Deserts and desertification in the tropical and subtropical environments.
Joao Guimaraes Rosa
Grande sertao: veredas (1956)
1 The arid space
1. What is a desert?
1.1 The loose concept of a desert
The term desert is an ambiguous word with many meanings. Etymologically, desert comes from the Latin verb deserere, which means to leave or abandon, and from its past participle desertum (also used as an adjective), which means deserted or abandoned. As a noun, used only in the plural, deserta refers to uninhabited, desolate places without human settlements (the opposite of the Greek term oikoumene, which means of the inhabited Earth). In modern usage, the word desert refers to a region with little or no vegetation and thus lacking human presence, though the relationship of cause and effect--whether direct or indirect--is not as clear as previously thought. In practice, the term desert has come to mean an area uninhabited due to the difficult prevailing conditions, especially climatic and soil-related ones.
Subdeserts and false deserts
In modern scientific usage, the term desert has been increasingly restricted in meaning, becoming almost synonymous with a hyperarid area. Yet some regions traditionally considered as deserts are not subject to a hyperarid climate. These areas, called pseudodeserts or false deserts, include the Sonoran Desert in the southwestern United States and northwestern Mexico, the Chihuahuan Desert (also in the U.S. and Mexico, but farther east), the Sind Desert and the Thar Desert (between India and Pakistan), the Kalahari Desert (in Botswana, Namibia, and South Africa), Australia's Gibson Desert, Simpson Desert, the Great Victoria Desert, and the Great Sandy Desert, and even the central Asian Karakum Desert and Kyzylkum Desert. Pseudodeserts also cover smaller areas and include the Desierto de Vizcaino in the center of the Baja California Peninsula, the Mojave Desert (between California and Arizona), and the Chalbi Desert (east of Lake Turkana, in northern Kenya). Other equally arid areas not popularly considered as deserts include the subdesert grasslands (often known as steppes) in North Africa and the Near East, the Sahel, the highlands of northern Mexico, Patagonia, and the monte in Argentina, the caatinga in northeastern Brazil, and the Karoo in South Africa.
All these regions are arid zones or subdeserts, with relatively abundant permanent vegetation and often with trees or large shrubs. These areas are generally unsuitable for dry farming because rainfall is scarce and irregular, but extensive stockraising based on seasonal migration (transhumance) is well developed and is by far the most important agricultural activity. The average annual rainfall in pseudodeserts and subdeserts is 4-16 in (100-400 mm); the pseudodeserts also include some areas with lower rainfall (2-12 in [50-300 mm]) but whose location on the western coastlines of the continents (Baja California, Peru, Chile, Morocco, Mauritania, southwest Angola, Namibia, South Africa, and northwestern Australia) means they receive precipitation in the form of dew or fog.
In the true climatic deserts such as the Sahara, the Arabian Desert, the Dasht-e Kavir and the Dasht-e Lut (also known as the Kavir Desert and the Lut Desert) in Iran, the Namib, the Atacama, and the Lower Colorado Desert, the plant cover is neither permanent nor continuous either in space or time. The perennial vegetation is restricted to the lower parts of the relief, with vast bare interfluvial areas and no perennial plants, although they may be covered in ephemeral annual plants for a few weeks after one of the scarce rainfalls. Thus, true deserts are distinguished from false deserts by the distribution of the vegetation: the vegetation in true deserts is restricted to certain areas within the relief, while in false deserts the vegetation is distributed more regularly in the interfluvial areas and on the pediments. Annual rainfall in hyperarid areas is below 4 in (100 mm), and below 2 in (50 mm) in the littoral (coastal) oceanic deserts such as the Namib, western Sahara, and Atacama.
1.2 A landscape dominated by geology
The absence or extreme scarcity of vegetation in deserts means that the soil and geological substrates are always exposed. Desert landscapes are not shades of green but of yellow, ochre, and red. The desert landscape is essentially geological in nature.
The forms of the relief
Desert geomorphology is closely related to wind erosion. The forms of relief that are characteristic of deserts are carved by the wind from the previous relief, as the modern deserts have not always been hyperarid, especially not during the Quaternary (over the last two million years). The forms of relief inherited from the Quaternary and earlier periods were reshaped after the establishment of the modern desert climate, occurring in most cases only 3,000-4,000 years ago. This shaping by the wind creates several patterns of erosion and forms of deposition or sedimentation, including regs (desert surfaces) and hamadas (structures that are flat or slightly sloping in one direction), as well as the dunes and the rocky and stony slopes of mountains.
Autochthonous regs are the result of in situ fragmentation of rock into roughly centimeter-sized pieces; all the particles less than 2 mm in diameter (less than a tenth of an inch) are then blown away by the wind, leaving the ground surface covered in gravel and stones. Allochthonous ergs are formed by wind erosion of preexisting alluvial material--different types of colluvium, alluvium, fluvial terraces, and a variety of plains and depressions.
Regs are generally bare, and the vegetation is restricted to the low points of the relief (fossil river beds, different types of depressions) where occasional runoff compensates for the low rainfall. Regs cover immense areas in the Sahara and other hot and cold deserts. They have different local names: serir in eastern Sahara, gibber plains in Australia, gobi in China and Mongolia, mreyye in Madagascar, etc.
Hamadas consist of horizontal or almost horizontal slabs that are between a tenth of a meter and a meter long. These slabs are formed by hard, often sedimentary rocks; limestone, sandstone, quartzite, and even basalt, andesite, trachyte, rhyolite, and other flows of volcanic rock. As a whole, and other things being equal, the soil environment in hamadas is less dry than in regs. Hamadas lose less water by runoff, as their rough surface has many small cracks and fractures where pockets of loose soil can develop. The rough surface also reduces wind erosion, thus lowering potential evapotranspiration at ground level between the slabs. The gaps between the slabs may be occupied by perennial plants, shrubs, and even trees, and when the scarce rains fall, the plants can benefit from the runoff from these slabs. Thus, part of this runoff is protected from evaporation within the pockets of loose soil that develop between the slabs. This is why plants and animals are much more abundant in hamadas than in regs. Hamadas represent about 20% of the surface area of the Sahara Desert.
Rocky slopes on hills and mountains have some features in common with hamadas. Because of the roughness of the ground surface and the presence of fractures and folds in the outcrops of rock, pockets of loose soil can accumulate the water from the runoff; this benefits the plants and animals that are thus concentrated on a very small part of the rocky outcrops and at the base of screes. These rocky slopes represent about 10% of the area of the Sahara.
Dunes are the result of deposition or sedimentation by the wind and almost always consist of sand, but there are also clay dunes. Sand dunes may adopt many different forms (crescents, pyramids, star shapes, linear dunes). Small, mobile, crescent dunes (333 ft [1-10 m] tall and 7-164 ft [2-50 m] long) are known as barchans. The tallest dunes, which are pyramidal or star-shaped, are called rhourds and may occasionally reach a height of 1,640 ft (500 m), though dunes in dune massifs are usually 492-820 ft (150-250 m) tall. They have many different names in different areas: edeyen, ramla, goz, nefud, kum, peski, and shamo, among others. Dune chains may intersect to form a network of parallel sheets called akle, which looks like a series of fish scales from the air, or they may blur into undifferentiated sandy layers, which may or may not show signs of undulation or ripple marks.
Dunes are classified as longitudinal or transverse, depending on whether they are parallel to or perpendicular to the dominant wind. The largest sand structures (rhourds, linear dunes or seifs, elbs, and silks) are stable, while barchan dunes may move dozens of meters a year (even hundreds of meters in the sand tongues of Patagonia). Dunes can also be divided into symmetrical and asymmetrical groupings. Symmetrical dunes are longitudinal erosion dunes (sand ridges) derived from Quaternary paleodunes. They are found mainly in the southern Sahara and the Sahel and are considered dead dunes. Asymmetric dunes, to the contrary, have formed recently or relatively recently and are known as live dunes; they have a slope of 6-12[degrees] on the side where the wind blows and 20-33[degrees] on the other side.
Small accumulations of sand may also form around trees or shrubs that grow at a rate compatible with the deposition of sand behind or around them. These sand structures are known as nebkas or rebdu and rehbub. Nebkas are fixed, circular or ovoid, and reach a height of 3-16 ft (1-5 m) and a diameter of 16-164 ft (5-50 m). Rehbub (singular rehba) are arrows of mobile sand that accumulates beside a plant, normally a shrub.
The norm for rehbub is that the length of the arrow is five times greater than the height of the plant-screen. They are typically 4 in to 3 ft (0.1-1 m) tall and about 20 in to 16 ft (0.55 m) long. The slope in the direction the wind blows is 35-40[degrees]; on the other side of the obstacle, it is 10-15[degrees].
Clay dunes form leeward depressions, generally saline ones that lack vegetation. The accumulated pseudosand is derived from the wind erosion of sodic soils and alkaline saline soils where the clay particles have flocculated (aggregated in a loose organization), forming pseudosand aggregates due to the highly alkaline pH (8.5 to 11.0). The dunes formed in this way are barchans, nebka, rehbub, and occasionally belts.
The most extreme deserts are usually located in depressed basins surrounded by raised areas. These basins may be endorheic, meaning they do not drain outwards. Watercourses form after rainfall and flow to the lowest point in the depression; they are finally lost when they infiltrate into the sand or flow into a lake occupying the bottom of the basin. These lakes, such as the Great Salt Lake in the United States, Lob Nor in central Asia, and Lake Eyre in Australia, are often extremely salty due to the dissolved substances from the surrounding rocks.
Desert depressions may or may not be saline. Saline depressions (sebkhas, salars, salinas, chotts) are formed by the evaporation of surface runoff water or by the evaporation of spring water from deep aquifers of variable age. They are characterized by the temporary or permanent presence of a layer of free salty water, generally shallow and varying in salinity between 5 g/l soluble dry residue (approximately one-seventh as salty as seawater) and saturation. Some of the largest saline lakes in deserts are extremely saline; the Dead Sea contains 270 g/l of soluble dry residue and the Great Salt Lake contains 260 g/l.
Alluvial depressions also have a permanent or temporary crust of salts, generally sodium chloride, as well as an external strip of halophilous (salt-loving) vegetation dominated by chenopods (members of the Chenopodiaceae). This halophilous vegetation is known as chott, which means shore in Arabic, and also refers to the salt pasture around the shore. The term has been erroneously used for the large sebkhas in North Africa (Chott Djerid, el Gharsa, el Fedjadj, Melrhir, el Hodna). Some depressions that do not show the features mentioned above have saline and/or sodic soils and characteristic halophilous and succulent vegetation.
Nonsaline depressions (known variously as kavirs, playas, bajadas, creeks, mallins, and canadons) are periodically flooded by runoff waters and in general have a relatively deep water table. The water table allows perennial vegetation to grow, sometimes even with trees, since many desert trees are phreatophytes, meaning they grow precisely where the water table is accessible to their deep root systems.
1.3 The distribution of aridity on the planet
The distribution of temperatures on Earth's surface can be broadly conceived as a relatively smooth gradient from cold at the poles to hot at the equator, but the distribution of aridity requires a much more complex explanation. Aridity depends on the balance between precipitation (as well as some forms of condensation such as dew) on the one hand and evaporation and loss by runoff on the other. Precipitation in turn depends on the circulation of the atmosphere and oceans (see vol. 1, pp. 322-325). These circulations are mainly determined by the above-mentioned heat gradient but are modified by the presence of the continental landmasses that warm up in the summer months alternately in the Northern and Southern hemispheres, generating a rising movement of hot air that has to be replaced by colder air from the surroundings. In the respective winters, however, the surface of the interior of the continental landmasses cools down, and the air in contact with it undergoes an increase in pressure, generating an outward air movement from the center of the continental landmass.
At the latitudes around the Tropic of Cancer and the Tropic of Capricorn, cold, dry air sinks; these are the zones where rainfall is lowest within the global atmospheric circulation. These zones contain the world's main deserts, separating the areas of high rainfall around the equator from those at the mid-latitudes in both hemispheres. In the Northern Hemisphere, the enormous deserts of northern Africa and the Middle East as well as the deserts and pseudodeserts of the United States and northern Mexico all lie within this zone. Their counterparts in the Southern Hemisphere, the deserts in Australia and the Namib-Kalahari-Karoo group in southern Africa, are also at these latitudes. All of them are characterized by average annual temperatures of 68-86[degrees]F (20-30[degrees]C) and hot or very hot summers. The subequatorial deserts and arid zones of eastern Africa, northeast Brazil, Ecuador, Colombia, and Venezuela can also be included here, although they occur at lower latitudes.
Other deserts that form at these latitudes are due to more local factors. The cold winds from the land mean that breezes blowing from the sea lose their humidity before they reach the coast, so very little rain falls on the coastline. Fogs are very frequent in these areas and there may even be dew. Such is the case of the Namib Desert, bathed by the Benguela current; the deserts of Peru and northern Chile, influenced by the Humboldt current; and, in part, the subdeserts of Baja California, influenced by the California current (see vol. 1, p. 326). In these cases, however, the global atmospheric circulation also plays a role: these regions would probably be dry, but not extreme deserts, even without the influence of these cold currents.
The temperate subdeserts and deserts
Other deserts are the result of the monsoon circulation in addition to the relief. When an almost-saturated wind from the sea (bearing water vapor from its passage over the ocean) encounters a mountain mass, it rises, its pressure falls, and the partial pressure of the water vapor reaches the condensation point; then, precipitation occurs. The rain falls on the slopes of the mountain, but when the air flows over the mountain and reaches the other side, the pressure increases again. At this point, the vapor pressure of the remaining water is far below the condensation point, meaning that rain is very uncommon. This phenomenon, called the foehn effect, causes a rain shadow in the lee of a mountain range (see vol. 9, pp. 202-203). Deserts form where the rain-bearing winds always blow from the same direction (the other side of the mountain) or when a zone is partly or completely surrounded by mountains that intercept rain-bearing winds from every direction.
This is true of the many deserts of central Asia, where the rain-bearing monsoon winds from the Indian Ocean discharge their moisture onto the Himalayas and the other mountain ranges forming a barrier to the south. In North America, the Great Basin Desert--the region between the Sierra Nevada and the Cascade Range on the Pacific coastline and the Rocky Mountains farther east--is isolated from most of the moisture derived from both the Pacific and Atlantic oceans. In South America, the puna of the Andes (see vol. 9, p. 235) and the monte in Argentina are similar zones. They fall into the zone in the rain shadow of the Andes, isolated from the main source of moisture in the winds from the west, although the eastern limit of these subdeserts lacks any mountain system equivalent to the Rocky Mountains in North America.
The subdeserts with cold winters
In the deserts at temperate latitudes between the 35th parallel and the 50th parallel (middle Asia and central Asia, and the Great Basin in North America), the vegetation struggles against an additional, extremely adverse factor not present in the tropical deserts--a harsh winter. The average annual temperatures are 32-41[degrees]F (0-5[degrees]C), though the summers may be temperate (50-68[degrees]F [10-20[degrees]C]), hot (68-86[degrees]F [20-30[degrees]C]), or very hot (above 86[degrees]F [30[degrees]C]), because the winter temperatures fall far below freezing. There is precipitation in the winter in the form of snow, but this does not provide even a sporadic opportunity for photosynthesis. Even algae and lichens have to enter dormancy to survive the winter, and their only chance to revive and reproduce comes from conditions created by cloudbursts that fall in the summer months. But summers when cloudbursts occur are rather harsh in themselves--they are marked by high temperatures, strong winds, and intense evaporation, meaning that the rains are even less effective for plant growth than they would otherwise be. Thus, for a given amount of annual precipitation, temperate zone deserts may be even more inhospitable for plant life than those in the tropical zone.
Precipitation in the cold subdeserts of the tropical and subtropical zones is unpredictable, and therefore very hard on plant life. Precipitation in the cold deserts is not only very low but also very variable. Rain, when it occurs, may fall as a downpour that causes sudden freshets and floods as the normally dry watercourses overflow. However, there can be an interval of many years between these events, so the plants have to survive these periods in a state of dormancy or semidormancy. During this phase, they are unable to photosynthesize but are exposed and vulnerable to herbivores.
The polar and high mountain deserts
In some deserts, plant life is limited more by low temperatures than by the lack of water. Below freezing point protoplasmic activity is reduced to a minimum and, in many cases, the tissues die. If they survive, they can only return to activity after unfreezing. Thus, plant life is almost totally excluded from microclimates that are permanently below the freezing point. In sites where temperatures rise above zero for short periods, plants that can rapidly resume activity may be able to photosynthesize for a short time and then return to dormancy when temperatures fall again. The shorter the periods with temperatures above freezing, the smaller the number of species able to support these conditions, until all that remains are algae, mosses, and lichens, as occurs in extremely arid areas. Lichens are the great survivors in extreme conditions of cold and aridity; only a few species of vascular plants have a comparable ability to survive. In any event, the main requirement is an ability to make use of the short periods of activity separated by long intervals of adverse conditions, when freezing or desiccating conditions force them to remain inactive.
Low-temperature regions occur at both high latitudes and high altitudes. The higher the latitude, the lower the angle of incidence of the sunshine, meaning that the energy supply per unit surface area is lower and is absent for much of the year. At higher altitudes, temperatures also decrease (see vol. 9, pp. 199-200). Thus, moving toward the poles or to higher elevations implies passing through a zone where plant growth declines (although the plant cover may be continuous, as in the tundra) and then entering a zone where phanerogams (seed or flowering plants) are sparse and lichens are often abundant. These are the polar or high mountain deserts, zones where plant growth is limited mainly by the low temperatures, though precipitation may also be low, since the air can hold little water vapor as it is below freezing point. Such areas usually have an annual average precipitation of less than 10 in (250 mm; meaning they would also be classified as deserts in the hot-temperate or tropical regions), but rarely below the 4 in (100 mm) common in the most extreme deserts at lower altitudes and lower latitudes.
Frozen deserts occur in Antarctica, mainly on the Antarctic Peninsula and in the dry valleys of Victoria Land, where there is some sort of plant life (see vol. 9, pp. 171-174). The plants do not grow where there is a continuous cover of snow or ice; only a few microscopic algae can grow under such conditions. In the Northern Hemisphere, the cold deserts occupy the northern tip of Greenland, much of the Canadian Arctic Archipelago, the northern coastline of northeastern Canada and Alaska, the northern coastline of Siberia, the neighboring islands, and the Svalbard Archipelago.
In mountainous regions cold deserts are much less widespread. They are situated at altitudes of around 16,400 ft (5,000 m), where the average annual temperature is below the freezing point, possibly as low as 23[degrees]F (-5[degrees]C) or 19[degrees]F (-7[degrees]C). These deserts only cover large areas in a few large plateaus, such as Tibet, the Pamirs, or the Andes, but the difference in altitude between the upper limit of the alpine vegetation and the perpetual snows may be very narrow. Climatic changes that occur in the polar regions over hundreds of kilometers are usually compressed into a vertical distance of just 328-656 ft (100-200 m) in mountains.
2. The problems raised by aridity
2.1 The ecophysiology of the desert
To understand why a given area of Earth's surface is a desert, it is first necessary to identify why vegetation does not grow there. The main cause in most deserts is usually the lack of available water. It is thus worth distinguishing between the concepts of aridity, drought, and desertification. A climate's degree of aridity is a permanent and measurable characteristic, despite fluctuations. Drought, on the contrary, is always relative and temporary, making it hard to define precisely except in relation to an average rainfall from which any one year or season deviates by making the water deficit worse. Desertification, unlike drought, has lasting consequences that are often permanent and irreversible.
Water constraints on plant life
Land plants contain a high percentage of water, mostly more than 50% and sometimes more than 95%; more importantly, water is absolutely necessary for their physiological activity. Exchanging carbon dioxide with the atmosphere in respiration and photosynthesis requires the presence of channels that are open to allow the exchange of water vapor. The cytoplasm of the photosynthetic cells (containing at least 75% water) have to be exposed to the atmosphere in order to capture carbon dioxide and fix it; for example, at a temperature of 68[degrees]F (20[degrees]C) and a water vapor pressure of 15 mbar, the protoplast shows an average net loss of water of up to 10 mbar.
The risk of losing the water essential for protoplasmic activity was an inevitable consequence of the migration of plants from their original aquatic environment to dry land about 500 million years ago. Land plants have developed mechanisms to reduce water loss, generally by surrounding the cytoplasmic membrane with an impermeable cuticle through which gas exchange is limited to pores that can be opened and closed (the stomata; singular, stoma). In some species, the stomata close during the day and open at night, when external atmospheric humidity is relatively high. This greatly reduces water loss but does not prevent carbon dioxide exchange taking place.
In addition to its role in maintaining protoplasmic activity, water is also essential for the uptake of nutrients from the soil and their transport within the plant to the centers of metabolic activity, a process driven by transpiration. For transpiration to occur, liquid water has to be present in the soil and available for uptake by the roots, organs especially adapted for this purpose. The nutrients are then transported together with the water due to the soil-plant-atmosphere continuum that is established. Land plants have thus retained a memory of their ancestral aquatic environment in their dependence on the availability of water in the environment 1) for nutrient transport and 2) to replace the water lost because of the necessary exposure of their photosynthetic tissues to a far-from-saturated atmosphere. For these reasons, the colonization of dry land started in areas where the atmosphere was nearly saturated and where the soil supplied constant and abundant water. The higher plants did not become adapted to grow in areas of low water availability and low atmospheric saturation until much later on, after the acquisition of many adaptations.
All this applies to the higher plants with differentiated leaves, stomata, and root systems, and to their reproductive processes, which no longer rely on the release of reproductive cells into the aquatic environment. However, other photosynthetic organisms probably colonized dry land long before this. These were organisms that could survive dry periods in a state of desiccation but then revive and return to activity as soon as water was available. This pattern of activity is shown by lichens, some algae, and many bryophytes--organisms that are not dependent on a constant water supply and can limit their activity to the sporadic opportunities provided by the rains, when the soil surface is wet and the water film contains the dissolved carbon dioxide and nutrients they need for their growth in the short period before the soil surface dries out again.
The temperature limits for plant survival
In addition to water, plants have certain requirements with respect to temperature. The range of temperatures in which plants can survive is relatively narrow, and the range in which they are active is even narrower (for most species, roughly 32-104[degrees]F [0-40[degrees]C]). The primeval ocean was presumably always liquid, so its temperature must have been between 32-212[degrees]F (0-100[degrees]C). Given that water has the highest known specific heat capacity of any liquid, any variations in its temperature due to energy exchange would have been very small and would have taken a long time to express themselves. Plants thus started evolving in an environment with a very limited range of temperatures but had to deal with a much wider range of temperatures when they colonized dry land. (Temperatures on land could fall below freezing for much of the winter, while some microhabitats could be much hotter than the oceans.)
Temperature extremes, like the lack of water, would have been less severe in the more humid terrestrial environments--wetlands and other moist spaces--that were undoubtedly the first places on land to be colonized by plants. From these humid areas, plants started their migration to the harsher terrestrial environments. The last zones colonized were those that were the most inhospitable because of their aridity and extreme temperatures. The impact of plant life on deserts has, so far, been only slight.
The privileged metabolic pathways
Light and solar radiation generally are not limiting factors for plant production in the deserts and subdeserts, since the energy flux is typically greater than 140,000 cal/[cm.sup.2] per year. Only locally, in the foggy zones on oceanic coasts and on some mountains, are light intensity and energy flux ever limiting. Yet water availability and the dynamic status of the desert and subdesert ecosystems greatly condition primary production; to put it another way, productivity is limited by aridity, the scarcity of vegetation, and the soil conditions, and not all photosynthetic pathways are equally efficient in the conditions of aridity and low soil water availability that prevail in the deserts and subdeserts.
Most plants photosynthesize using the [C.sub.3] metabolic pathway. The other two photosynthetic pathways found in vascular plants, [C.sub.4] and CAM, are physiological adaptations to high temperatures and low humidity. The terms [C.sub.3] and [C.sub.4] refer to the different products of photosynthesis. In plants following the [C.sub.3] pathway, photosynthesis leads to the formation of 3-phosphoglyceric acid, an acid with three carbon atoms, while in [C.sub.4] plants, it leads to production of oxaloacetic acid, which contains four carbon atoms (see vol. 1, fig. 121). The CAM pathway (Crassu-lacean Acid Metabolism) was first discovered in members of the succulent family Crassulaceae. CAM plants take up carbon dioxide at night and store it as malic acid that is decarboxylated the following day; the carbon dioxide released is then fixed using the [C.sub.3] pathway. These plants can thus keep their stomata closed during the daytime, when water loss is greater because temperatures are higher; this means their use of water is extremely efficient. (Sometimes they only need 110-220 lb [50-100 kg] of water to produce 2 lb [1 kg] of dry matter, though they generally require 440-660 lb [200-300 kg].)
CAM species, like [C.sub.4] ones, are very widespread in nature. In addition to the members of the Crassulaceae, the same metabolic pathway has been found in members of the Agavaceae, Poaceae, Asclepiadaceae, Bromeliaceae, Cactaceae, Asteraceae, Cucurbitaceae, Didiereaceae, Euphorbiaceae, Liliaceae, Orchidaceae, Polypodiaceae, Portulacaceae, and Vitaceae. It is very probably also present in the Geraniaceae, Lamiaceae, Oxalidaceae, and Piperaceae, while the evidence is not so clear in some species of the Buxaceae, Caryophyllaceae, Convolvulaceae, Plantaginaceae, and Chenopodiaceae. It seems certain that it does not occur in a single member of the grass family, Poaceae. Most CAM plants are tropical succulents belonging to families such as the Agavaceae, Bromeliaceae, Cactaceae, and Euphorbiaceae, although a single family may contain both CAM and [C.sub.4] plants. For example, the tropical African species of Euphorbia use the CAM pathway, while the subtropical North American species of the same genus use the [C.sub.4] pathway.
Though the CAM pathway is usually associated with a succulent growth form, there are succulent plants such as halophytes that do not possess it, and some CAM species are more accurately described as semisucculents. Some species, including the Mesembryanthemum crystallinum (Aizoaceae), M. forsskalii, and M. nodiflorum, use the CAM pathway when they are subject to extreme conditions of drought or salinity but revert to [C.sub.3] metabolism when conditions return to normal. CAM plants use water much more efficiently than [C.sub.3] and [C.sub.4] plants, but their productivity is much lower. CAM metabolism has also been associated with a slow rate of growth, but the energetics of carbon dioxide fixation show this need not be so in all plants using the CAM pathway (though many CAM plants such as dwarf cacti and several epiphytes are very slow growing). In fact, CAM plants are most characteristic of relatively constant environments with moderate nighttime temperatures and relatively higher humidity at night, such as coastal deserts and subdeserts and the zones at higher elevations where clouds condense.
2.2 The dysfunctions of the physical environment
The discussion so far has focused on the role that the climate and the plant responses to it play in desert formation. But what role does the substrate play? In fact, the substrate's role is very small: it plays a part in determining which type of desert ecosystem will develop, but not if a desert or some other type of biome will develop. Geology also has a clear influence, but only on a regional scale. As already pointed out, movements of Earth's crust that have led to mountain formation have played a very important role in the creation of many deserts in the area of their rain shadow.
The unfavorable substrate conditions
Although the bedrock does not determine whether a desert will develop or not, it may modify the vegetation and determine what type of desert will form. The interaction between the plants and the substrate is of great importance in this process. In other climatic conditions, some soil forms, so the substrate where plants grow is very different from the rock or the type of sediment originally formed by geological processes.
Water flow and animal activity incorporate plant remains into the soil, and they are then further modified by other animals and microorganisms; plant roots and their secretions help to break down the mineral particles; the final result, after many millennia, may be a layer of soil a few meters thick over the bedrock, differing greatly from it in composition and structure. In deserts, soil formation is greatly restricted by the limited plant growth. Furthermore, the low plant biomass means that the biomass of microorganisms and invertebrates is also low, and they too play a very important role in soil formation and the development of soil structure. The structure and properties of desert soils are often little different from those of the underlying parent materials. The organic matter content is normally below 1%, even in the topmost layers, and the little organic matter present rapidly decomposes when temperatures are high. Soil development is thus so imperceptibly slow that it is almost impossible to talk of a desert soil.
The high salinity of desert substrates usually greatly limits plant establishment and growth. Many desert soils, especially in depressions, contain high levels of common salt or gypsum derived from the rocks over or through which the rainwater has flowed and then evaporated. Even without the adverse climatic conditions, this salinity alone makes deserts extremely inhospitable for plants.
Because of their immaturity, desert soils are usually unstable and mobile, exemplified by the fact that sandstorms occur in deserts. Winds are often strong and can blow away the unconsolidated particles before they can form a true soil. These particles may be transported many kilometers. The wind also sorts the particles: at higher speeds, the larger particles are deposited before the smaller ones. Once the desert surface has been consolidated to some extent, for example by a crust of algae and lichens, this may be sufficient to stop wind erosion and protect the rudimentary soil. However, once broken (perhaps by the effect of animal hooves), this crust becomes highly vulnerable to erosion.
Like everywhere else, in deserts normal geological erosion takes place at roughly 0.1-1.0 t/ha per year. This erosion may be greatly accelerated by human activity, especially by bad land management and its negative impact on vegetation and soils. Uncon-trolled erosion may reach rates of 10-700 t/ha per year, proportional to the surface runoff. The normal geological rate of runoff is on the order of 1%, but it may be increased to 20% or even to 80% by the above-mentioned causes. Clearly, geological erosion may be much more active in some locations for a variety of reasons, among them orogenic folding (mountain formation by the folding of Earth's crust), cyclones, solifluction (soil flow), lava flows, mudslides, and prolonged torrential rains. In one example of an arid area, an experimental station in southern Tunisia, the average annual runoff is 14-25%, and average annual soil loss is 8.2 t/ha with an average rainfall of only 6 in (150 mm); on a loamy soil on a 5% slope after rainfall of 10 in (250 mm) in 26 hours (which is likely to occur about once a century), runoff of 80% was recorded and erosion reached 39 t/ha in a single day.
Vulnerability to erosion by water and wind
Erosion by water is proportional to total precipitation. It is thus lower in arid areas; likewise, certain arid areas with clay-gypsum geological formations show annual rates of abrasion of 200-300 t/ha, combining gully erosion, erosion by surface runoff (whether concentrated or diffuse), and gallery erosion due to subsurface runoff.
Water erosion in deserts and subdeserts is largely due to torrential rains and has to be considered on two different spatial scales: the scale of the area of land in question (on a scale that may range from a few hundred square meters to a few hectares [1 hectare=2.5 acres]) and that of the drainage basins (ranging in size from a few hundred square kilometers to several thousand). Their rates of erosion are inversely proportional to their area. There is nearly an order of magnitude of difference between the values obtained at the level of the plot of ground and those obtained at the level of the catchment basin. Erosion at plot level may be evaluated by calculating Wischmeier's universal soil loss equation (USLE), which averages five independent factors: climate, soil type, relief, plant cover, and conservation practice. Other indexes can be defined such as the climatic erosion index, corresponding to the annual runoff--the ratio of the annual average runoff (E) to the average annual rainfall (P). For the drainage basin itself, the reference index is that of Fournier, p2/P where p is the rainfall in the rainiest month and P is the average annual rainfall. In the arid areas of the northern Sahara, normal rates of water erosion are 10-20 t/ha per year (the equivalent of the annual abrasion of an imaginary thickness of 0.7-1.4 mm).
Wind erosion is characteristic of subdeserts and especially of true deserts. Normally negligible when the average annual rainfall exceeds 24 in (600 mm), in true deserts wind erosion plays a decisive role in shaping the geomorphology. There is little information on wind erosion. Rates of 150-300 t/ha per year have been recorded in southern Tunisia (and also in Mongolia, in a cold desert) as a result of the clearing of subdesert sandy steppes. As in water erosion, wind erosion results from the bareness of the soil surface. There is a critical value, 25% permanent canopy cover, at which wind deposition compensates deflation.
1 Dunes with little or no vegetation, such as these dunes in the Erg Chebbi in the Moroccan Sahara, are the image suggested to most people by the word desert. In the most typical deserts, the almost total lack of plant cover leaves the geological substrate and soils exposed, coloring the landscape warm shades of ochre, red, and yellow, in keeping with the high daytime temperatures. The hostile environment of this biome may offer very little to human beings, but they have their own beauty. Deserts are among the world's most fascinating landscapes.
[Photo: Cyril Ruoso / Bios / Still Pictures]
2 Sand dunes vary greatly in shape, depending on the direction and strength of the wind. There are three main shapes: The most common are linear dunes (upper left), undulating crests of sand with the same slope on each face, formed by strong winds that always blow in the same direction. Barchan dunes (upper right) are crescent-shaped dunes that form where sand is not particularly abundant, through the action of moderate winds blowing in the same direction all year round; barchan dunes move in the direction the wind is blowing, and the leeward side is steeper than the windward side. If the winds do not blow from a single dominant direction but blow from all four quarters, then irregular stationary dunes form that are pyramidal or star-shaped. These are known as star dunes or rhourds (lower center) and can reach heights of up to 1,640 ft (500 m).
[Drawing: Jordi Corbera, bas-ed on several sources]
3 Saline depressions are seasonal lakes, sometimes very large ones, that, depending on the period of the year and the rises and falls of the underlying water table, turn into marshy plains of brackish mud, as in this photo of Death Valley (United States). The intense heat of the sun evaporates the water, drying out the lake and forming a hard surface crust, which seems solid enough but in fact covers extremely soft ground. In the deserts of the Old World these treacherous muddy plains have swallowed entire caravans of camels.
[Photo: Francois Gohier / Ardea London]
4 The Hadley cells, convection cells that form between the equator and 30[degrees]N and 30[degrees]S, are responsible for the arid conditions of the high-pressure subtropical zones, and thus the presence of hot deserts. On the equator, where sunshine is most intense, the air heats up and rises, lowering the pressure at ground level. The space formerly occupied by the equatorial air masses is then occupied by warm air masses that release intense rainfall. This is why the typical plant formation on the equator is the tropical rainforest. But the masses of equatorial air that have risen are then driven toward the poles by the new masses of rising air, and as they advance they cool down. Eventually, they become denser than the underlying air, at about 30[degrees]N and 30[degrees]S. As they sink to ground level, they dry out and create a high pressure zone that prevents the entry of moist air. This is the zone where hot deserts form. In the end, these air masses return toward the equator at ground level.
[Drawing: IDEM, based on Allan & Warren (eds.), 1993]
5 The dunes and steep high mountain desert slopes in the valley of the Brahma-putra River (Tibet) at an altitude of more than 13,123 ft (4,000 m). The Plateau of Tibet receives very little precipitation, with the western half receiving an average annual precipitation of less than 4 in (100 mm), most of which falls as snow, and the eastern half receiving only 1-2 in (20-50 mm) annual precipitation. This low rainfall, together with the low temperatures (the average annual temperature is below freezing), limits plant growth and leads to the appearance of a desert landscape--a cold desert, with little vegetation concentrated almost exclusively around the water points.
[Photo: Galen Rowell / Moun-tainlight Explorer]
6 Mirages are optical illusions that are very common in deserts. They are caused by the refraction of light rays as they cross the warmer, denser layer of air in contact with the soil surface. They are most common in large, flat areas lacking vegetation that get very hot in the direct sunshine. In mirages, the images of distant objects are upside down, as if they were reflected in a sheet of water at their base, as can be seen in this photo of a mirage in a salar in Bolivia. When the observer approaches, however, the mirage disappears.
[Photo: Ildefonso Barrera]
7 The different plant strategies to fix the carbon from atmospheric carbon dioxide are adapted to different environments. In some plants, carbon is fixed when it reacts with the five-carbon sugar RuBP (ribulose 1,5-diphosphate), forming two molecules of three-carbon ([C.sub.3]) sugars. These are known as [C.sub.3] plants, while the pathway is known as the Calvin cycle. In others, the [C.sub.4] plants, carbon is fixed by carboxylation of the three-carbon acceptor PEP (phosphoenolpyruvate, see also vol. 1, fig. 121) to produce four-carbon sugars. In the CAM plants, (upper diagram) C[O.sub.2] is fixed at night (blue arrows) and stored temporarily as organic acids, mainly malic acid; during the daytime (red arrows) they continue to produce sugars using the Calvin cycle. The route used by each species depends largely on the local climate. The CAM strategy has the advantage that C[O.sub.2] is fixed at night, meaning the stomata need only open at night, and this reduces water loss, the reason why it is very common among desert plants. Though the CAM strategy is excellent in environments where it almost never rains, it does not operate well at very high or very low temperatures. CAM plants are thus often restricted to the coastal regions of deserts, where the climate is milder due to the influence of the sea. This is clearly shown in a graph (lower diagram) representing the percentage of the plant cover using each one of these strategies along a transect of southern Africa at 30[degrees]30'S, running inland from the western coast to 27[degrees]S. [C.sub.3] and [C.sub.4] plants account for most of the plant cover in the inland eastern regions where the summer rains are relatively abundant, but CAM species are much more abundant near the coast where rainfall is scarce or concentrated in the winter.
[Drawing: Jordi Corbera, based on several sources]
8 Comparative studies of erosion and abrasion in two arid zones of Africa with very similar climatic conditions show that regions with very similar ecological conditions may suffer from very different problems of erosion, depending on their management. The two zones are the Karoo in southern Africa and the semidesert steppes of northern Africa. The rate of runoff is 35% greater in the Karoo than in the North African steppes, but the water erosion is 4.6 times greater in North Africa. In the Karoo there is almost no desertification, while in the steppes of North Africa erosion is advancing at a rate of 0.7% per year. Since both regions receive a very similar amount of rainfall--4-16 in (100-400 mm) per year--this contrast can be explained by differences in the plant cover. The Karoo has a perennial plant cover of 20-30% and a perennial biomass of 1,000-1,500 kg dry matter/ha. Furthermore, there is almost no dry farming in the Karoo (which leads to much greater infiltration of water), as it has been banned below the 16 in (400 mm) isohyet, except on soils more than 4 ft (1.2 m) deep. In the North African steppes, however, 50% of the area has been cleared to cultivate cereals, though the volume of production is very unpredictable. The perennial cover and biomass, initially comparable to those of the Karoo, reach only 5% and 250-300 kg dry matter/ha, respectively.
[Drawing: IDEM, based on data provided by the author]
9 Sprouting and flowering immediately after a rain shower, like this member of the Amaranthaceae in the Atacama Desert, is a good way of adapting to the dry conditions of the desert. It will bear and shed its seeds within a few weeks of germinating. This plant solves the problem of water shortage by avoiding the drought, remaining dormant in the form of seeds until the drought is over. Other species do not avoid drought but resist it; they have developed mechanisms to absorb as much water as possible (with special storage organs) and reduce their losses due to evapotranspiration.
[Photo: Tony Morrison / South American Photos]
10 Part of the Atacama Desert (Chile) is covered in volcanic materials. They have been eroded by the rains, which are scarce but can accumulate because of the large size of the watersheds. These materials are an indication of the volcanic activity in the Tertiary and Quaternary that accompanied the formation of the Andes, when subduction of the Nazca Plate beneath the South American Plate began. This volcanic activity still occasionally threatens the people of the Andes. The geomorphology of the areas of the Atacama Desert linked to volcanic activity are large and spectacular: the western limits of the Andes, near the Domeyko Cordillera, contain several volcanic cones, some of which reach a height of almost 16, 400 ft (5,000 m).
[Photo: Ramon Folch / ERF]
11 The risk of erosion by wind and rain depends on the average annual rainfall (above) and the percentage of plant cover (below). Logically, water erosion is directly proportional to average annual rainfall. Thus, in deserts rainfall is so low one might expect erosion by water to be limited, but in fact the nature of the substrate and the topography also play a major role. If rainfall is very scarce, almost all erosion is by the wind. In deserts, wind erosion plays a major role in shaping the landform and landscape. But when rain falls, wind erosion diminishes rapidly, as wet soil is not blown away as easily by the wind. Furthermore, in areas where rainfall is greater, a plant cover that protects the substrate from wind, and to a lesser extent from the impact of raindrops, gully erosion, and runoff, grows rapidly. When rainfall is much higher, the thick plant cover greatly reduces wind erosion.
[Drawing: Jordi Corbera, bas-ed on data provided by the author]