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The boreal conifer forests or taiga.

Cool, clammy gloom reigns within the forest. If a single cloud covers the sun, the taiga darkens and everything turns gray. Yet on a clear day, the sunlit tree trunks, the glistening conifers, flowers, moss, and multicolor lichens form an incomparable setting. The vegetation is so thick that sometimes you can not see the sun between the branches.

Vladimir Arseniev

Dersu Uzala (1921)

1 The kingdom of the conifers

1. The cold forests

1.1 The concept of taiga

The word of Russian/Mongolian origin, "taiga" (in Yakut tia means "forest"), has been adopted by almost all the world's languages. There are several different scientific definitions and everyday uses of the word taiga, but it is most commonly used to mean the conifer forests running in a broad strip around the north of the Northern Hemisphere and subject to the harsh climatic conditions of the northernmost parts of the Temperate Zone.

What is and what is not taiga

The taiga is also known as the boreal forest, derived from the Latin boreas (boreaV in Greek), the name of the god of the north wind. Coniferous forests that could be referred to as taiga also occur to the south at higher altitudes, and in the subalpine layers of the mountains in the tropics.

To the contrary, not all coniferous forests can be considered as taiga. Large areas of the Mediterranean Basin, for example, are covered by forests of pines, such as the cluster pine (Pinus pinaster), the Calabri-an pine (P. brutia), or the Aleppo pine (P. halepensis), whose structure, appearance, and flora are more similar to the neighboring perennial oak woodlands of the Mediterranean (see vol. 5, p. 75) than to the pine forests of Siberia or northern Europe. The Southern Hemisphere also contains many forests dominated by conifers such as podocarps (Podocar-pus), dammar or kauri pines (Agathis), monkey puzzles (Araucaria), Rimu pines (Dacrydium), etc., that have nothing at all in common with the taiga, and which are more comparable to temperate laurel forests ("laurisilva") (see vol. 6).

The true taiga

Taiga covers about 10% of the world's dry land, and is the most widespread vegetation in the Northern Hemisphere outside the tropics. Wherever it occurs, the presence of taiga forests is due to the same conditions, and this is why it looks similar in different regions of the world (Canada, Siberia, the Pyrenees, and the Himalayas). The coniferous trees of the taiga all have long, thin, needlelike (acicular) leaves with well-developed strengthening tissue (sclerenchyma). They are mainly members of the pine family (Pinaceae), such as spruces (Picea), firs (Abies), pines (Pinus), larches (Larix), and hemlock spruces (Tsuga). The boreal forests dominated by spruces, firs, hemlock spruce, and pine species of the subgenus Cembra--i.e., trees with dark green leaves--are known as dark taiga. The forests dominated by larches and most other pine species with light green leaves are known as light taiga. Taiga ecosystems sometimes contain conifers from other families, such as junipers (Juniperus, Cupressaceae).

The taiga has a small number of tree species, all of them belonging to genera almost entirely restricted to the Northern Hemisphere. There is hardly a single tree in the taiga that is not a conifer. There are almost no deciduous species, only the occasional birch (Betula) and poplar (Populus), genera which are not very representative of the deciduous forests (see vol. 7, pp. 85 and 89). Except for these deciduous broad-leaved trees (and the larches, the only deciduous conifers), all the trees of the taiga are evergreen. In most taiga ecosystems, the tree cover is dense and lasts throughout the year. The taiga biome thus shows a comparatively high level of annual production, despite the low temperatures during half the year. The taiga forests are also of great importance to humanity, as they are the world's largest source of timber.

1.2 Snow almost all year round

The essential condition for taiga landscapes to develop is a humid climate, in which the water coming from precipitation is greater than that lost by evaporation. In a typical taiga forest, 50-70% of the precipitation evaporates, the remaining water activates drainage, and where drainage is impeded, wetlands form. In the taiga of east central Europe, for example, average annual precipitation is about 30 in (750 mm), but evaporation is only about 18 in (450 mm). In many regions, a stable layer of snow forms in early September that does not melt until the beginning of the next summer. The trees and shrubs do not bear new leaves until late May, and there are nighttime frosts even in July.

The cold and the tree vegetation

The type of taiga landscape is largely determined by the amount of energy that it uses. In the boreal forests, the total annual quantity of energy received as sunshine is about 450645 kcal/in2 (70-100 kcal/cm2), and the annual energetic balance between the radiation received and that reflected is around 160-195 kcal/in2 (25-30 kcal/cm2). These trees cannot grow where the average temperature of the hottest month is less than 50[degrees]F (10[degrees]C); in the far north, the hottest month is usually July, though in coastal areas it is often August. For a forest landscape, even 50[degrees]F (10[degrees]C) is insufficient. The most dense or closed boreal forests grow at the more southern latitudes (or at lower levels on mountainsides) of the 54[degrees]F (12[degrees]C) and 55[degrees]F (13[degrees]C) mean July isotherms (the line connecting areas that have the same mean temperature at the same time). In the dense band of open boreal (tundra) forest between the 55[degrees]F (13[degrees]C) and 50[degrees]F (10[degrees]C) mean July isotherms, the trees show signs of stress and are stunted, low, and scarce. Increasing summer temperatures, combined with decreasing precipitation, limit the distribution of the conifers. As a general rule, it can be said that the dark-colored conifers (firs, spruces, hemlocks) do not grow on flat plains to the south of the 64[degrees]-66[degrees]F (18[degrees]-19[degrees]C) isotherm for July, and only occur in valleys and on shady slopes.

The higher the latitude, the greater the importance of temperature as an ecological factor determining the physiognomy of the vegetation. Temperatures only play a key role in the tundra. South of the tundra, in eastern Siberia, adjacent areas with identical temperature conditions may support boreal forests or steppes. Like everywhere else on the planet (except the polar regions), whether these areas support forest, steppe, or desert ecosystems depends basically on the moisture conditions. If a mountain chain blocks the movement of moist air masses from the ocean, the area behind the mountain will support steppe, not forest (see vol. 9, p. 203). Differences in the soil class may also determine whether moisture conditions are favorable. Clay soils prevent the water from precipitation from filtering into the soil, and so trees can grow on them. Sandy soils allow the water to filter into the soils, and so trees do not have enough water to grow.

The availability of water and the decisive role of the snow

Normally, if annual precipitation is below 16 in (400 mm) and there is no additional supply of water, trees, especially conifers, grow weak and stunted, as in the tundra forest. Yet in eastern Siberia, and especially in central Yakuty-Sakha, forests grow in sites where the average annual precipitation barely reaches 12 in (300 mm), and in some years is less than 8 in (200 mm). The continental regions of northwest Canada, where the Yukon Basin is located, also have very low average annual average precipitation, 9-10 in (240-260 mm), comparable to many of the world's deserts. These forests exist because permafrost is present under the soils in Yakuty-Sakha and the Yukon, and the tree roots can obtain water from the melting permafrost and from the condensation of water from the air on the cold soil surface.

The taiga is often considered as a single area with a very continental climate, but the North American area of the biome is very varied. It contains many adjacent ecoclimes, from the subhumid taiga in the prairie provinces through humid taiga in northern Ontario to perhumid (very humid) taiga in eastern Canada. The energy, precipitation, and length of the growing season are all sufficient in all these sites for trees to grow, though the dominant species in the forests may differ. The unifying feature of all these ecoclimes is that conifers are favored over broad-leaved deciduous trees. Broad-leaved trees are only dominant or codominant with conifers where the conditions are subhumid or temperate, where there have recently been forest fires, and in wetlands along rivers.

The snow dramatically decreases the amount of variation between the temperature of the soil and that of the layers of air nearest the soil, so that snow cover is very beneficial for the vegetation because it keeps the soil warm, and in the winter the soil is always warmer than the air (at a depth of 20 in [50 cm], the soil temperature is 27[degrees]-36[degrees]F [15[degrees]-20[degrees]C] warmer than the air temperature), thus saving the plant roots from freezing solid. In extremely cold winters, many taiga plants cannot live without an even, loose, protective layer of snow (which is a very bad conductor of heat) that lasts all winter long.

In Eurasia, the mean annual temperature and the thickness of the snow layer determine the distribution of the permafrost. In the European taiga zone, where the average annual temperature is above 32[degrees]F (0[degrees]C) and precipitation is abundant in winter, the southern limit of the permafrost is further north than the limit of the taiga, in the tundra zone. There is evidence, however, that during the Quaternary glaciations permafrost was present throughout what is now the taiga area of Europe. Yet to the east of the Yenisey River, where the climate is exceptionally continental and the snowfall in winter is scarce, the southern limit of the permafrost moves sharply to the south.

The different degrees of climatic tolerance among the conifers

The boreal forests grow in areas where mean temperatures for July are between 54[degrees]F (12[degrees]C) and 68[degrees]F (20[degrees]C)--usually between 59[degrees]F (15[degrees]C) and 64[degrees]F (18[degrees]C)--and in these areas average annual precipitation varies between 8 in (200 mm) and 39 in (1,000 mm). One of the most critical requirements for the growth of the taiga, especially the dark taiga, is high atmospheric humidity in the summer months. Taiga trees cannot grow in arid regions, even with abundant irrigation water. In Tbilisi Botanical Garden (Georgia), most of the spruces 60 years old have not reached a height of 33 ft (10 m), although in boreal forests trees of the same species reach a height of 98 ft (30 m) at a much younger age. At this age they become old and die. Even when soil moisture is abundant, taiga species cannot compensate for the water loss due to intense transpiration in hot dry air.

Most conifer species are, on the other hand, extraordinarily cold-resistant. The temperature in eastern Siberia in winter can fall as low as -76[degrees]F (-60[degrees]C). In continental regions where very cold winters alternate with very hot summers, the temperature range over the course of the year may be as great as 180[degrees]F (100[degrees]C), but even this does not prevent the Dahurian or Gmelin larch (Larix gmelinii) and some other conifers from growing.

In North America, the typical climate of the open boreal forest with lichens, the coldest boreal forest, ranges from the conditions at Lake Cree (Saskatchewan), where the average annual temperature is 27[degrees]F (-2.7[degrees]C) and the precipitation is 16 in (414 mm), to those at Nitchequon, Quebec, which is wetter and where the average annual temperature is 25[degrees]F (-4.1[degrees]C) and the average annual precipitation is 31 in (783 mm). Despite being warmer, with mean annual temperatures of 32[degrees]-39[degrees]F (0[degrees]-4[degrees]C), the northern conifer forest varies greatly in precipitation from east to west. The Pas (Manitoba) has an average annual precipitation of only 18 in (454 mm), but Cameron Falls (Ontario) receives some 31 in (796 mm), and in the oceanic climate of St. John's, Newfoundland, precipitation may exceed 59 in (1,500 mm). Conditions are even warmer in southern Ontario and Quebec and in the Maritime Provinces of Canada, where mean annual temperatures are 39[degrees]-45[degrees]F (4[degrees]-7[degrees]C), which allows the appearance of mixed forests; in Mount Forest, Ontario, for example, the mean annual precipitation is 38 in (964 mm).

It is not the severity or mildness of the winter that is decisive in the establishment of the taiga forest, as in the taiga of western Scandinavia the average monthly temperature in January is 37[degrees]F (3[degrees]C), while this average may be as low as -62[degrees]F (-52[degrees]C) in some regions of Siberia. The really vital condition is the alternation of seasons and the long winters. It is the long winter that makes the taiga ecosystem so different from the cloud forests of tropical mountains, some of which they resemble in many respects.

Finally, it is worth pointing out that in the most continental regions of western Europe, where mountain chains prevent precipitation from reaching the intermountain depressions, the dark conifer forests so typical of European mountains are replaced by forests of the European larch (Larix decidua), for example in the central Alps. Obviously the central Alps have a much less continental climate than Siberia, but the fact that large forests grow in the Alps reflects an important generalization: within the taiga area, similar climatic changes cause similar responses in the vegetation.

== 2. Inhospitable, cold, half-formed soils

2.1 Soil components and processes

In the cold winter, the taiga soils may freeze to great depth, and they only start to melt in late spring and early summer. But except in some highly continental regions, permafrost is not common in the taiga, although seasonal freezing means the soil has special morphological features. The climate is too cold for microorganisms to be active, and so leaf decomposition is slow, taking several years, and this leads to the formation of a thick layer of leaf litter.

Soil texture and acidity

The distribution of the sizes of the soil's constituent mineral particles plays a major role in the distribution of the boreal forests. Conifers prefer light, open soils with a high water retention capacity. Sandy soils cannot retain as much water and are dominated by pines, which do not require a lot of water. Firs are present throughout the European taiga and require moisture, and prefer to grow on open clay soils.

Taiga soils are highly acidic (pH 3.5-4.5) for a variety of reasons. The main reason is the parent materials on which the soils have formed; in much of the biome, these are rich in silica and poor in basic cations. Outcrops of basic rocks such as limestone are scarce but they are interestingly diverse. The south of Yakuty-Sakha, for example, is dominated by a monotonous boggy taiga of larch that is floristically poor and unproductive, but the limestone outcrops are easy to detect because they are covered by productive pine forests, where diversity is high. The second reason for the soil acidity is that the leaf litter of conifer needles, branches, cones, etc., is very poor in minerals, its C/N ratio is very high (often greater than 70), and so it does not release the basic cations that might compensate for the acidity of the soil.

The leaf litter that accumulates on the soil surface (the O horizon) plays a vital role in the life of the taiga. This horizon can be divided into subhorizons (OL, OF, and OHi) that vary greatly depending on the greater or lesser degree of decomposition of the litter, which in turn gives rise to stable organic compounds that combine with the mineral components that the plants need to grow. The nutrients stored in the organic remains are gradually released by the action of microorganisms, making them available to the plants gradually. In acid media the main organisms responsible for this breakdown are fungi, though they are only active at certain temperatures.

This leaf-litter horizon consisting of dead plant remains contains most of the roots of the taiga plants, which explore its entire volume in order to obtain as much as possible of the limited fertility. If a permafrost layer forms near the soil surface, the leaf litter is even more important, as the roots cannot grow in the frozen horizons. The leaf-litter layer is not, however, always so positive for the plants; for example, the seeds shed by pines, firs, and other trees with small seeds cannot germinate successfully in the leaf litter. In the taiga, the seedlings of firs, larches, and pines frequently grow on the wood of rotten branches and decomposing tree trunks, i.e. where litter has not accumulated.

Podzol formation

When a soil is subject to percolation of excess water through the soil profile, in cold environments where the parent material is sandy and nutrient-poor, the process of podzolization occurs. In fact, two processes occur: the first is the formation and migration of soluble chelate compounds of humus with iron and aluminum cations (cheluviation), and the second is their accumulation at depth. Fulvic acids are the dominant complex organic compounds in these environments, where environmental conditions delay the decomposition of the organic matter and favor the activity of acidophilic fungi. The carboxyl and phenol groups on the fulvic acids form sites where polyvalent metal cations, such as iron and aluminum, are trapped very efficiently.

Fulvic acids are soluble, but only slightly so. As they become saturated with iron and aluminum, their solubility decreases until they precipitate out. If the soil does not contain many of these metallic ions, these acids may be carried considerable distances by the water, up to several hundreds of meters, forming a discharge of "black water." In most cases, they only migrate to a depth of a few centimeters, forming a dark accumulation horizon of organic matter, iron, and aluminum known as a spodic B horizon. The washed surface horizons are highly acidic, and this causes clays to break down by acid hydrolysis. After this, the most resistant minerals accumulate in the eluviated horizon, and in extreme cases all that is left is almost pure quartz sand.

Gleying

Waterlogged conditions occur when there is an excess of water in the soil, sometimes for part of the year and sometimes all year round. Waterlogged soils vary in their appearance due to differences in the process of gleying. When the soil is waterlogged and organic matter is present, the organisms (roots, microorganisms) rapidly use up the little oxygen available, and thus favor the action of anaerobic organisms. Conditions cease to be oxidizing (producing increased positive charge) and become reducing (producing decreased positive charge), and elements like iron, manganese, and sulfur are reduced.

In oxidizing conditions, iron and manganese are usually present in the soil in the form of insoluble oxides, which are usually yellowish, brown, or reddish. In reducing conditions, iron compounds are bluish or grayish, typical of waterlogged soils; moreover, the reduced iron compounds are soluble and migrate in the water. When waterlogging is very severe and the water table drains slowly, the soil loses the iron oxides and turns gray or olive green: in the absence of iron oxides, the soil reveals its own color. When soil water with dissolved reduced ions reaches a crack or the empty space formerly occupied by a root, it enters an oxidizing environment, and the oxides of iron and manganese precipitate out of solution. The soil horizon thus becomes full of orange patches within a gray matrix. This kind of morphology indicates alternating oxidizing and reducing conditions.

Waterlogging of the soil affects the vegetation, which consists of plants adapted to such conditions. It is important to know the intensity of waterlogging of the soil, because the presence of a reduced horizon, even if it only lasts for a few months of the year, severely restricts root growth. The presence of colored patches does not always indicate the soil suffers waterlogging, because in some sites these colored patches may be due to waterlogging in the past. Geomorphological analysis, some chemical tests, study of the formation from the parent material, and the current vegetation all provide additional information on determining how far the soil is suffering from waterlogging.

Bog formation

Bogs form and accumulate when plant remains decompose very slowly in comparison to the rate they are produced. Bogs exist at almost every latitude and in almost every biome, but they are especially abundant in the taiga, where low temperatures, excess moisture, and highly acidic or oligotrophic soils create conditions in which decomposition of organic matter is very slow, a feature characteristic of peat formation.

Most peat bogs form in endorheic (inward-flowing) depressions where, because of the factors mentioned above, organic matter decomposes very slowly. Differences in the degree of saturation and the depth of the water table mean that there is often a zonation of vegetation types from the edge of the bog to its center, with the vegetation in the center most adapted to waterlogging. Even when the depression is full of water, the peat can continue growing, as long as conditions remain oligotrophic and acidic or organic toxins persist. The peat grows above the original water level, and the external input of nutrients is even further diminished as water drains away from the center, not toward it. The bog can thus spread beyond its initial area and create a wet band around it. In relatively marshy areas with a gently rolling relief, the bogs that initially form in small depressions may eventually join together, forming a single continuous layer of peat.

If a peaty depression is shallow, the vegetation can absorb nutrients from the underlying mineral materials. When the peat has become deeper than the roots, the losses by washing or fixation force the plants to survive on fewer and fewer nutrients. The vegetation that develops is adapted to this situation, and generally shows low diversity. In general, at greater depths the plant remains are more decomposed. Peat can be split into three types: fibrous material, in which the plant species and organs can still be recognized; rotted material, in which the botanical characters can no longer be recognized; and humic material, which is intermediate in structure.

2.2 The different types of soils in the taiga

Unlike the soils of the steppe and prairie biome, the soils of the taiga are not fertile, and are of little agricultural interest. The sterile podzols are in sharp contrast to the fertile chernozems. Chernozem and podzol, as well as gley, podbur, and many other types of soil, were originally the common names given by Russian peasants, but have entered the scientific vocabulary and provided the terms used in soil science. The Russian naturalist and soil scientist Vasily V. Dokuchayev (1846-1903) standardized these popular terms and gave them a more precise scientific meaning.

Podzols

Podzols cover almost 1.2 billion acres (480 million ha), most of them in the temperate and boreal regions of the Northern Hemisphere occupied by the taiga forest. The Russian term podzol was originally used for the grayish ash left in a stove after burning wood. The Russian peasants extended it to refer to a particular type of soil that was the same color.

A typical well-developed podzol consists of a O organic horizon of organic material overlying a mineral A horizon with an accumulation of humified organic matter. Below this is a light-colored E horizon that is highly eluviated and known as an albic E horizon. Below this, there is an undifferentiated dark brown or black spodic B horizon (Bhs), or one consisting of eluviated organic matter (Bh) on top of an accumulation horizon of iron or aluminum sesquioxides (Bs).

The A horizon consists of a mixture of organic matter partially converted into humus and mineral materials, often grains of quartz, covered by a layer of leaf litter 0.4-2 in (1-5 cm) thick (O horizon). The albic E horizon (E), whose ash color is the origin of the name given to these soils, consists almost entirely of highly washed grains of quartz. The E horizon is often stratified into layers 2-3 mm thick that form when the soil freezes in winter. There are almost no roots in this inhospitable medium, where acid hydrolysis of minerals is very active. The other components are eventually hydrolyzed until all that remains is quartz, the most resistant of the minerals present. This high sand content means few nutrients are available to the plants. The underlying spodic B horizon is reddish in the drier regions of the taiga, where sesquioxides of iron and aluminum predominate in the organic matter, but the spodic B horizons in wetter areas tend to be darker in color as a result of the dominance of the organic material. The physical properties of the podzols are conditioned by their sandy structure, which means they cannot retain much water. Their chemical fertility is also low, the result of their heavy washing and low ability to retain cations. The surface horizons are acidic, between pH 3.5 and 4.5, which increase slightly to a maximum of 5.5 in the lower horizons. In these acidic conditions, aluminum is mobile and can reach levels toxic to plants. There is little biological activity. Organic matter is mainly decomposed by fungi, together with some insects and a few other small arthropods. These characteristics mean that, as a whole, podzols are not suitable for agriculture.

Podzoluvisols

In a strip in the south of the biome, between the true podzols of the taiga proper and the luvisols of the deciduous forests, there are soils that share some of the formation processes that occur in the two areas, podzolization and the illuviation of clay (see vol. 7, pp. 32 and 34). They are called podzoluvisols. They cover about 618 million acres (250 million ha), most of them in a strip running from Poland to central Siberia.

The most typical sequence of horizons in a podzoluvisol is a thin dark (ocherous) A horizon on top of an albic E horizon. This E horizon forms tongues that penetrate the underlying Bt horizon, where clay accumulates. Below this is the C horizon, where soil formation processes have had little effect. The presence of an E horizon on top of a Bt horizon shows that water moves down through the soil for much of the year. The progressive obstruction of the drainage pores in the Bt horizon by the illuviated clay eventually prevents drainage and leads to waterlogging of the surface horizon. The presence of frozen horizons, or even permafrost, greatly affects the water regime in podzoluvisols for several months of the year. As with podzols, the high acidity of podzoluvisols, their low nutrient content, and the fact that they are frozen for much of the year mean that they can be used for little other than forestry.

Histosols

Swampy soils are also present in the taiga. Two related processes are responsible for bog formation: the formation of peat in the upper layer and the gleying of the lower layer. Peat forms when vegetation decomposes in anaerobic conditions, i.e. in the absence of oxygen. In other words, the soil of the swamps is so saturated with water that all the air is expelled. In these conditions, the decomposition of organic matter is incomplete and intermediate products accumulate, such as lignins, waxes, etc. In most swampy soils, the organic (histic) H horizon is 39-79 in (1-2 m) thick and is usually present over a mineral layer that has undergone gleying.

Raised bogs are oligotrophic and acidic, and most consist of the remains of sphagnums (Sphagnum). Valley bogs in depressions contain more nutrients and are less acidic. Peat is a valuable resource that is widely used by human beings. Low bog peats are good fertilizers, and raised bog peats are used as fuel, as bedding for livestock, as raw material for the production of raising agents, and even to flavor alcoholic drinks.

Gleysols

When the relief of the site causes waterlogging, the soils become full of excess water and the lack of oxygen may start gleying. The external signs are the presence of a hydrophilous vegetation and the presence in the soil of bluish, grayish, and olive green colors; these soils are known to Russian peasants as gley. They have a loose layer of leaf litter on a dark gray A horizon, which changes abruptly into a Bq horizon that is blotched gray and olive green, has undergone severe gleyification, and which is increasingly anaerobic at greater depth.

In the world as a whole, gleysols occupy about 1.5 billion acres (625 million ha), about two-thirds of them in the boreal regions. They are unfavorable for plant growth because the prolonged waterlogging and the lack of oxygen inhibit root growth.

Permafrost and podburs

Soil formation on permafrost (see vol. 9, pp. 25-28) is completely different. Permafrost, when soil is permanently frozen, is very characteristic of the most continental regions of Eurasia and America, especially in the taiga and tundra regions. They occur over a huge area of more than 1.2 billion acres (500 million ha) and are characterized by the surface accumulation of humic acids and iron oxides, both of which are highly mobile. Frozen soils in the taiga frequently show gleyification.

The development of frozen soils usually takes place over hard parent materials, and is determined by the extremely cold conditions, in which organic substances decompose very slowly. In permafrost soils, water may rise or fall, depending on the periods of freezing and thawing and the thickness of the permafrost, which is a barrier to the water and causes gleying. Because water moves both up and down in these soils, which are also influenced by alternate freezing and thawing, the soil profile is almost homogeneous. In similar climatic conditions but where the soils are lighter, a different type of soil develops, known in Russian as podbur. It may form in the permafrost area, as light soils thaw to a much greater depth than heavy soils. Podbur soils are intermediate between permafrost soils and podzols. Like the permafrost, podbur soils have a homogeneous profile due to cryoturbation. Despite this, gleyification does not occur, and where drainage is good the water tends to percolate downward, and this may cause podzol formation.

3. The taiga in the world

3.1 The Eurasian boreal conifer forests

The taiga zone occupies the north of Eurasia, from the Atlantic Ocean to the Pacific, in a broad strip covering a huge area, more than 1.7 billion acres (700 million ha), 85% of it in the Russian Federation. At the latitude of the Arctic Circle, the taiga zone runs about 4,350 mi (7,000 km) from west to east, and at the 60th parallel (60[degrees] N) it is more than 4,970 mi (8,000 km) long. The average north-south width of the taiga is about 620-745 mi (1,000-1,200 km), but in some regions, such as the area between the Yenisey and the Lena Rivers, it may be up to 50% wider. The Eurasian taiga includes the lowland conifer forests of eastern Europe and the western Siberian Depression, as well as the montane taiga, which occupies large areas in northwest Europe and in central and eastern Siberia.

It is difficult to state precisely the southern and northern limits of the taiga biome, because there are always transition zones. To the north, the boreal forests always turn into tundra. In some areas, the frontier is clearly defined, but the conifer forest is usually present in patches within the tundra. This transition is an ecotone, it occupies large areas in central and eastern Siberia, and is called tundra forest or tree tundra. The southern edge of the taiga is normally in contact with deciduous forests or steppes.

The taiga in northern Europe

In Europe, the north-south width of the taiga zone is 435-497 mi (700-800 km), and this includes much of Norway, Sweden, and Finland, as well as the northern part of European Russia. To the south, the European taiga borders the deciduous forest biome, with a very broad strip of mixed conifer and broad-leaved forests that is sometimes called southern taiga. The southern taiga occurs in some areas of southern Scandinavia, in northern Scotland, in the Baltic states, in some areas of Poland and Byelorussia, and in the central regions of European Russia. Variations in the soil mean the vegetation of the southern taiga is frequently a mosaic of alternating patches of coniferous and broad-leaved forest. As a whole, the southern edge of the boreal forest in Europe coincides approximately with the northern limit of the distribution of the English oak (Quercus robur).

The taiga of the Atlantic coastline of the Scandina-vian Peninsula has some unusual features. The narrow patch of taiga on the western slopes of the mountains of Scandinavia, between 62[degrees] N and 66[degrees] N, differs from the other areas of taiga, as it has a mild, wet, oceanic climate. In the mountains, annual precipitation may reach 79 in (2,000 mm), with maximums in autumn and winter, meaning that a thick layer of snow forms. The frost-free period lasts for five or six months, longer than in other boreal forests. The summers are usually cool, and the air temperature in mid-July does not reach 59[degrees]F (15[degrees]C). In the north of the Scandinavian Peninsula, the boreal conifer forests grow very well far north of the Arctic Circle.

Further east, the Scandinavian mountains cast a rain shadow. The precipitation is lower, the peak is in summer, the annual temperature range increases, and the climate becomes typically boreal, moderately continental, or even temperate. Podzol formation is very frequent in the soil, there is almost no permafrost, and there are many peat bogs, but not as many as in western Siberia. The plant cover is totally dominated by forests of spruce, with a significant amount of birch. The three latitudinal subzones--northern taiga, central taiga, and southern taiga--are represented. In comparison with other areas of boreal forest, those in eastern Europe are relatively densely populated.

The taiga in Siberia

Broad-leaved forests and mixed conifer and broad-leaved forests gradually disappear as the climate becomes more continental, and they are almost absent to the east of the Urals. There, in the south of the flat West Siberian Plain, the taiga is in direct contact with the steppe. In the transition zone is a band of steppe forest and of primary forest of birch and aspen. The width of the taiga zone decreases to 373-404 mi (600-650 km), because the taiga's northern limit moves further south the greater its distance from the warming influence of the Atlantic; its southern limit moves to the north, due the dry climate and the fact that the summers are hotter. The climate is greatly influenced by the winter Siberian anticyclone (high-pressure system) and the frequent arrival of cold air masses from the Arctic. The joint effect of the flat relief and other factors has caused bog formation to take place on a huge scale on the plain. Bogs may cover entire watersheds, restricting the forest to the valleys. The main species forming the boreal forest in western Siberia are the Siberian spruce (Picea obovata) and the Siberian pine (Pinus sibirica).

In central Siberia, between the Yenisey River and the lower stretches of the Lena, the northern limit of the taiga approaches the Arctic Circle, and in some areas crosses it. The eastern Tamyr Peninsula contains the world's most northerly forests: larch forests in the basin of the Khatanga River, at 72[degrees]23' N. The southern limit of the taiga reaches as far south as 52[degrees] N, at which latitude steppe dominates the plains of western Siberia. In central Siberia, the taiga zone may thus stretch up to 930 mi (1,500 km) from north to south. This is not because the Central Siberian Plateau is very high (1,640-2,297 mi [500700 m]), but because the climate is highly continental. The average air temperature in summer is higher than in western Siberia, which means that the taiga can grow further to the north. In continental climates, the snow layer is thinner in winter, meaning that the area of permafrost is greater. In summer, this permafrost supplies the trees with additional water, and the forests can grow further south than usual. In the continental climatic conditions of central and eastern Siberian, taiga is mainly represented by light conifer forest, especially forests of larches, although pine forests are also very typical, especially in the southern regions of central Siberia.

In eastern Siberia the climate is extremely continental (the difference between the average temperature in July and January is about 108[degrees]F [60[degrees]C]) and the taiga zone is much wider, up to 995 mi (1,600 km) from north to south. However, the highly continental climate has also led to the appearance of enormous nonforested areas. Forests alternate with bogs in western Siberia, but there are many patches of steppe in eastern Siberia, because of the high summer temperatures and low precipitation. These patches of steppe are especially notable in depressions in the relief, on the south-facing slopes of mountains, and in the arid rises in the lowlands of central Yakuty-Sakha, and occur within both the true taiga and the most northerly areas of tundra forest. The largest area of permafrost in the world is in eastern Siberia. Hardly any podzol formation takes place, and the dominant soils are permafrost taiga soils. Unlike eastern Europe and western Siberia, there are few bogs, because of the mountainous relief and the low precipitation.

The Far Eastern taiga

In the Far East the taiga does not reach as far north as in other regions of Eurasia, but reaches further south. Its northern limit is at less than 60[degrees] N, in the contact between the Djudjur lowland and the Kolymskiy Mountains, while the southern limit is about 49[degrees] N, in the middle stretches of the Amur River, and even further south on Sakhalin Island and the northern islands of Japan. On mountain chains running north to south, the taiga reaches even further south in northeastern China and the Korean Peninsula. There is an almost continuous strip of montane taiga in the mountains of Japan, starting in northern Hokkaido at sea level and reaching an elevation of 8,202 ft (2,500 m) in central Honshu, at 36[degrees] N.

In the Far East taiga, the summer monsoon's arrival from the Pacific Ocean is usually accompanied by abundant rains. In July and August, the average monthly rainfall is more than 4 in (100 mm). Winter temperatures are very low and there is almost no snowfall, because the area is influenced by the Siberian anticyclone. Moving west from the coast, oceanic influences rapidly decrease, due to the north-south alignment of the mountain chains. The wet summer and cold winter prevent the taiga vegetation from spreading further north, but do allow coniferous species to spread to the south, to latitudes that in central Eurasia are occupied by desert. The zone with a monsoon climate coincides with the area of distribution of the two main tree species of the Far Eastern Taiga, the yeddo spruce (Picea jezoensis) and the Siberian white fir (Abies nephroplepsis). In the southern Far East taiga zone there are forests with many species of trees, shrubs, and climbers, dark forest soils, and a rich and unusual fauna.

3.2 The boreal conifer forests in North America

The boreal forest zone in North America is very similar to the boreal forest zone in Eurasia, as it is subject to most of the factors determining the geography and ecology of the Eurasian taiga. The taiga occupies a large area in southern Alaska and is the dominant forest cover in the whole of mainland Canada, especially on the Canadian Shield. It occupies a huge continuous arc 3,915 mi (6,300 km) long, from the north of Yukon Territory and the south of Hudson Bay, passing through central Ontario and Quebec, and on to Newfoundland.

The curious distribution of the North American taiga

As in Eurasia, the taiga zone in North America forms an unbroken strip, 621 mi (1,000 km) wide in some places, and runs from the Pacific to the Atlantic. However, the North American taiga forest's distribution, in the form of an arc, is very different from the east-west distribution found in Eurasia.

In North America, the topographical barrier formed by the mountain ranges running parallel to the coastline (north-south), the southern prolongation of the Arctic Ocean south to Hudson Bay (which is frozen all winter), and the absence of transverse (east-west) barriers to impede the flow of humid tropical air from the Gulf of Mexico, together lead to the establishment of a relatively stable zone of contact between tropical and polar air masses. The influence of this special feature of the North American climate on the distribution of the boreal forest is so important that the transition zone between the boreal forest and the tundra (to the north of the open forest with lichens) coincides with the average summer position of the polar front, while the southern limit of the boreal forest coincides with the polar front's average winter position.

Latitudinal zonation

The North American boreal forest is usually divided into two subregions, the true boreal forest (with spruces, pines, American larch, firs, aspens, and birches) and the open taiga forest (with lichens, spruces, and American larch). Further south, between the boreal forest in southern Ontario and Quebec and the temperate deciduous forest of south and southeast Canada, there is an ecotone of mixed forests, with a mosaic of conifers (spruces, firs, white pine, and red pine) and deciduous trees (oaks, maples, ashes, and beeches) that many authors consider part of the boreal forest. As in the Eurasian taiga, the North American taiga also contains many bogs, but unlike Siberia and Europe, they support dark conifers, especially black spruce (Picea mariana). Apart from this, they share a very similar flora.

The boreal forest in Alaska grows in lowlands from the Brooks Range in the south to the mountains of British Columbia, at 57[degrees] N, where a subalpine forest separates it from the moist temperature forests of the Pacific coastline. Near the mouth of the Mackenzie River in the northern area along the frontier between Canada and Alaska, the North American taiga forest reaches furthest north, at about 69[degrees] N. It runs about 1,120 mi (1,800 km) south-southeast in the Yukon Territory, from the subarctic region to the prairie with poplars in Alberta. Apart from the western region of the Northwest Territories, its northern limit is clearly displaced southeast and it runs along the southern edge of Hudson Bay, whose cold water notably lowers the air temperature (at 51[degrees] N, the latitude of Cologne, Ghent, and southern England, the average temperature in July is 60[degrees]F [15.5[degrees]C]) and permits tundra ecosystems to replace taiga ones at 54[degrees]-55[degrees] N (roughly the latitude of Belfast, Kiel, and Moscow). The southern limit of the taiga forest, which is between 50[degrees] N and 52[degrees] N, is displaced south to 49[degrees] N, to the south of Lake Winnipeg in southeast Manitoba, almost reaching the United States border.

Boreal forests dominate central and northern Ontario and most of Quebec, except the Ungava Peninsula and the St. Lawrence Lowlands. They run east-northeast from Quebec to the Atlantic coast of northern Labrador, covering much of Newfoundland. Mixed forests, known in Canada as the Great Lakes, St. Lawrence, and Acadia forests, cover large areas of southern Ontario and Quebec, most of the Maritime Provinces (New Brunswick, Prince Edward Island, and Nova Scotia), and some areas of the states around the Great Lakes and New England in the United States.

3.3 The subalpine conifer forests

Coniferous forest ecosystems very similar to the taiga forests can be found on mountains far to the south of the southern limits of the true taiga biome. It is well known that altitudinal zonation on mountains shows many similarities with latitudinal zonation. The mountains of central and western Europe, the Caucasus, central Asia, the Himalayas, China, and the Rockies all have altitudinal zones where the canopy is dominated by conifers. These mountain conifer forests have a flora that is dominated by the same genera as the lowland taiga zone, and often the same species. The climate in these subalpine zones is also similar to that of the true taiga, although the temperature range over the course of the year decreases as one moves toward the equator. The further these mountain areas are from the lowland taiga zone, the fewer typical taiga genera and species present, and the higher the number of nontaiga floristic elements.

The Eurasian subalpine taiga

Subalpine conifer forest is widespread in the Pyrenees and Alps and the composition of the tree vegetation is very similar to that of the taiga zone of northern Europe. For example, the pines and firs are represented by the same species, and the distribution of the pines of central Europe joins up without a break to their area in northern Europe. Human intervention favoring these fast-growing trees makes it very difficult to say which contemporary subalpine pine forests are natural and which are artificial.

The subalpine conifer forest story that covers the greatest difference in altitude is in the Caucasus, especially in the western regions with a wetter climate. Thus, for example, the conifer layer occurs between 4,593 and 6,234 ft (1,400 and 1,900 m) on the northern side of the Caucasus and between 3,937 and 6,234 ft (1,200 and 1,900 m) on the southern side. The main tree is the Caucasian fir (Abies nordmanniana), which can reach a height of 260 ft (80 m) and a trunk diameter of 5-6.6 ft (1.5-2 m) in rich soils and where air humidity is high. One interesting feature of the fir and spruce forests of the Caucasus is the presence of an understory where typically boreal plants coexist with plants typical of deciduous forests, which is partly explained by the relatively high nutrient levels of the dark forest soils of the Caucasus, which favor the growth of deciduous woodland species that are very demanding in regard to nutrients.

One very unusual story of subalpine forests occurs in the highly arid conditions of the Tian Shan Mountains in central Asia. It is a very atypical spruce forest that never forms canopies as dense and closed as those of the dark lowland taiga. This is because of the ecology of the dominant species, Schrenk's spruce (Picea schrenkiana), also known as the Tian Shan spruce. It requires more light than other spruces, and does not demand such high air humidity. In its evolution, this spruce has adapted to resist the harsh dry conditions of late summer. This adaptation includes a growth halt in early July, meaning that the plant's growing season is only 50-55 days long. Its root system is deeper than that of other spruces, and grows deep into the soil and into cracks in the granites and schists. This is why the spruces of the Tian Shan are very resistant to strong winds. This boreal forest story occurs in the Tian Shan at elevations of 5,577 to 8,858 ft (1,700 to 2,700 m) above sea level (Schrenk's spruce sometimes reaches an elevation of 10,500 ft [3,200 m]) and it grows best on the wet north-facing slopes (in Kyrgyzstan, for example, 96% of the spruce forest is on north-facing slopes).

The North American subalpine taiga

The subalpine or mountain taiga covers large areas in the Rocky Mountains. Broadly speaking, this conifer forest zone runs along the Rocky Mountains and occurs at higher altitudes further to the south. It runs from the lowlands in the boreal regions of Alaska and Canada at 65[degrees] N to the high peaks of the volcanic mountain ranges of Mexico at 19[degrees] N, and possibly further south in the highlands of western Guatemala. Different tree species dominate these forests at different latitudes, though some, such as Engelmann's spruce (Picea engelmannii) and the Douglas fir (Pseudotsuga menziesii), are distributed along almost the entire length of the strip of coniferous forests of the Rocky Mountains.

In most cases, the conifers in the Rocky Mountains form the timberline. At 61[degrees] N, the timberline is at 4,593 ft (1,400 m), and gradually rises (roughly 330 ft [100 m] for every degree of latitude) to 9,843-11,483 ft (3,000-3,500 m) in Wyoming and Colorado (between 38[degrees] N and 43[degrees] N) and to 13,123 ft (4,000 m) in central Mexico (20[degrees] N). In the southern Rocky Mountains (Colorado), the montane conifer forest story is between 6,890 and 9,843 ft (2,100-3,000 m) above sea level. Here, the average annual temperature is 46[degrees]F (8[degrees]C) at 4,593 ft (1,400 m) and 26[degrees]F (-3.3[degrees]C) at 12,303 ft (3,750 m); average annual precipitation is 20 in (500 mm) in the lower part of the conifer forest layer and 39 in (1,000 mm) in the upper part.

In the arid zones of the southeastern United States and in northern Mexico, the taiga strip of the Rocky Mountains directly borders the dry prairies and subdeserts. When there is a shortage of water, open low conifer woodlands develop, dominated by pinyon pines (Pinus edulis, P. cembroides) and other small trees, such as cherrystone and alligator junipers (Juniperus monosperma, J. deppeana), which rarely exceed 23 ft (7 m) and have a rounded crown and almost shrublike growth form, with trunks that branch very close to the base.

154 Pine forest in the Finnish taiga at sunset. Conifer forests are the typical plant cover in the taiga, a biome that covers the northern regions of Eurasia and North America, between the tundra to the north and the deciduous forests, steppes, and temperate grasslands to the south. They form a dense forest where the trees grow so closely that they prevent light from reaching the soil. The vegetation of the lower layers is thus usually poor, and consists mainly of low evergreen shrubs (especially members of the heath family, Ericaceae) covering a layer of humicolous (plants that inhabit medium-dry ground) grasses, mosses, and lichens. Trees are thus the most abundant plants in the taiga. Most conifers are evergreen, such as firs (Abies), spruces (Picea), and northern pines (Pinus), but some are deciduous, such as larches (Larix). There are occasionally small thickets of broad-leaved species that can withstand the low winter temperatures and snow, such as willows (Salix) and birches (Betula). The forest cover is frequently broken by bogs, sphagnum (Sphagnum) bogs, or small lakes with an abundance of insects. The fauna of these forests consists of small animals, such as hares (Lepus), voles (subfamily Microtinae), and many species that eat conifer seeds, such as squirrels (family Sciuridae) and crossbills (Loxia). There are some larger animals that depend on moving into the neighboring biomes to feed, such as reindeer (Rangifer tarandus, known as caribou in North America), elks (Alces alces, moose in North Ameri-ca), and lynxes (Felis lynx). Many insectivorous birds also spend the summer in the boreal forests, but they migrate far to the south in the winter.

[Photo: Michel Gunther / Bios / Still Pictures]

155 Snow often accumulates to a considerable depth in the taiga, as shown in this photo of a snow-covered spruce (Picea) forest in the Riisitunturi National Park in Finland. The low temperatures in this biome mean that the precipitation almost all falls as snow, forming a stable white layer that covers the soil and vegetation throughout the winter. When the melt starts in spring the evergreen conifers (which have kept their leaves through the winter) can start photosynthesizing immediately, and can thus make best use of the very short growing season. This is one of the reasons why the taiga consists of evergreen forests. Deciduous trees could not survive in these conditions because they cannot produce their foliage in the very short growing season.

[Photo: Jan Tove Johansson / Planet Earth Pictures]

156 One-sided trees looking like flags, such as the white spruces (Picea glauca) in this photo, are typically found in areas where the dominant winds always blow from the same direction. The branches all grow in the opposite direction. This is an example of how climate and weather conditions determine not only which species of plant can grow in a given region, but also the way in which they grow. Other examples include trees with a "skirt" and stunted trees. Trees with skirts are usually small, with most of their branches on the lower part of the trunk, below the level of the snow cover in winter, meaning that they do not grow straight upward but spread horizontally, reaching a limited height; they occur above the timberline. Here, the negative influence of the low temperatures is increased by the strong winds and the thinness of the snow layer, and any shoots above the snow are battered by the wind or frozen solid. This forces the tree to adopt a low, flat, and compact shape.

[Photo: Konrad Wothe / Oxford Scientific Films]

157 The taiga looks most cheerful when summer arrives. The snow melts, the trees are bright green, and the understory is full of flowers, as shown in this photo of a spruce (Picea) forest in Denali (Alaska, USA), part of the second largest forest mass on the planet, after the Eurasian taiga. These forests typically consist of the black spruce (Picea mariana) and the white spruce (P. glauca), which occurs throughout the area in mixtures with some deciduous species, such as aspens (Populus) and the paper birch (Betula papyrifera). The tamarack (Larix laricina) grows on the wettest soils, and the preferred habitat of the balsam fir (Abies balsamea) is on the banks of lakes and rivers. Several species of pine (Pinus) also occur in the region. As in the taiga, there is little understory vegetation and the soils are often waterlogged, especially in the northern zones, where there are often bogs and peat bogs.

[David A. Ponton / Planet Earth Pictures]

158 The taiga biome only occurs in the Northern Hemi-sphere. It forms a continuous strip south of the tundra biome. Its climate is highly seasonal, with long, very cold winters and short, hot summers. These temperature and rainfall diagrams from four sites in the taiga show that precipitation (which falls as snow in winter) is low and mainly falls in the warm season. Average annual precipitation is normally 16-24 in (400-600 mm), though in northern zones it may be less than 6 in (150 mm)--for example in Verkhoyansk, at 67[degrees] N. The only other biomes receiving less precipitation are the tundra and the deserts. As the temperatures are so low, evaporation is also very low, and this makes plant growth possible. There are frosts during most of the year (in blue and vertically shaded in the diagrams), but temperatures may be high in the short summer, and so the temperature range over the course of the year is very high. In Verkhoyansk, the annual temperature range is 113[degrees]F (63[degrees]C), but in the larch (Larix) forests of eastern Siberia it may be more than 180[degrees]F (100[degrees]C).

[Drawing: IDEM, based on several sources]

159 Podzol formation is linked to the accumulation of lignin (the cellulose-binding part of wood). Lignin decomposes slowly in cold climates to produce humic complexes, which attack the minerals and cause their migration through chelation (when a metal ion forms a chelate compound). Three different horizons can be distinguished in podzol soils, as shown by this profile of a micropodzol near St. Petersburg (Russia). The surface horizon consists of a blackish layer of humus that is usually fibrous in structure. Below this there is a whitish A horizon, never more than 4 in (10 cm) thick, that looks like it contains ash (pod in Russian means "under," and zola means "ashes"). The clays and other silicates in the A horizon are attacked by the fulvic acids, and the breakdown products form complexes and accumulate in the lower B horizon (see fig. 161), which is dark or sometimes reddish (see fig. 162).

[Photo: O. Spaargaren / ISRIC, Wageningen]

160 Bogs and flooded soils are typical of the taiga landscape, often covering large areas--in Canada alone, they cover more than 247 million acres (100 million ha) and dominate northwest Quebec and the north of Alberta, Manitoba, and Ontario. Their existence is because the climate is wet and evaporation is low, and also because, in geological terms, these boreal forests appeared relatively recently (after the last glaciation). Most of these flooded and marshy areas are basic bogs or acidic peat bogs, like this one in Luce County, Michigan (USA), in the middle of a forest of black spruce (Picea mariana) and tamarack larch (Larix lariciana). The peat bogs form as a result of excess precipitation, high humidity, dystrophic conditions, poor drainage, and slow decomposition, factors that speed up the accumulation of sphagnum (Sphagnum) bogs and help turn them into raised bogs (see figs. 196 and 197).

[Photo: Rod Planck / NHPA]

161 Podzols, like this podzol from Canada, form in cold wet environments with an acidifying vegetation (usually conifers) and very permeable parent materials that are resistant to breakdown and rich in silicates, features that are found in the entire taiga biome. They are the result of podzolization, a process in which iron and aluminum, together with organic matter, are washed from the A horizon (see fig. 159) and deposited in the B horizon. The long-lived conifer needles decompose slowly, and, together with the poor understory and low temperatures (which slow down microbial activity), mean that podzols have much less humus than the soils of the prairies and temperate forests and are not suitable for agriculture.

[Photo: D. Creutzberg / ISRIC, Wageningen]

162 Humoferric gleyic podzols are among the most common soil types in the taiga. They only form in very wet sites and are characterized by the reddish color of the underlying B horizon, as can be seen in this humoferric podzol near St. Petersburg (Russia). This reddish color is due to the accumulation of iron sesquioxides. This is only one of the types of podzol that occur in boreal forests, where differences in weather, in the composition of the parental material, and many other factors lead to edaphic (soil composition) differences.

[Photo: O. Spaargaren / ISRIC, Wageningen]

163 Gleysols usually show greenish or grayish mottling, are poorly developed, and are predominantly hydromorphic in the top 20 in (50 cm), as shown in this photo of a soil in the Canadian taiga. They form in conditions of waterlogging, and are dominated by the reduction of ferric iron oxides to ferrous iron oxides, because micro-organisms use up most the oxygen. Ferrous compounds are soluble, and they are easily washed from the horizon, which is bleached. If anaerobic and aerobic conditions alternate, at some times of year there may be enough oxygen for the iron to precipitate into ferric compounds. Sometimes gleysols form on moraine deposits, and they often support a plant cover of members of the heath family (Ericaceae), sphagnum (Sphag-num), and other wetland plants.

[Photo: D. Creutzberg / ISRIC, Wageningen]

164 The conifer forests of the Eurasian taiga run in a belt below the Arctic Circle, between roughly 50[degrees] N and 70[degrees] N. They form a long narrow strip from the Scandinavian Peninsula to Siberia. They are south of the tundra biome and north of the steppes of central Asia and the deciduous forests of Europe and western Asia. Though the forest mass is continuous, it is not homogeneous, because in this large area (more than 1.7 billion acres [700 million ha]) there are major climatic differences, from north to south and to an even greater extent from east to west. For example, a distinction is usually made between the northern taiga (which is in contact with the tundra), the central taiga, and the southern taiga (which borders the steppes and deciduous forests). Large geomorphological units also determine the presence of different types of taiga. The Ural Mountains, for example, clearly separate the European taiga from the Asian taiga.

[Drawing: IDEM, based on several sources]

165 The most northerly forests in the world are the larch (Larix) forests in central Siberia near the Taz River, shown in this photo covered with snow. They are almost pure forests of Dahuri-an larch (Larix gmelinii), a species that is very resistant to ice, can grow on very shallow soils (39 in [1 m] or even just 20 in [50 cm]), and is well adapted to the zone's extremely continental climate, with relatively hot summers, very cold winters, and low precipitation. It is accompanied by the less abundant Siberian spruce (Picea obovata), which is considered a preglacial relict species. The understory is very poorly developed and is dominated by low shrubs, several species of herbs and moss, and abundant lichens (see vol. 9, fig. 15).

[Photo: Andrey Zvoznikov / The Hutchison Library]

166 The autumn coloration of the taiga in the Numanodaira region of Hokkaido (Japan). This area of conifer forest, and those on the other northern islands of Japan, are among the most southerly areas of taiga in the Far East. Conifer forests occur from almost sea level to 8,202 ft (2,500 m) in the highest mountains on Honshu, where they are dominated by the firs Abies mariesii and A. veitchii. Abies mariesii grows on east-facing slopes, which are colder, while A. veitchii grows on the western slopes, where the high temperatures prevent thick layers of snow from accumulating. Other common species in the conifer forests of Japan are the yeddo spruce (Picea jezoensis), the Sakhalin spruce (P. glehnii), the Japanese larch (Larix kaempferi), and the hinoki cypress (Chaemaecyparis obtusa), which is less abundant.

[Photo: Orion / Bruce Cole-man Collection]

167 The boreal conifer forests in North America form an arc, about 620 mi (1,000 km) wide at its widest, that runs along the north of the continent from Alaska through Canada to the Atlantic coastline of Labrador. The taiga lies between the tundra to the north and prairies (in the west and center) and deciduous forests (in the east) to the south. The area of transition with the prairies is also covered by forests, but dominated by quaking aspen (Populus tremuloides) rather than conifers. The transition with the deciduous forests consists of mixed forests of conifers and deciduous species. This huge area of continuous forest is very uniform in its appearance, but it can be divided into two broad types. The boreal forest is closed and dense, while the open boreal forest consists of smaller, scattered trees that are in turn replaced by tree tundra. [Drawing: IDEM from several sources]

168 Some of the altitudinal zones on mountains support conifer forests similar to the lowland taiga forest, like the forest shown in the photo, which is in the Teberda Reserve near Dom-bay in the Autonomous Province of Karachayevo-Cherkesskaya (Russian Federation). These coniferous forests in the mountains are different from the typical northern lowland taiga. For example, in summer temperatures decrease with increasing altitude (1[degrees]F [0.5[degrees]C] per 328 ft [100 m]), while they increase in winter (up to 2[degrees]F [1.6[degrees]C] per 328 ft [100 m]), due to temperature inversion. On the other hand, the mountains in the taiga show an unusual type of altitudinal zonation. Though it may vary considerably depending on the conditions of each site, normally the lowest zone is a continuation of the lowland taiga, and in the middle mountain zones it is displaced by subalpine scrub, stunted forests, and cloud forest. The high mountain supports a montane tundra. In the mountain taiga, orographic factors (altitude above sea-level, orientation, and slope) are particularly important, as they directly influence the microclimate, the water regime, and thus the nature of the plant cover. For example, in mountain ranges that run north-south, as one moves further south the conifer forests grow at increasingly high altitudes.

[Photo: Vadim Gippenreiter]
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Publication:Encyclopedia of the Biosphere
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Date:Aug 1, 2000
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