dry out in March, shortly after shedding their seeds. The moisture content gradually declines during the dry season, and by September the plants of the lower layer have dried out completely.
The woody savannahs and grassy meadows
Some of the many variants of open woody formations in Australia are worth mentioning because they are so typical and cover such large areas. These include the eucalyptus forests and eucalyptus woodland savannah, the tree savannahs, and the tussock and hummock grasslands.
The woody savannahs in the form of savannah woodland and thickets occupy the wettest areas. For example, the lateritic soils of the wettest part of the Cape York Peninsula and the northeastern zone of Arnhem Land are covered by woody savannahs with open populations of eucalyptus and a herbaceous layer dominated by Heteropogon triticeus and Sorghum leiocladum. Yet in the sandy and stony soils of the wet part of north and northwest Australia (Arnhem Land and the Kimberley region), the herbaceous layer of the eucalyptus woody savannah is formed by several annual sorghums (Sorghum intrans, S. stipoideum, and S. australiense) and Aristida holathera.
The subhumid areas correspond to grassy savannahs with isolated trees, of which there are many examples in both the northwest (the Kimberley region and the regions bordering Northern Territory) and the northeast (the south of the Cape York Peninsula). In these cases, the lower layer of the eucalyptus populations, much more open than in the savannah forests, is dominated by Chrysopogon latifolius, Heteropogon triticeus, Mnemesia rottboellioides, and Sorghum leiocladum.
In the driest environments such as the black cracking clay soils found from Victoria River Downs (in the northwest of Northern Territory) to the center of Queensland, the characteristic vegetation is tussock grasslands dominated by Chrysopogon fallax, Dicanthium fecundum, Heteropogon contortus, and Themeda triandra, with the occasional eucalyptus tree. Tussock grasses (Eleocharis, Oryza) dominate both the seasonally flooded black soils in the coastal plains of the north of the continent and the clay soils of the arid zone, the latter being covered in Mitchell grasses (Astrebla), a genus endemic to Australia. Finally, the less fertile soils of Australia's arid interior are covered in hummock grasslands (generally with dispersed shrubs and small trees of the genera Eucalyptus and Acacia) that are dominated by Triodia and Plectrachne. Hummock grasses form characteristic mounds of sclerophyllous leaves that may grow into rings more than 33 ft (10 m) in diameter (known in Australia as spinifex). Other vegetation types occur locally within the Australian savannahs. Complex formations such as monsoon forest and vine thicket may occur on sites within the relief where soil humidity is relatively favorable or on sites that are protected from fire.
The savannahs of the Pacific islands
In the islands of the tropical Pacific, savannahs occupy areas that are generally small and often related to human action. The main type in New Guinea and Timor is woody savannah with eucalyptus, acacias, or species of Melaleuca, although in the highlands of New Guinea there is also a high-altitude grassy savannah. On the smaller islands, the savannah is associated with a seasonal climate or situations within the relief on the leeward side of the rainbearing winds.
The lowland savannahs of New Guinea are dominated by the grasses Themeda australis and T. novoguineensis, with a tree layer of several species of eucalyptus (Eucalyptus alba, E. confertiflora, E. papuana), and sometimes with a lower woody layer of acacias (such as Acacia leptoclada), banksias (such as Banksia dentata), and other small trees. There are also floodable grasslands with a herbaceous layer of large grasses, including Phragmites karka, Leersia hexandra, and several species of rice (Oryza), and a tree layer dominated by different species of Melaleuca (M. cajuputi, M. leucadendron) and other trees such as Barringtonia tetraptera (Barringtoniaceae), Erythrina fusca (Leguminosae), or Mitragyna speciosa (Rubiaceae), and also some arborescent species of screw pine (Pandanus). The herbaceous formations found at high elevations in New Guinea can be considered savannahs because of their physiognomy and especially their floristic composition, but in fact they are tropical mountain grasslands similar to the Andean paramo or other herbaceous formations in the tropical high mountains. The grassy savannahs at high elevation are very characteristic, with tree ferns (Cyathea, Dicksonia) and a herbaceous layer with several species of tussock grass.
There are also occasional woody savannahs with niaouli (Melaleuca quinquenervia, Myrtaceae) in New Caledonia. Presently, the different varieties of this savannah--distinguished by the density and greater or lesser height of the tree layer of niaouli and by the different lower woody and herbaceous layers--occur in very different climatic and topographic conditions throughout the islands.
1 When the sun sets, the crowned cranes (Balearica pavonina) retire to sleep in the scattered trees on the savannah of Tanzania's Tarangire National Park. Although the savannah is typically associated with safaris and big game hunting, birds are abundant and varied in almost all savannahs. This type of tropical biome, then, is not just an immense expanse of grass and scattered trees, where large herds of zebras, elephants, and giraffes graze while lions wait for their prey or bask in the sunshine. There are many very different types of savannah, and their flora and fauna vary greatly. Some consist of almost pure stands of grass, others have a more or less open woody cover, and others are densely covered in trees. Each one supports its characteristic natural system. Although the large herbivorous mammals are the most conspicuous, many other zoological groups are also represented, including birds and small invertebrates, some of which--like termites--cannot pass unnoticed despite their smallness.
[Photo: O.C. Jones / Natural Science Photos]
2 Changes in species' composition and physiognomy in plant communities over the forest-savannah border in the central zone of the Coastal Range (Vene-zuela). The inventories correspond to 15 parallel transects perpendicular to the savannah-ecotone-forest vegetation gradient. The green spheres show the presence of a species, while the green outlines indicate its absence. The ecotone contains some species from both the neighboring communities, together with others that are exclusive to this transitional community such as the tree species Adenanthera peregrina (Mim-osoideae subfamily of the Leguminosae). The first six inventories show the overwhelmingly dominant presence of the savannah species and the total absence of forest or ecotone plants. The next five are characteristic of the ecotone community, as they include both savannah and forest species (although from left to right savannah species decrease and forest species increase). The last four transects are clearly forest, al-though the first one (12) still shows the presence of a considerable number of ecotone species. The physiognomic changes are directly related to the greater moisture content of the increasingly deep savannah soils, which are less stony and show a higher proportion of fine particles (loam) in the surface horizons.
[Drawing: Jordi Corbera, from data provided by the author]
3 The curatella (Curatella americana), a slow-growing evergreen sclerophyllous tree, often forms very open woody savannahs in contact with typical forest communities. The curatella's need for strong sunshine means it cannot grow within closed forest. This is clearly shown in the trunk growth of the specimen in the photo (taken in the Orinoco llanos): the trunk curves toward the sunnier side of the woody savannah in which it grows.
[Photo: Bruno Pambour / Bios / Still Pictures]
3 The curatella (Curatella americana), a slow-growing evergreen sclerophyllous tree, often forms very open woody savannahs in contact with typical forest communities. The curatella's need for strong sunshine means it cannot grow within closed forest. This is clearly shown in the trunk growth of the specimen in the photo (taken in the Orinoco llanos): the trunk curves toward the sunnier side of the woody savannah in which it grows.
[Photo: Bruno Pambour / Bios / Still Pictures]
The distribution of the dry and rainy seasons in tropical and subtropical zones. At the equator, rainfall is distributed evenly over the entire year, so there is no dry season. Toward the tropics, there is a clear and increasingly long dry season. On the tropic of Cancer, the rains are concentrated from May to September, precisely when the tropic of Capricorn has its dry season. The rains always occur when the Sun is at its zenith at midday. Thus, the position of the rainbelt changes with the position of the Sun. In the Northern Hemisphere summer, the Sun reaches the tropic of Cancer; in the southern summer, it reaches the tropic of Capri-corn. The two positions of the Sun at its zenith approach each other with increasing distance from the equator. Near the equator, in addition to the winter dry period, there is a further short--but not very intense--summer dry season that disappears at latitudes of about 15[degrees]. These gradual climatic changes (with increasing distance from the equator) and increasingly long dry seasons also cause changes in the vegetation. The equatorial rainforests are replaced by semievergreen and evergreen forests and finally by open savannahs.
[Drawing: Jordi Corbera, based on Water & Breckle, 1984]
5 Savannahs, distributed throughout the tropical and subtropical regions with arid and semiarid climates, are one of the biomes adjacent to the tropical forest biome. Their main climatic feature is an alternating dry and wet season, with the average monthly temperature remaining constant over the course of the year. This is illustrated by the heat and temperature diagrams for eight savannah sites in different parts of the world. Each diagram shows the altitude, latitude and longitude (in black), the average temperature (in red), and the average annual precipitation (in blue). The average temperatures are between 70[degrees]F (21[degrees]C) and 84[degrees]F (29[degrees]C), while total annual rainfall varies between 20 in (500 mm) and 62 in (1,600 mm). The sites in the Southern Hemisphere have their dry season around July, while in the Northern Hemi-sphere the dry season begins around January and can last until March or April. In latitudes closer to the equator, the temperature range is not so clearly seasonal, as is shown by the diagrams for Calabozo, Goias, Lamto, and Antseranana, with average monthly temperatures that only vary 7[degrees]F (4[degrees]C) or 9[degrees]F (5[degrees]C) between the coolest month and the warmest. Farther from the equator, variations in average monthly temperature may reach 68[degrees]F (20[degrees]C). The more arid continental conditions of the stations at Santiago del Estero and Niamey are responsible for their lower annual rainfall. Furthermore, in Niamey the rains are concentrated during a very short period as it is close to the Sahel, while in Lamto, at the savannah's southern border with the tropical forest, rain falls almost all year round. These differences in rainfall and temperature are responsible for the appearance of very different savannah plant formations.
[Drawing: IDEM, from several sources]
6 In Queensland, Australia, the borders between the Eucalyptus savannah and the rainforest are very sharp and cause gradients of temperature, light, and humidity. The climatic changes of the last few million years are clearly reflected in the displacement of the ecotone between the dense forest and the open savannah, which has been reconstructed from fossil pollen analysis. The reduction in rainfall, among other climatic changes in the late Cenozoic, allowed the savannah to expand at the expense of the tropical rainforest, shaping the current landscape of small isolated patches of surviving rain-forest surrounded by a typical sclerophyllous savannah vegetation. The increase in fires, probably related to Aboriginal activities, has also played an important role in the savannah's expansion.
[Photo: David Barret / Planet Earth Pictures]
7 The physiognomic and floristic features of diver- se savannah communities known as cerrado make them ecotone formations; they exist between the grasslands of the campo limpo (meaning "clear field") and the xeromorphic forest (able to live and grow with limited water) known as cerradao. If environmental conditions prevent the growth of cerradao, savannah formations appear dominated by herbaceous and shrub components, which are better able to resist the adverse conditions. The campo limpo is open grassland without trees (grassy savannahs). Campo cerrado (meaning "closed field" ) has a relatively closed tree cover. Between the "open field" and the "closed field" is the campo sujo, the "dirty field," a type of shrub savannah containing woody vegetation but not as dense as that in the campo cerrado. The cerrado is the transition community between the campo cerrado and the cerradao. These different physiognomic types share a common flora, and there is continuity between them; some authors use the term cerrado to refer to all of them.
[Drawing: Jordi Corbera, from several sources]
8 The appearance of sa-vannah with a very open tree layer such as this one in the Orinoco llanos is often determined by geomorphological, topographical, and soil factors. The peaks and higher parts of mountain slopes are usually dominated by grassy savannahs, but at lower elevations the density of the woody elements increases. (The lower parts are better drained and are not so affected by the lack of water in the dry season.) The soil humidity also influences nutrient content, which in turn determines the type of vegetation present.
[Photo: Bruno Pambour / Bios / Still Pictures]
9 The plant-soil cycle in the Guinea savannahs in contact with the forest shows the effects that soil factors may exert on the plant communities. Initially (diagram a), the forest covers both valleys and raised tables; the underlying ferruginous crust does not prevent the growth of the plants, as it is at some depth. Later (diagram b), human intervention destroys the forest on this crust, and it is replaced by a savannah formation. Soil erosion due to human activities (diagram c) brings the hard pan closer to the surface, making it difficult for plants to root deeply. A treeless savannah or bowal then forms, dominated by xeromorphic grasses. Finally (diagram d), regressive erosion of the edges of the raised table wears the crust away, and the forest spreads to cover the former bowal.
[Drawing: IDEM, based on Schnell, 1972]
10 Recurrent fires are an inevitable consequence of the savannah's organization and seasonality. Fires shape the savannah's structural and functional characteristics so greatly that they are among the key factors determining the presence of this biome. By favoring the growth of herbaceous species over tree species, fire helps in the maintenance and succession of the different plant layers. Furthermore, it promotes faster and more intense plant growth and synchronizes the phenological manifestations of many plants, including the timing of flowering, leaf fall, and fruit ripening. The savannah's extreme aridity, especially at the end of the dry season, means spontaneous fires often occur. Over the past 25 million years, most species of the savannah have been able to develop fire-resistance mechanisms such as thickened or corky bark or the ability to sprout from underground organs. But several species have gone further than this, developing ways to take advantage of the heat of fires and even growing dependent on the occurrence of fires, as happens in the Mediterranean area. For instance, the seed cases of some trees need the heat of a fire to open and to allow the seeds to germinate. Some species only flower after a fire, whatever time of year it occurs.
[Photo: Jonathan Scott / Plan-et Earth Pictures]
11 The changes in the woody cover of the Ivory Coast's Lamto Reserve over the 25 years from 1963 to 1988. The area within the circles with broken edges represents the sections covered in 1963 by each type of savannah and semievergreen forest; the circles with unbroken edges show this information for 1988. The arrows indicate the change from one ecosystem to another, and their width is proportional to the number of hectares (ha) affected (1 hectare=2.5 acres). The diagram shows that the woody cover of the Lamto savannahs increased threefold during this period. In 1963 the dense shrubby and woody savannah occupied one-twelfth of the reserve's area, but by 1988 they accounted for a quarter. These two ecosystems and the forest increased in area, while the grassy savannah and the less densely woody savannahs declined greatly. The shrub savannah was reduced by a quarter; the slightly shrubby savannah was reduced by two-fifths. The grassy savannah lost only one-tenth of its surface, basically in favor of the forest, as shown by the fact that the arrow from the grassy savannah to the forest is much thicker than the arrow in the opposite direction. The ex-changes with the other types of savannah are either very small or balanced. The greatest transfers are between the slightly shrubby savannah and the shrubby savannah, and between the shrubby savannah and the densely shrubby savannah. Over the 25 years of the study, the forest cover increased by a total of 60 ha, confirming the hypothesis that the fires--when they occur periodically--do not prevent reforestation.
[Drawing: IDEM, based on Gautier, 1990]
12 In Los Llanos Biologi-cal Station (Calabozo, Vene-zuela), a thicket of Cassia moschata once formed a small island of dense forest vegetation amid the savannah but was totally destroyed by fire in 1993. The accumulation of dead branches, trunks, and other woody material caused by an earlier fire, together with the high production of the (introduced) dominant grass Hyparrhenia rufa, provided abundant fuel for the 1993 fire.
[Photo: Valois Gonzalez]
13 Grasses are the plants that sprout with the greatest ease after a fire and thus become the dominant vegetation, as shown in this photo taken in Corocito (Puerto Ayacucho, Venezuela). Many grass species have underground rhizomes and ground-level stolons (offshoots) that are not damaged by flames and can sprout from underground or basal buds immediately after a fire, usually any time of the year, allowing the plant to recover rapidly. This suggests that the treeless savannahs dominated by grasses have always been linked to periodic fires.
[Photo: Teresa Franquesa]
14 Large herbivorous mam-mals, both wild and domesticated, modify the savannah vegetation substantially as they alter the composition and density of the plants in these ecosystems. One of the most active of the large wild herbivores is the African elephant (Loxodonta africana), shown in the upper photo stripping the bark from a baobab in Tsavo (Kenya). This herbivore, like other browsers, feeds by stripping the leaves from trees--and may even knock the tree over to get at the highest branches. Further-more, they browse on the saplings, preventing them from growing high enough to escape the destructive effects of fires. Browsing herbivores make the vegetation's regeneration more difficult; so do the grazing herbivores, especially the domesticated ones, as they are present in large numbers. If they form large herds, they may affect the dynamics of the forest-savannah border, as in the case of the zebu (Bos taurus indicus or Asiatic ox) in the Vene-zuelan llanos (lower photo). Large herbivores also alter the physical properties of the soil: they always follow the same paths and trample the same site repeatedly. This effect is not so clear in open savannahs, as the animals have greater freedom of movement, but savannahs with a denser tree cover force the herbivores to graze the same sites, and they create paths of compacted earth where grass cannot grow.
[Photos: Ernest Neal / Planet Earth Pictures and Juan Carlos Munoz]
15 The clearing of land to bring it into cultivation, as seen in this photo of Senegal, creates a landscape mosaic of forest, crops, abandoned fields beginning to recover, and well-established savannahs. In general, the forest grows in areas that have never been cultivated or ceased to be cultivated a long time ago, while recently grazed or cultivated ground is occupied by savannahs. Areas of forest that are cleared for cultivation and then abandoned will eventually be covered once more by forest.
[Photo: Timothy Beddow / The Hutchison Library]
16 The shortening of crop rotation cycles exhausts the soil, leading to smaller and smaller harvests, until it is no longer worth cultivating the ground, as in this field of maize (Zea mays) in Tanzania. Excessive population growth has not only forced the clearing of new land for agriculture and grazing but has intensified soil cultivation, thus increasing human pressure on the environment so much that in some areas the ecological balance has been irreversibly altered.
[Photo: Michael McIntyre/ The Hutchison Library]
17 A section in the dry bed of a river in the Rift Valley in Kenya, showing the soil profile and its clearly differentiated horizons. The topmost horizon is darkened by the presence of humus, partially decomposed dead organic matter. Soils are one of the factors determining the presence and appearance of savannah vegetation. In turn, plants influence the soil's properties and are one of the factors intervening in its formation. One of the most important soil factors determining whether savannah appears on a given site is the moisture level, obviously closely related to the climatic conditions. Most savannah soils have a water regime of dry periods alternating with periods of water saturation, although savannah formations also arise in areas with higher rainfall where the tropical forest has been destroyed. Low soil fertility and hardpan formation are other factors favoring the development of savannah formations. Rainforest never establishes on nutrient-poor tropical soils; savannah vegetation develops instead. It has, however, been noted that hardpans are much more common in savannah soils than in tropical forest soils. The different types of savannahs have very different soils as well. Color, profile, structure, texture, moisture levels, drainage, pH, and other factors may vary greatly from one zone to another, and so it is difficult to define a typical savannah soil (see figures 20 and 21) or a black clayish vertisol (see figure 22).
[Photo: J.L. Klein & M. L Hubert / Bios / Still Pictures]
18 Diagram of the soil profiles and vegetation of a kopje zone (small outcrops of pre-Cambrian rocks) in Simba in the Serengeti National Park (Tanzania). In the eastern part of the park, which is drier with an annual average rainfall of 16 in (400 mm), petrocalcic castanozems and chernozems develop on unweathered volcanic ashes, with herbaceous vegetation including Sporobolus marginatus and Cyperus nervosa as the most characteristic species. On porous soils, hardpans form easily and prevent the water from filtering into the soil. Root growth is hindered by the presence of a hard, impermeable horizon not far below the surface, formed by the accumulation and cementation of calcium carbonate and the saline and sodic conditions in the subsurface horizons (1). Toward the west, rainfall increases (20-24 in [500-600 mm]) and two situations may occur: Mollic solonetz with clay and sodium (natric) subsurface horizons support grasslands dominated by Andropogon greenwayii (2). But when these horizons are absent, more rainwater filters into the soil and luvic and vertic castanozems form, with grasslands of tall grasses (3). The plains farthest west with higher rainfall (28 in [700 mm]), and especially the better-drained areas most favorable for plant root growth, support tall (up to 3 ft [1 m]) grasslands with Themeda triandra and Pennisetum mezianum. They are transition communities with the acacia savannahs covering large areas to the west and north. The tree species present include Combretum molle, Terminalia mollis, and Acacia claviguera. Floodable depressions with fine-textured soils (calcic vertisols and luvic chernozems) support Acacia seyal and A. drepanalobium (4). Balanites aegyptiaca is an indicator of the presence of subsurface sodic horizons.
[Drawing: Jordi Corbera, from several sources]
19 Termite mounds up to 7 ft (2 m) tall, built by termites of the Macrotermitinae subfamily, are highly characteristic of the many savannah areas in eastern Africa. In the best drained areas (such as the one shown in this photo taken in the Tsavo National Park in southern Kenya), better drainage and the absence of soil horizons that prevent plant growth mean that the areas around termite mounds favor the growth of shrubs and trees. In drier and badly drained regions (like those on the east side of the Serengeti National Park), the weathering of highly alkaline volcanic ash, combined with low rainfall and poor drainage, has caused the soils to develop unfavorable physical properties. They restrict root growth and the infiltration of rainwater, reducing the effectiveness of the rains and increasing surface runoff.
[Photo: Fred Hoogervorst / Foto Natura / Still Pictures]
20 Ferrasols or ferralitic soils are reddish in color, highly weathered, and form in the wettest zones of the savannahs. They form mainly over basic rocks rich in iron and magnesium minerals (pyroxene, amphibole, biotite, and chlorite). Heavy hydrolysis releases the cations of silicon, potassium, sodium, calcium, and magnesium, which are gradually leached if the drainage is adequate. The resulting low levels of silica allow the new formation of clay minerals rich in aluminum and the oxides and hydroxides of iron, aluminum, and titanium. This is why these soils are known as ferrasols, ferralitic soils, or oxisols. There are several types of ferralitic soils, which often differ in color, as shown in these soils of the Brazilian savannahs (see figure 21).
[Photos: J.H. Kaufmann / ISRIC, Wageningen]
21 This ferralitic soil profile in Sierra Leone (western Africa) is quite typical: the horizons show very little differentiation. The A horizon is darker due to the humus it contains. Below it is a second horizon (Bt) with a higher clay content than the surface horizon and showing all the features of an argillic horizon. This second horizon is very firm when wet and very hard when it dries out. It also shows ferric properties, as it consists of a mixture of clay minerals and oxides of iron and/or aluminum. At greater depth, the second horizon gradually gives way to a regolith formed of fragments of weathered rock mixed with the soil, and this extends to great depths. The regolith-mottled red, yellow, and black--is highly weathered and partly ferruginous. Beneath it is the underlying unaltered material. Ferralitic soils are especially abundant in wet savannahs. These slightly washed and very nutrient poor soils may be almost totally lacking in organic matter (see figure 20).
[Photo: W.A. Blokhuis]
22 An example of a vertisol from the central zone of western Sudan between 10[degrees]N and 16[degrees]N. The soils in this area are exceptionally uniform, although the region's climate is not homogeneous, and thus the savannah vegetation growing on them is not uniform. These vertisols contain 60-80% clay, 20-30% loan, and 10-20% sand. The cation exchange complex is saturated with basic ions, and the soil's pH is alkaline. Moving from north to south, there are small, gradual changes in the soil characteristics. These changes are related to the increasing precipitations (6-32 in [150-800 mm] per year) and to the length of the dry season (2-5 months). In the northern area, the red soils are slightly saline and sodic and contain free carbonates and gypsum, but not in abundance. The soils in the southern area, however, like the one in the photograph, are never saline or sodic and do not contain gypsum, al-though free carbonates may be present. Nor are they such a reddish color. Note the deep cracks caused by the retraction of the clay when it dries.
[Photo: B. Spiers]
23 The changes in the vegetation of the central-eastern savannahs of Sudan are due to the soil differences, not the annual rainfall, which is the same throughout the region. With a yearly average rainfall of 12-32 in (300-800 mm), the sandy areas are covered by an open woody savannah of deciduous trees, mainly Acacia seyal, with a discontinuous herbaceous layer. But on the clay plains where there are vertisols, the vegetation is predominantly herbaceous and there are only a few scattered trees or a mosaic of woodlands and grasslands. In flat clay sites, most of the rainfall accumulates on the surface and evaporates, so the moisture infiltrates only into the top 4-24 in (10-60 cm) and reaches lower only through the cracks; this is why most of the roots grow at the surface. In sandy soils, however, the water reaches greater depth. Part is lost through drainage, and very little is lost by evaporation. Thus, although the water retention capacity of these soils is low, water is always available. For this reason, plants with deep roots are more common in sandy areas.
[Drawing: Jordi Corbera, from several sources]
24 The soils that support the woody cerrado formations are generally deep and well-drained geric ferrasols and ferralic arenosols, as can be seen in the eroded walls of this seasonal torrent in central Brazil. The regolith is clearly visible beneath the uniform and well-structured soil formed by the red upper layer. Geric ferrasols, despite their high clay content, show a very stable microaggregate structure that makes them highly permeable, so that most of the rainwater filters into the subsoil and little is lost as surface runoff.
[Photo: Mundayatan Haradisan]
25 The distribution of the soils and the relation between relief and vegetation in the bolilands region of Sierra Leone. The bolilands form an intricate mosaic of depressions, low river terraces, and wetlands with a vegetation of grassland and rushes, as the periodic flooding prevents the growth of woody species. During the rains, most of the lands are flooded. Only the soils of the raised tables and the highest part of the slopes have relatively good drainage and allow the growth of more or less open forests, controlled both by fires and by the nutrient-poor soils. Where the fires are less intense and not so regular, the number of forest species increases, the trees are taller, and the tree layer is more closed.
[Drawing: Jordi Corbera, bas-ed on several sources]
26 The miombo grows mainly on ferric lixisols (lower photo). The surface horizon is highly washed and weakly structured, with a low content of organic matter; below it, there is a clay horizon with a finer texture. The pH is about 5 at the surface and increases with depth. One characteristic feature of these soils is the lack of cohesion between the organic components and the minerals, which favors the washing of nutrients. The 1988 revision of the FAO Soil Classification includes some of the soils previously (1974) classified as luvisols in a new group, the lixisols. The differences between them are related to the cation ex-change capacity, CEC, of the clay fraction--less than 24 [cmoles.sup.(+)]/kg soil in lixisols and greater than this in luvisols. The dark eutrophic soils found in Africa (upper photo) are clearly visible in the strips of cultivated land alternating with fallow fields occupied by tall grasses and scattered trees.
[Photos: B. Spiers]
27 Mosaics of forests and savannahs, shown here in Landsend Valley in Sri Lanka, are also common in India and throughout Southeast Asia. The savannah plant community consists of a tall vigorous grass layer dominated by the grass Cymbopogon confertiflorus and a discontinuous tree layer of small trees, most of them belonging of the genera Terminalia, Albizia, Careya, and Phyllanthus. The soils of these savannahs are moderately deep eutric cam-bisols and haplic lixisols that do not show toxicity due to aluminum, and with CEC (Cation Exchange Capacity) values of between 5 and 7 [cmoles.sup.(+)]/kg soil. The scattered patches of forest within these savannahs grow on acid soils (haplic acrisols and dystric cambisols) with a high percentage saturation with aluminum and a CEC value of less than 2 [cmoles.sup.(+)]/kg soil. The tree cover consists of typical rainforest species such as Dimocarpus [=Eu-phorbia] longan, Ceylon mango (Mangifera zeylanica), wild nutmeg (Myristica dactyloides), and Canarium zeylanicum. Some authors consider this Asiatic savannah a community that replaces forest, but in fact it occupies conditions with special soil conditions and is not derived from a hypothetical original forest cover.
[Photo: B. Spiers]
28 Outcrops of red clay formations can be seen in the Hamersley Range Nation-al Park in Western Australia. In general, Australian soils are poor in nutrients (especially nitrogen and phosphorus) and certain micronutrients (zinc), and so are not very suitable for agriculture. Yet the correlation of soil type to savannah structure and composition are better established in Australia than in Africa and Asia.
[Photo: J.M. La Roque / Aus-cape International]
29 The early morning light on the Kukenan Tepuy in southeastern Bolivar State (Venezuela). The region's landscape consists of large table blocks of pre-Cambrian materials that rise to varying elevations and appear to be gigantic steps carved away by the river valleys. The highest ones reach elevations of 6,562-9,842 ft (2,000-3,000 m)-like solitary plateaus--and are known by their Amerindian name tepuy. Tepuys run, in smaller and more isolated groups, into Guyana, Surinam, and Brazil.
[Photo: Luiz Claudio Marigo / Bruce Coleman Limited]
30 The distribution of savannahs in Central and South America. In some areas in the cerrado in Brazil and in the llanos of Colombia and Venezuela, the monotonous savannah landscape is broken only by occasional narrow strips of gallery forest along the rivers. In other regions, such as in Amazonia or in Central America, the opposite occurs; there are small isolated patches of savannah vegetation among the continuous rainforest cover. The distribution of the American savannahs and their relation to environmental factors are not simple but lack the complexity of the African and Australian savannahs. Their floristic composition is markedly uniform over the entire area of distribution, and there is continuity from tree savannahs through shrub savannahs to grass savannahs. This does not, however, mean that each type of savannahs lacks its own characteristic ecology and physiognomy. Thus, it is possible to distinguish initially between dry savannahs, grassy savannahs with a substantial tree presence and an annual dry period, and wet savannahs with a wetter and less seasonal climate that encourages the growth of woody species with forest affinities. The environmental conditions of each region are responsible for the existence of subtypes within these large groups.
[Drawing: IDEM, based on several sources]
31 The cerrados occupy the central part of Brazil and are characterized by a woody savannah vegetation that forms low open forests such as this one in Monte do Carmo. The heat and light of the sun easily penetrate this type of plant formation and permit the growth of a dense herbaceous layer consisting of annual and perennial species. The appearance of the highly seasonal cerrados varies greatly between the dry season (July to September), when most of the trees and shrubs lose their leaves and the herbaceous layer is dry, and the wet season (beginning in early October), when spring flowering starts. The cerrado is covered with a carpet of varied and colorful flowers, contrasting with the deep blue sky and the reddish soils. The arrival of the rains changes this variety of colors to a monotonous green.
[Photo: Dudu Tresca / Jacana]
32 The characteristic appearance of the Venezuelan llanos, a large plain mostly covered by natural savannah, seen here in Hato Pinero in the state of Cojedes (Venezuela). The llanos form an enormous sedimentary basin bounded to the north and west by the Andes, to the south by the Guaviare River in central Colombia, and to the east by the delta of the Orinoco and the Guiana Shield. The savannah vegetation is dominant, but there are also areas covered by tropical forests, and gallery forests follow the courses of the rivers.
[Photo: Roland Seitre / Bios / Still Pictures]
33 The different plant formations grouped together under the name of chaco spread over the subtropical plains of Argentina, Paraguay, and Bolivia. The climate is not uniform throughout the region, so that communities with different growth-forms appear. The eastern Chaco is wetter and more closely resembles a subtropical forest, while the western and southern Chaco consists of a xeromorphic scrub that is much poorer in species. The change from one to the other is gradual, passing through plant formations that are increasingly low, open, and sclerophyllous.
[Photo: Tony Morrison / South American Pictures]
34 The savannah pine-woods of Honduras pine (Pinus caribaea var. hondurensis), such as this one on Guanaja Island (the Bay Islands), are very characteristic of the coastal strip of Honduras but are also found throughout Central America and the islands of the Antilles. They are generally associated with sandy substrates formed by the erosion of quartzite on the banks of rivers and in coastal plains. They grow in areas with exceptionally high annual rainfall (102-138 in [2,600-3,500 mm], with 3 months of drought), confirming the hypothesis that they are secondary formations derived from the tropical rainforest.
[Photo: Adolf de Sostoa & Xavier Ferrer]
35 The grassy savannah in Gabon turns an intense green in the wet season. In some zones of Gabon, the equatorial rainforest contains small patches of tall grasslands scattered with isolated woody plants or in patches. These herbaceous patches usually appear naturally on nutrient-poor soils and consist of a relict savannah vegetation that penetrated the current forest zone when a major climatic change occurred.
[Photo: Michael Gunther / Bios / Still Pictures]
36 The distribution of savannahs on mainland Africa and Madagascar. In Africa, the different plant communities are distributed in a very complicated model, the result of major regional climatic changes and the area's complex geomorphological history, which has given rise to highly diverse relief and soils. As a result, the African mainland shows great diversity in its savannahs, mainly shrub and tree savannahs, and they vary greatly in their floristic composition. Furthermore, the large areas of rainforest in central Africa separate the northern savannahs from the southern ones, a further factor leading to diversification. Although some plant species are present in all the savannahs on the continent, others have a unipolar distribution (meaning they are present either north or south of the rainforest belt). The physiographic and climatic changes that have taken place in eastern Africa in recent geological time (since the early Quaternary) have led to further differences between the plant communities of eastern and western Africa. This is why some plants are exclusive to the east of the continent, while others with a very broad range do not occur in this area.
[Drawing: IDEM, based on several sources]
37 The savannahs of eastern Africa coincide with the classic image of the savannah, a vast expanse of tall grass dotted with trees with a flat crown. This is clearly visible in this photo of the Massai Mara Reserve (Kenya). The dominant herbaceous plants are grasses, and the tree layer consists mainly of members of the Mimosoideae subfamily of the Leguminosae, which grow in the valley bottoms, and members of the Combretaceae, which occupy the slopes with less regular drainage and the stony soils. The large mammal fauna is the richest and most varied of all the African savannahs, containing the most typical species that are known all over the world.
[Photo: Luiz Claudio Marigo / Bruce Coleman Limited]
38 One form of miombo found at low elevations is the woody savannah of mopane (Colophospermum mopane) found in central Africa and in the most northerly part of southern Africa, in hot areas with little rainfall (for example in the Nxai Pan region of Botswana). This formation is more or less open with a clear understory and an often discontinuous cover of tall grass. Mopane can resist high temperatures 118[degrees]F (48[degrees]C), as well as frosts 25[degrees]F (-4[degrees]C), and is also highly resistant to the fires that occur at the beginning of the dry season. However, the more intense fires occurring later may eliminate much of the tree layer and may even turn it into a very open plant formation, known locally as chipya. Mopane is a highly xerophytic Caesalpinoid legume. Its paired leaves, reminiscent of the wings of a butterfly, fold together during the hottest hours of the day to prevent excessive transpiration. Mo-pane produces winged fruit that are dispersed by the wind.
[Photos: Ildefonso Barrera and Gerald Cubitt / Bruce Coleman Limited]
39 The setting sun silhouettes the trees of the open savannah of the Kalahari Gemsbok National Park in South Africa. In the eastern Kalahari region, most of the space is occupied by very open grassy savannahs. The lack of permanent surface watercourses and the very low annual rainfall (less than 4 in [100 mm]) make it look like a subdesert. Despite the arid climate, the limited summer rains (when the temperatures are most favorable for plant growth) and underground water allow some degree of plant cover. The plants' adaptations to these conditions include long roots that grow deep down or laterally, leaves that are shed for several months a year, and seeds that only germinate when soil moisture is high enough.
[Photo: Anthony Bannister / NHPA]
40 The spread of the open savannahs in Madagascar, such as the one in Ankarame-na shown in this photograph, was not due to a climatic change in the island but to the arrival of human beings, who systematically used fire to protect their settlements and to clear land for crops and grazing. Initially deforestation was slow, but over the centuries it speeded up and the forests were eventually replaced by woody and grassy savannahs. This does not mean that some types of savannah were not already present.
[Photo: Olivier Langrand / Bios / Still Pictures]
41 The savannahs of the different regions of India vary greatly and can be grouped into five basic types depending on the rainfall regime and their floristic composition. (1) The peninsular zone, with a climate between dry and subhumid, is occupied by a savannah dominated by spiny shrubs, as shown in this photo of the landscape of Bandhavgarh National Park (Madhya Pra-desh). (2) The semiarid western part of the mainland is occupied by a grassy savannah with very sparsely scattered trees and shrubs. (3) The eastern mainland's higher rainfall allows greater growth of the shrub and tree layer. (4) A fourth type of savannah occurs at the transition between the two mainland areas. (5) Finally, in the mountainous regions, which are wetter, the abundant woody elements are more diversified.
[Photo: C. McDougal Tops / Ardea London Ltd.]
42 Savannahs with stands of the palmyra palm (Boras-sus flabellifer) cover the high parts of Rinca Island, part of the Komodo National Park (Indonesia). Herbaceous formations with a tree layer are common in Southeast Asia, but this area contains a range from dense woody savannahs to grasslands with very scattered trees. These plant formations form a mosaic with the tropical rainforests that occupy most of the territory. Relief, soil conditions, and bioclimatic features determine the appearance of one type of vegetation or the other, but anthropogenic factors also play a major role.
[Photo: Gerald Cubitt / Bruce Coleman Limited]
43 Grasslands and dense woody formations cover the plateaus of the Khao Yai National Park, 124 mi (200 km) northwest of Bangkok (Thailand). This landscape is repeated in many other places in Thailand and throughout mainland Southeast Asia. The class of plant formations are probably subclimax communities that periodic fires, shifting agriculture, and the increasing number of domesticated animals all prevent from developing any further and thus remain more or less stable.
[Photo: Roland Seitre / Bios / Still Pictures]
44 Distribution of the main species of Acacia and Euca-lyptus in the Australian savannah systems. The average annual rainfall and the soil's composition and texture determine which species of Eucalyptus or Acacia grow in each area. Eucalyptus argillacea, for example, grows in fine-textured soils in areas with average annual rainfall of 20-30 in (500-750 mm), while E. dichromophloia grows on rocky outcrops and on sandy soils where the average annual rainfall is greater. In the more arid areas, the eucalyptus trees are replaced by both shrub and tree acacias.
[Drawing: IDEM, based on Beadle, 1981]
45 Tussock grasslands are widespread in Australia and are also present in many other tropical savannahs. The term tussock describes the growth-form of many grasses from the Southern Hemi-sphere, forming a clump consisting basically of the old dry leaves from which the fresh young leaves grow. Tussocks may reach heights of over 40 in (1 m), but most are 20-28 in (50-70 cm) tall, although a few form pillars over 79 in (2 m) tall. The tussock grasses usually have hardened leaves that may be flat or inrolled, smooth or covered in hairs, with a large number of siliceous incrustations that may be of use in identifying species. Tussock-forming grasses include the species of the genus Astrebla, shown in the lower photo in the Sturt National Park (Tibooburra, New South Wales). These plants are endemic to Australia and are commonly known as Mitchell grasses. Hummock grasses show a variation on the tussock growth-form. The aerial stems have short internodes and hard sharp leaves and grow from rhizomes, forming a dense hemispherical mass. The mass increases in diameter until the center dies and the remaining ring structure fragments into new hummocks. In Australia, about 20 species of the genera Triodia and Plectrachne form this type of spiny cushion, such as the ones in the upper photo, growing in the Kimber-ley Block (Western Australia) and known as spinifex. Spinifex clumps cover (often continuously), areas of hundreds of kilometers, even in the driest areas, as they are very resistant formations.
[Photos: Purdy & Matthews / Planet Earth Pictures and Graham Robertson / Auscape International]
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|Publication:||Encyclopedia of the Biosphere|
|Date:||Mar 1, 2000|
|Next Article:||El Orinoco Ilustrado.|