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The subtropical cold deserts and subdeserts.

The sky suddenly turned black, and a moment later the storm hit the caravan with tremendous violence. Huge quantities of sand, mixed with small stones, were torn up in a flash, sucked into the air and thrown against the men and the animals. It got even darker and the claps of thunder were lost in the roar and howl of the storm. It seemed that all hell had broken loose.

Albert von Le Coq

Auf Hellas Spuren in Ostturken (1926)

1 Frozen drought

1. Dryness and extreme cold

1.1 The cold dry deserts

The cold continental deserts are found at the subtropical latitudes within temperate areas. They are always inland and, like the hot tropical deserts, are very arid, with a high rate of evaporation and a large difference between the temperature regime and the precipitation.

The unusual conditions of the cold deserts

Despite the similarities between the hot tropical deserts and the cold subtropical deserts (low rainfall, high temperatures, high moisture deficit, etc.), the main difference between them is the range of the seasonal temperature variations. In hot deserts, the air temperature remains almost constant all year-round, but in the cold deserts there is a large difference between the summer and winter temperatures, due mainly to the seasonal changes in the sun's elevation. On the other hand, in the tropical deserts, the rainfall deficit is linked to the special characteristics of the Intertropical Convergence Zone (ITCZ), while in the cold continental deserts, the scarcity of precipitations (rain and snow) is basically determined by the fact that the moisture-bearing oceanic atmospheric fronts are prevented from reaching them by the surrounding mountains.

Little rain and almost no snow

The moisture and the amount of precipitation in the areas occupied by the inland cold deserts depend mainly on the character of the moist oceanic air masses that may potentially reach them. The great diversity of natural factors preventing them from reaching these deserts (their distance from oceans, the existence of mountain barriers, atmospheric circulation patterns) determine the great diversity of rainfall conditions.

In Asia, the different cold deserts receive 2-12 in (40-300 mm) annual rainfall. Rainfall is lowest in the deserts of central Asia (Takla Makan, Bei Shan, and others), located in large depressions surrounded by high mountain chains that isolate them from the eastern monsoons. A larger volume of precipitations (up to 10-12 in [250-300 mm] per year) characterizes the northern parts of the deserts of middle Asia* and Kazakhstan, where moist Atlantic cyclones may reach the immense plains despite their distance from the ocean.

Most of the deserts in central Asia and North and South America receive 4-8 in (100-200 mm) of precipitation per year. However, the potential evaporation in these deserts is exceptionally high--98-120 in (2,500-3,000 mm) per year--almost 25 times greater than the moisture input from precipitation. This creates a severe water deficit in these deserts. The special nature of the inland cold deserts is shown in the seasonal distribution of their scarce rainfall. Thus, in the central Asian deserts, the rain falls mainly in the summer, due to the arrival of the monsoon rains. The southern zones of the central Asian deserts (Karakum, Kyzyl Kum) receive most of their rainfall during the winter and early spring, because at this time subtropical Mediterranean winds are dominant. But the more northerly deserts of Kazakhstan depend on cyclones from the western Atlantic and are characterized by a relatively regular distribution of precipitation over the course of the year. In the deserts of the Great Basin in North America, the seasonal distribution of precipitation is also evenly distributed.

The seasonality of precipitation is of great ecological significance, as it determines the effective evaporation rate and the amount of water entering the soil. In the summer, the very dry conditions and high potential evaporation mean that the rainfall is not at all effective and is almost totally evaporated. Even heavy rainfall only wets the topmost layers of the dry soil, and the moisture evaporates rapidly. In the deserts of middle Asia (Kyzyl Kum, Karakum), with annual average precipitation of 3-6 in (70-150 mm), the moisture from the scarce summer rains does not reach the soil surface because the air is so dry; this means that atmospheric moistening during the summer period is almost nonexistent. Clouds often gather, there is thunder and lightning, but no rain falls. This is called dry rain. In the more northerly deserts and subdeserts, where there is no difference in rainfall between the seasons, the summer rains are more abundant and more widespread, but the moisture only wets the bone dry topsoil and is rapidly lost by evaporation; therefore, it represents little or no input of water to the soil.

The situation is totally different in the winter. In this period, the temperatures are low, and even the slightest rainfall provides moisture that is not lost by evaporation. The rain penetrates the soil and may even form large reserves. Thus, for example, in the Karakum, with an average temperature of 14[degrees]F (-10[degrees]C) in January, the relative humidity of the air reaches 70-80%--almost four times higher than the average for July in the same area. Observations taken in these areas over 50-60 years show that the maximum soil wetting (to a depth of 20-26 ft [6-8 m]) occurred in 14% of cases; wetting to a depth of 10-16 ft (3-5 m) occurred in 30% of cases; wetting from 3-5 ft (1-1.5 m) occurred in 16%; and wetting less than 2 ft (less than 0.5 m) occurred in the other 40%. After maximum soil wetting, the water lost over the year by evaporation was only 34% of the entire evapotranspiration. In the northern Ciscaspian Desert, the winter precipitations total about 6 in (160 mm). When the snow melts, the water penetrates deep into the soil, entering the water table and causing it to rise at the same time as protecting the water from evaporation. In this case, the precipitation in winter and spring represents the entire annual input of water to the soil. Thus, even scarce winter precipitations are more useful than abundant summer rains.

In many deserts, where in winter the temperatures are below the freezing point, precipitation falls in the form of snow. In the southern region, where the climate is milder, precipitation is mainly as sleet mixed with rain; there is never a permanent snow cover, and all the moisture is rapidly absorbed by the soil. In the harsher, more northerly deserts, the precipitation falls mainly in the form of snow, and the snow cover is roughly 4 in (10 cm) thick. The snow blows around in the wind, accumulating in depressions, ravines, and in the areas of tall vegetation. Plains often lack vegetation, and the snow falling there is totally or partly blown away by the wind, so they do not benefit from even the small amount of winter precipitation normal for the area. After the snow melts, the moisture from the winter precipitations accumulates very unevenly; much of it drains to low-lying areas and spots where the vegetation is higher, while little accumulates on the flat plains, where the plant cover is scarce or nonexistent. The fact that the soil moisture reserves depend on the distribution of the snow cover is useful in agriculture. In order to increase soil freshwater reserves, trees and shrubs are planted to trap the snow, and snow is heaped into piles in the fields.

The spring snow melt is so fast that the moisture is not absorbed by the soil, especially where the soil is not very permeable. The excess water is lost as surface runoff, which distributes the water locally or to other areas. Even the low snowfall typical of deserts may accumulate in considerable quantities in depressions. This is especially relevant in takyrs, flat depressions in the desert surface. A takyr in an area with average annual precipitation of about 4 in (100 mm) may thus receive 5,000-31,000 m3 of water per [km.sup.2] per year. At the same time, much of this water is lost by runoff in temporary watercourses and is lost to the region of origin, thus increasing the already large water deficit.

1.2 Cold winters and cold summer nights

The cold continental deserts have a very dry summer, high potential evaporation, and very high air and soil temperatures. These deserts cool down at night, when air and soil temperatures fall sharply and the relative humidity of the air increases. The existence of this cold winter period in the continental cold deserts is a radical transformation of the usual properties of the deserts, something that is shown mainly in the considerable drop in the water deficit. This, in turn, has a positive influence on the structure of the plant cover, on the animal life, and on the productivity of the ecosystems.

Extreme temperature changes

The cold continental deserts are characterized by extreme temperature changes linked to the large seasonal changes in the intensity of solar radiation. In the deserts of middle Asia, the direct sunshine in December is only about 5,000 kcal/[cm.sup.2], while in July it is 24,500 kcal/[cm.sup.2], almost five times greater. The radiation balance--the amount of incoming solar heat minus what is lost by reflection and radiation from Earth--is 7-10 kcal/[cm.sup.2] per month in July, and around zero in January; the annual value of the radiation balance is 30-45 kcal/[cm.sup.2]. In the summer period, the days are much longer than in winter, so the amount of heat received in the cold deserts may even be greater than at the equatorial latitudes, where the monthly values of the radiation balance do not exceed 8 kcal/[cm.sup.2] in winter, and the short day and the high albedo value (that determines the reflectivity of the surface) mean that the real quantity of heat received is very small in winter. All this means that in the summer, air temperatures are as high or higher than around the equator, and extremely low--below zero--in winter. In the deserts of central Asia, the absolute maximum temperature during the summer is about 113[degrees]F (45[degrees]C), and may reach 122[degrees]F (50[degrees]C), while the absolute minimum, in the month of January, is between -40[degrees]F (-40[degrees]C) and -49[degrees]F (-45[degrees]C), meaning that the annual temperature range may be as great as 162[degrees]F (90[degrees]C). In the Great Basin deserts of North America, the temperature extremes are, respectively, 104[degrees]F (40[degrees]C) and -6[degrees]F (-21[degrees]C). Thus, in the summer, the climate of the cold continental deserts is as arid (high temperatures and very dry conditions) as that of the hot deserts.

In addition to these seasonal contrasts, the cold continental deserts are characterized by a huge diurnal temperature range, which may be as much as 86[degrees]F (30[degrees]C) in the summer. This is due to the intense radiation of heat during the night. The high air temperature during the day means evaporation is intense (98-138 in [2,500-3,500 mm] per year) and the relative humidity of the air is low, an average of 20-30% in the summer. The result is that during the daytime, the sunshine on the ground surface reaches the maximum values possible at these latitudes; at night, this heat is lost rapidly, as the transparent dry air retains little of the infrared radiation emitted by the desert surface. Daytime heating and nighttime cooling depend on the surface features of the desert relief; bare sand warms up faster in the daytime and cools down faster at night than sand with plant cover. In the Karakum Desert in the Repetek Nature Reserve, by 2:00 P.M., even sand with herbaceous plant cover reaches 165[degrees]F (74[degrees]C), while between 4:00 and 6:00 in the morning, it cools down to 70[degrees]F (21[degrees]C). The air temperature between 2:00 and 4:00 in the afternoon at a height of 3 ft (1 m) above the ground reaches 115[degrees]F (46[degrees]C), and around 2:00 A.M. it falls to 63[degrees]F (17[degrees]C)--a drop of 52[degrees]F (29[degrees]C). Likewise, the relative humidity of the air falls to 7-9% between 2:00 and 4:00 in the afternoon and rises to about 52% at 2:00 A.M.

In winter, inland deserts get very cold, mainly because of the seasonal decrease in solar radiation. At the latitude of Tashkent (41[degrees]N, about the same as Naples, Barcelona, or Oporto), for example, the sunshine in July is almost five times stronger than in December. The lack of mountain barriers to the north allows masses of cold air from the Arctic to reach the deserts of middle Asia, and even in the southern parts of these deserts (for example the region of the city of Ashgabat, at a latitude of 37[degrees]N, roughly the same as the Catania region of Italy or the city of Seville), temperatures sometimes fall to -22[degrees]F (30[degrees]C), with heavy snowfalls and storms. Despite this, the winters are usually relatively mild, with an average monthly temperature in January of between 35[degrees] and 7[degrees]F (1.4[degrees] and 14.1[degrees]C). The most northerly regions of these deserts get much colder in winter; the average monthly temperature in January is between 18[degrees] and 5[degrees]F (-8[degrees] and -15[degrees]C), with absolute minimum temperatures between -40[degrees] and -54[degrees]F (-40[degrees] and -48[degrees]C). The soil freezes to a depth of 3 ft (1 m). These regions are characterized by a 4-8 in (10-20 cm) deep layer of snow that lasts between two and a half and three months.

The deserts of central Asia are distinguished by their exceptionally harsh climate, especially the Mongolian part of the Gobi Desert, which is exposed to cold anticyclones from Siberia. With an almost total lack of precipitation in winter, the anticyclones bring cold air at temperatures that may be as low as -40[degrees]F (-40[degrees]C). In the deserts of North America, the winters are not so harsh; precipitations are often in the form of snow, but temperatures do not usually fall below 7[degrees]F (-14[degrees]C). The winters are even milder in Patagonia because it has a more maritime location. The average minimum temperature in July is between 30[degrees] and 9[degrees]F (-1[degrees] and -13[degrees]C) and precipitation in the form of snow is uncommon.

The action of wind and fog

One of the special features of the cold deserts is the presence of constant winds. Gentle winds, with speeds of 2-3 m/s, blow regularly every day. Strong winds of 15 m/s raise dust storms, a common and extremely unpleasant phenomenon in the deserts. The dust in the air is strongly heated by the sun, so it increases the high air temperature and makes conditions even drier. In the deserts of middle Asia, there may be 30-50 days a year with sandstorms; in some places--the coastline of the Caspian Sea and the Aral Sea, for instance--these winds are constant.

The harshness of the frosts is made worse by the strong winds. These often raise snow storms, blowing the snow around above the ground. The visibility declines to almost zero, the snow forms drifts, and roads and buildings are buried. The desert landscapes, in this period, are surprisingly similar to the snow-covered tundra, and this similarity is emphasized by the presence of overwintering birds typical of the tundra.

Cooling during the night and the higher relative humidity causes the formation of fog, dew, and, especially during the winter, hoarfrost. This represents additional moisture that can filter into the soil. In the Repetek Nature Reserve in the Karakum, when hoarfrost and dew are deposited, the sandy soil is wetted to a depth of less than an inch (1-2 cm); in the case of prolonged fogs, the moisture that runs down the trunks of the shrubs may wet the soil to a depth of 4-6 in (10-15 cm). In total, the moisture input derived from dew, hoarfrost, and fog into the soil is noticeable and is equivalent to 4-6 mm of rainfall per year.

The persistent winds in the Chinese deserts and subdeserts that blow northwest-southeast at an average speed of 4-8 m/s for the nine or more rainless months of the year are responsible for the formation of the gobis or shamos, the coarse sand deserts of Xinjiang and Mongolia, products of the very active desertification that is occurring there. They are also responsible for the accumulation of a thickness of 10-984 ft (3-300 m) of loess over an area of 50,000 [km.sup.2] during the Pleistocene and Holocene. In addition, the winds sculpt the yardangs (streamlined wind-rocks) of Lob Nor and the Qinghai Nanshan. In the same way, the violent and continuous winds (average annual speed 6-10 m/s) that blow westeast over Patagonia throughout the year cause the formation of fronts and tongues of sand that may advance hundreds of meters a year, blanketing everything in their path.

2. Gypsum, salt, and fine dusts

2.1 Little-developed soils

The soil formation processes occurring in the cold deserts are not fundamentally different from those in the hot deserts, although the special climatic features of the cold deserts (high soil temperatures in the summer, extreme aridity and high evaporation in the summer, insufficient soil wetting) lead to even heavier physical weathering and further limit the biological processes in the soil. The most abundant soils show 1) little development and a weak structure, 2) an insignificant humus content, and 3) a hard, porous surface crust. Almost all the cold desert soils show accumulations of salt, calcium carbonate, and chalk. The limited soil formation processes is reflected by these soils' shortened profile and by genetic horizons that are little differentiated due their low degree of wetting and the shallow penetration of the water.

Humus-poor soils

One of the most characteristic features of desert soils is their low humus content, usually only 0.5-1% or less. This low accumulation of humus is due to the very nature of the desert, which mineralizes the organic matter rather than turning it into humus. This predominance of mineralization over humus formation is linked to the composition of the microbial population typical of desert soils.

Despite the extremely unfavorable environmental conditions, the desert soils are rich in many groups of microorganisms. Thus, nitrogen-fixing bacteria and nitrifying bacteria are very common in these soils. Groups like Nitrosomonas, Nitrosococcus, and Nitrobacter are more resistant to high temperatures and dry soil conditions than higher plants. Desert soils are especially rich in actinomycetes, which can also tolerate high temperatures very well and are better adapted than other microorganisms to desert conditions. At the same time, desert soils are very poor in soil fungi and invertebrates that turn organic matter into humus. There are grounds to believe that the abundance and activity of microorganisms like actinomycetes and nitrifying bacteria, together with the low number of fungi and invertebrates, account for the rapid mineralization of plant remains and the slowness of humus formation.

Soil salts and gypsum

The most specific feature of soil formation in deserts is the accumulation of readily-soluble salts and the formation of highly saline soils called solonchaks. The main reason for salt accumulation is the environment's high potential evaporation, which causes the saline soil solution to rise to the surface by capillary action, favoring its evaporation and leading to the accumulation of salts in the surface horizons. These salts are mainly chlorides, sulfates, sodium carbonate, and, to a lesser extent, magnesium carbonate. They have their origin in the erosion of salt-containing marine sedimentary rocks and rock salt deposits, and when they are eroded, they dissolve and end up in the water table. This type of soil salinization and solonchak formation is especially characteristic of plains of marine origin, as exemplified by the Ciscaspian Depression, which, until relatively recently, was covered by marine waters.

The saline soil profile of solonchaks is determined by great accumulation of sodium chloride and sodium sulfate in the topmost soil horizon, but considerably less in the deeper horizons near the water table. Depending on the soil class, surface horizons only suffer salinization if the maximum depth of the water table is from 5-11 ft (1.5-3.5 m). If the water table is deeper than this, the process of salinization is slower or nonexistent.

Thus, a high soil salt content combined with a deep water table shows that in the past the water table was closer to the surface. Salt accumulation in the soil may reach very high values and depends on the rate of evaporation of the soil moisture. For example, in the solonchaks of the deserts of central and middle Asia, the total quantity of highly soluble salts (such as sodium chloride, sodium nitrate, magnesium chloride, magnesium sulfate, and sodium sulfate) may account for 15-25% of the weight of the soil; in subdeserts, it is 5-8%.

One very characteristic feature of the soils of the hot and cold deserts is the accumulation of gypsum (CaSO42H2O, hydrated calcium sulfate). In many sites, an extremely gypsum-rich horizon may even form. Known as a hypergypsic horizon, it forms as a compact subsurface horizon (a crust) about 5 ft (1.5 m) thick. This type of gypsum crust is found in Kazakhstan (Ustyurt Plateau) and middle Asia (the southern Kyzyl Kum). In most desert soils, gypsum accumulations take the form of individual crystals (desert roses), concretions, veins (vermiform gypsum), or nodules, all of which are relatively recent. When the hypergypsic horizon is white, very porous, snow-like to the touch, and mixed with fragments of rocks and pebbles, it is not a recent product and is associated with marine limestone sediments.

Takyrs

Takyrs or nurs are one of the strangest phenomena of the cold deserts. Water plays a major role in takyr formation, just as it does in solonchak formation. The takyrs, the cold desert's equivalent of sebkha in hot deserts, are hydromorphic soils with a very compact surface crust that is almost impermeable to water and devoid of vegetation. They form on flat undrained plains and on flat depressions with clay or loam-clay soils. Because of their endorheic drainage and clayey texture, the water derived from precipitation remains there and causes temporary waterlogging.

Takyrs generally occur in former alluvial plains or piedmont depressions at the base of a mountain, and they often occupy large areas (several square kilometers). When they are waterlogged, vast areas are covered with a thin sheet of water (16-20 in [40-50 cm] deep) with an absolutely flat bottom. The volume of water accumulated is sometimes very large, reaching 2,500 m3/[km.sup.2]. The salinity of the water in the takyrs is only 1-2 g/l of salts, mostly chlorides and sulfates. When they are full of water, large numbers of cyanobacteria and other microscopic algae grow there.

After the water has evaporated, the topmost soil horizons dry out rapidly to form a very hard crust separated by fissures into regular polygonal plates. Takyr soils consist of two horizons under a dense large-pored surface crust that may be 1 in (3 cm) thick. The first horizon is loose with a layered pattern to a depth of 2 in (7 cm), and the second lower horizon between 2 and 6 in (7 and 15 cm), a granular porous horizon with accumulations of salt. The special physical features of takyrs are determined by the existence of swelling clays that expand when they are wetted and make the soil impermeable. The autumn, winter, and spring rains only wet the takyr soil to a depth of 8-12 in (20-30 cm), causing the surface to become waterlogged. How long the water remains varies; it may only be a single day or a few hours, but the water is always temporary, and its duration depends on the class of the takyr soil and on the gradient of its surface (completely flat or with a slight slope).

Loess

Loess is a wind-deposited unconsolidated sediment that does not show stratification and consists almost entirely of silt-sized quartz particles. Hardly any particles exceed 0.25 mm in diameter (the size of sand) and nearly the entire soil consists of fine dust with a particle size of 0.05-0.001 mm. Loess formation is linked to the erosion of rocks to very fine particles, the long-distance transport of the particles by the wind, and the deposition of the particles in layers. Loess sediments are typical of the piedmont plains of central Asia.

The cold desert soils that form on loess are very special, with a microaggregate structure that makes them highly permeable with high capillarity. These soils absorb water very well but lose it just as quickly.

2.2 The different soil types

The soils of the cold deserts are characterized by their high diversity, largely due to the fact they are derived from a wide range of parent materials. In the desert landscapes, there are four types of parental materials. The first group is the sandy rocks derived from lake, marine, or river sediments; these substrates are the most widespread (representing more than 40% of the area of the deserts of middle Asia and Kazakhstan).

The second group is comprised of outcrops of sandstone, limestone, and argillite (lutite). The third group consists of clay loess materials on lacustrine and alluvial-delta plains. The final group is the clay or clay-loam materials of plains of marine origin, alluvial plains, or on undrained solonchak depressions.

The special nature of the desert soils, shown mainly in their relatively low fertility, their poorly developed structure, and many other factors adverse to life, make the already extremely harsh desert environment even worse. The makeup of the soil does, however, allow some uncommon plant cover to grow--plant cover that simply could not exist without the biological components of the desert soil.

Sandy desert soils

Sandy desert soils are poorly developed, with few clear signs of soil formation processes; they are found everywhere on sandy massifs, especially in the sand fixed by vegetation. In the areas with a poor plant cover, the soils are less developed and the profile is scarcely differentiated into distinct horizons. Typically, they all have a low humus content (around 0.5%), low salinity, and a poor structure. These soils consist largely (5060%) of particles of fine sand; there are few clay or silt particles and the amount of silt in the top layer is normally 4-6%.

There is also a wide variety of sandy desert solonchak soils. These differ from the other desert sandy soils in that they have a saline surface crust, the salts of which are derived from the leaf litter of halophytic Haloxylon spp. (Chenopodiaceae), characteristic shrubs of the cold Asiatic deserts. The high concentration of sodium salts in their leaves and stems causes surface salinization, and the crust that forms prevents other plants from germinating. Herbaceous plants are absent from these soils, but lichens and bryophytes are abundant.

As they seem to show few of the features of soil formation, should these sandy desert substrates really be called soils? Several processes take place that indicate the substrate is undergoing soil formation: 1) slow but continuous mineral uptake and photosynthesis by plants, 2) the biochemical breakdown and mineralization of organic matter by soil animals and microorganisms, 3) the vertical migration of solutes due to the action of the vegetation, and the redistribution of the solutes in the vertical profile of the sand, and 4) the presence of signs of the soil's fertility. It should be borne in mind that sandy desert soils have a richer plant cover than clay ones because the vegetation of sandy areas makes more effective use of the water from atmospheric precipitations; furthermore, the sand is rich in nutrients because of the more intensive mineralization of organic matter by the microorganisms. The involvement of the biological component in the functioning of the sand-desert ecosystems overlying a sandy substrate shows quite clearly that soil formation processes have taken place.

Clay soils

Soils that form over clay materials containing some gravel are also very characteristic of deserts. The soil is 2-7 ft (0.5-2.0 m) thick; under it, there are compact sedimentary rocks (limestones, sandstones, calcilutites or fine-grained limestone) and, in areas of piedmont, fine-grained rock materials. This soil group embraces a wide group of soils that vary in the extent of soil formation from the original materials.

The most developed soils form on plains on loose loam-clay or loam-sand sedimentary rocks. The characteristic feature of the profile of these soils is the presence of a largegrained crust 0.8-2 in (2-5 cm) thick, a loose stratified horizon 1.2-2.4 in (3-6 cm) thick, a horizon with a compact porous granular structure 6-8 in (15-20 cm) thick, and, finally, a loose horizon with accumulations of salts and gypsum. Below this is solid horizon containing up to 50% gypsum. There is a large quantity of gravel, which increases with depth. The carbonates, which are of biogenic origin and derived from the leaf litter, are found mainly in the upper parts of the profile. These soils, like most desert soils, are shallow (up to 16 in [40 cm] thick). The upper horizon is not saline, or only slightly so; it is clay with little humus (0.1-0.8%). The salinity appears at a depth of 12-16 in (30-40 cm) and may reach 1-2%.

Takyr soils and solonchaks

Takyr soils are a special type of desert soils; they form on stratified sediments, mainly of clay or loam. The composition of this type of sediment ranges from 50-80% clay and 2030% silt. Almost all takyr soils are solonchaks with moderate to high salinity. The amount of highly soluble salts is roughly 0.5-3.0%. The humus content is about 0.5%.

Solonchaks, as a soil type characteristic of desert soils, form in depressions in the parent material and in depressions between sand deposits. The size of these depressions varies, but they may often cover several square kilometers. In the wet period, the surface of the solonchaks is covered by water; in summer, some dry out, and a salt layer forms on top of the soil. (The salt content may be as high as 15-20%.) Solonchaks show different morphologies, depending on the composition of the salts and the depth of the water table.

When the surface layer of soil dries out in solonchaks that contain sodium sulfate in the soil solution, mirabilite (Na2SO4x10H2O) crystallizes out of solution, causing the soil matrix to swell and the surface layer of the solonchak to become dry and fluffy. When walking on these surfaces, the feet sink into the surface layer as if it were eiderdown. The colorless salt crystals make the surface layer of the solonchak look white. Solonchaks with a shallow water table (less than 3-7 ft [1-2 m]) are almost permanently waterlogged and are therefore highly saline.

Solonchaks are the least favorable soils for plant growth, as the soluble salts present are toxic to plants. Even if they are not toxic, the high quantity of salts means that the osmotic pressure of the soil solution is extremely high and the plants cannot absorb the water, even when it is present in abundance.

Nonsaline gray soils

The most characteristic soils of the cold deserts include the ones called sierozems (gray soils) by Russian soil scientists (and equivalent to the Greyzems in the FAO terminology). They normally form on the piedmont plains where the water table is deep. Sierozems have a well-developed fine structure, are highly permeable, and are well washed by atmospheric precipitations so they are not saline.

This type of soil is found throughout the piedmont zones of middle Asia and in regions where there are loess accumulations on the edges of the cold deserts, where precipitations are greater. This greater depth of soil wetting means plants can grow deeper roots, thus enriching the soil with humus at depth. The abundant heat and light and the high fertility of loess soils favor agriculture; if the presence of a river makes irrigation possible, good harvests of cereals, cotton, vegetables, and fruit can be obtained.

The properties of these soils very greatly depending on the distribution of the atmospheric humidity, which in turn depends on the absolute elevation of the plains where they occur. The humus content may vary from 0.8-1.7% on the low piedmont plains and 1.5-2.5% on the high piedmont and low mountains; the total depth of the soil is, respectively, 12-24 in (30-60 cm) and 39-51 in (100-130 cm). The soluble salt content, even on the lowest plains, does not exceed 0.25%. All sierozems are characterized by deep wetting in the spring and severe drought in the summer. The depth of annual wetting is 3-5 ft (1-1.5 m). During the dry period, from May to October, soils covered in vegetation lose all the moisture they had received in the wet period. This type of soil is not affected by the presence of the water table.

3. The world's cold deserts and subdeserts

3.1 The cold deserts and subdeserts of central Asia

The temperate latitudes of central Asia are one of the largest areas of desert on the planet: they include almost all of central Asia, middle Asia, and Kazakhstan, between 36[degrees]N and 48[degrees]N, covering an area of about 2,800,000 [km.sup.2] and accounting for more than half the territory. They include very different kinds of landscape but are dominated by sandy deserts of varying dimensions with irregular distribution.

The deserts of central Asia

Central Asia is of one of the oldest regions of Earth's crust, with a long and complex geological history that is reflected in its structure. There are large mountain ranges, plateaus formed by Cretaceous sediments, and recent alluvial plains, profoundly transformed by the action of the wind. The special features of the relief--large and deep depressions (Tarim, Tsaidam, the southern Helan Shan Plateau) surrounded by the high mountain massifs of the Tian Shan, Kunlun, Altun Shan and Nan--create an extremely dry climate, as these mountain ranges isolate the land from the moist air masses coming from the Pacific and Atlantic oceans.

Central Asia is a set of endorheic basins. Only at the eastern tip does the Yellow River (Huang He), which flows into the China Sea, drain a small part of the area. The other rivers (the Tarim, Charchan Dar'ya, and others) flow into lakes or end in dry beds. Most of the area has absolutely no surface runoff. Only the piedmont plains at the bases of mountains are really affected by the rivers; when the rivers flood, the depressions between the dunes fill temporarily with water and the reserves in the water table are replenished. These reserves are very important because the large plains of central Asia receive almost all the runoff from the surrounding mountains. The water table is shallow, at a depth of 20-33 ft (6-10 m).

The deserts of central Asia are separated from each other by massifs, but, between the mountains, the desert lands occupy a large area. In western central Asia, sand deserts predominate, while the central part is dominated by stone and sand-pebble deserts (gobis). The most characteristic feature of this group of deserts is the unity of their formation. Throughout their long and varied geological history, the highlands of the deserts of Ordos, Bei Shan and Gashun, the Mongolian Gobi and other gobis, and the western Tsaidam have been caught in a cycle of abrasion of the original relief and the subsequent transport of these erosion products beyond their frontiers. They are actively eroded and denuded deserts. The sand deserts of A-la Shan, Takla Makan, Dzungaria, and the eastern Tsaidam are just the opposite: these are areas where materials from the erosion of the surrounding mountains have been accumulating since the Cretaceous.

The deserts of central Asia are located in the temperate zone, with cold winters and hot summers. The eastern part, as far as the Bei Shan Desert, receives some rain from the monsoons of the China Sea. For this reason, most of the total precipitation occurs in the summer. The western zone (Dzungaria, the Tarim Depression) is exposed to the effect of the Atlantic winds, and the seasonal distribution of precipitation is more regular.

The Takla Makan Desert

The Takla Makan Desert is one of the largest sand deserts in Asia and the entire world. It covers about 270,000 [km.sup.2] and is located in the Tarim Depression--between 37[degrees]N and 42[degrees]N, and between 76[degrees]E and 88[degrees]E--in the territory of Kashgaria, at altitudes of 2,6254,921 ft (800-1,500 m). The sand massif runs east-west in a strip 620 mi (1,000 km) long and up to 261 mi (420 km) wide. This desert was formed by alluvial deposits covered by a layer of fine sand up to 984 ft (300 m) thick. The long rows of mobile crescent-shaped dunes (barchans) are often 98-492 ft (30-150 m) high and 820-1,640 ft (250-500 m) wide, with a separation between rows of 1-2 mi (1-4 km). Some pyramidal dunes may rise 656-984 ft (200-300 m) above the relief of the barchans. The peripheral zones of the desert are dominated by sands that are almost fixed, which alternate with clay areas.

The central zone of the desert is the most inaccessible area of Asia; it is arid and uninhabited. (Takla makan means place of no return in Uygur.) In the summer, precipitations are usually only 2-3 in (50-75 mm) and in some places are less than half an inch (as low as 9 mm). The average monthly temperature in January is between 12[degrees] and 20[degrees]F (-11[degrees] and -6.8[degrees]C); in June, it is 76[degrees]-81[degrees]F (24.4[degrees]-27.3[degrees]C). The maximum temperature is 99[degrees]F (37[degrees]C) in the summer; the minimum is -17[degrees]F (-27[degrees]C) in winter. The Takla Makan is a region of endorheic drainage. The Khotan River, which originates at the confluence of a large number of tributaries that flow down from northern Tibet, divides again into different beds in the lower stretches of the river and slows down, but some years it joins the Aksu River to form the Tarim River. There are other rivers that flow down from the mountains and water the piedmont plains and even the areas of sand. The rivers of the Tarim Basin bear abundant water in the summer, during the period when the snow and glaciers in the high mountains melt. These rivers thus provide substantial soil water reserves. The abundant heat and light, the rich loess soils, and the supply of irrigation water from the mountains are all factors favoring the development of agriculture in the peripheral zones of the Takla Makan.

The extremely low total precipitation means that the Takla Makan Desert is almost totally lacking vegetation. On the rocky slopes there are small patches of shrubs such as Ephedra przewalskii (Ephedraceae), Gymnocarpos przewalskii (Caryophyllaceae), and several chenopods (mainly species of Sympegma and Anabasis) and Zygophyllum spp. (Zygophyllaceae). At the contacts between river valleys and the areas of sand is a sparse psammophilous vegetation (plants that thrive in sandy soil). On the river terraces and in the deltas, there is halophytic vegetation consisting mainly of annual chenopods (Salsola, Suaeda, Halogeton) or shrubby chenopods (Halostachys), the occasional specimen of zaisan (Haloxylon ammodendron), tamarisk, and Nitraria schoberi (Zygophyllaceae). Only on the flood plains of recent rivers and in the oases in the peripheral regions is there a rich mesophyte tree cover. The entire area of the flood plains is covered by thickets that are very similar to the tokai of the banks of the Amu Dar'ya, with poplars (Populus diversifolia, P. pruinosa) and Siberian elms (Ulmus pumila).

The A-la Shan Desert

The A-la Shan Desert occupies another of the depressions of central Asia (in north-central China). It occupies an area of roughly 170,000 [km.sup.2] at the interior of a plateau between 2,625-3,937 ft (800-1,200 m) in altitude. It is bordered to the east by the Ala Range, west of the Yellow River; to the west by the Ruo Shui River, which drains into Lake K'a-shun and separates it from the K'a-shun Desert (or K'a-shun Gobi); and to the south by the Great Wall of China, which separates it from the Kansu Corridor. The A-la Shan Desert's geographic coordinates are roughly 39[degrees]-40[degrees]N and 101[degrees]-107[degrees]E. Most of the A-la Shan Desert is occupied by an accumulation of mobile barchans. The sand is only slightly fixed, and the relief is moderate or large barchan dunes, which in some places can reach a height of 656-820 ft (200-250 m). The central and southern regions of the desert are dominated by sand-pebble plains called gobis. In some places, among the plains of sand and pebbles and the barchans, there are small rocky outcrops. The A-la Shan Desert is, in fact, a set of endorheic basins that often have abundant underground water. This water may be very near to the surface in the depressions and can form small temporary lakes, or it may be at a depth of several tens of meters in elevations of tectonic origins. There are many lakes, both freshwater and saline, and many freshwater springs in the A-la Shan Mountains. The climate is exposed to the influences of cold air masses from Siberia and warm air masses from southern China. In the cold period of the year, anticyclones from Siberia and constant winds from the northwest are dominant. In the summer, however, winds from the southeast dominate, bearing moisture from the monsoons of the China Sea. Most of the precipitation (about 90%) falls as rain during the summer. The western part of the A-la Shan Desert is drier and hotter than the eastern part, and average annual precipitation falls from 9 in (219 mm) in the east to 3 in (68 mm) in the west. The average annual temperature is 46[degrees]F (8[degrees]C), with an absolute maximum of 104[degrees]F (40[degrees]C) in summer and an absolute minimum of -8[degrees]F (-22[degrees]C) in winter.

The most widespread plant of the barchans is Nitraria schoberi (Zygophyllaceae), while Kalidium spp. (Chenopodiaceae) and boxthorn (Lycium spp., Solanaceae) are less abundant. In the wetter areas, there are abundant grasses or reeds such as Stipa [= Lasiagrostis] splendens, Phragmites australis [=P. communis], and Typha minima. On the banks, there are remnants of forests of the poplar Populus diversifolia and the Siberian elm (Ulmus pumila). On the accumulations of river alluvium, there are also thickets of Haloxylon aphyllum. Stony soils have a shrub cover of Ephedra przewalskii (Ephedraceae), Calligonum mongolicum (Polygonaceae), Convolvulus fruticosus (Convolvulaceae), Prunus [=Amygdalus] mongolica (Rosaceae), Potaninia mongolica (Rosaceae), Gymno-carpos przewalskii (Caryophyllaceae), and Ammopip-tanthus mongolicus (Leguminosae).

The Bei Shan Desert

The Bei Shan Desert covers a total area of about 150,000 [km.sup.2] of an extensive but not very high plateau consisting of Carboniferous and Cretaceous sediments. It is located at the west of the A-la Shan Desert, from roughly 40[degrees]-42[degrees]N and 91[degrees]-100[degrees]E. The relief is an unordered combination of low mountains (less than 6,562 ft [2,000 m]) and hills with numerous depressions between them that have filled up with rock debris. The stony surface, the strong sunshine, and the lack of vegetation mean that the lower layers of the air are intensely heated, compounding the processes of weathering and formation of small and large screes. The wind bears the sand particles to the neighboring Takla Makan and A-la Shan deserts, while the smallest particles (dusty silt) are blown south, where they cluster as loess accumulation. The gravel stays in the Bei Shan, leading to the formation of a stone desert (hamada). The Bei Shan is not influenced by the monsoons coming from eastern China or by air masses from the Atlantic. Average rainfall does not exceed 2-3 in (40-80 mm) a year, and most of it occurs in the summer. The absolute maximum temperature in summer is 102[degrees]F (39[degrees]C), and the absolute minimum in winter is -11[degrees]F (-24[degrees]C).

There are no rivers in the Bei Shan, and springs are very scarce. Hardly any water is present in the region, and there are only a few wells with drinkable water, sometimes tens of kilometers apart. These wells used to determine the caravan routes and still supply water for the sparse local population.

The vegetation of the Bei Shan Desert consists of different desert shrubs common in central Asia: Ephedra (Ephedraceae), Nitraria (Zygophyllaceae), Zygophyl-lum (Zygophyllaceae), Calligonum (Polygonaceae), Atraphaxis (Polygonaceae), Lycium (Solanaceae), Caragana (Leguminosae), Haloxylon (Chenopodi-aceae), Salsola (Chenopodiaceae), Anabasis (Chenopo-diaceae), Tamarix (Tamaricaceae), Reaumuria (Tamari-caceae), and Artemisia (Asteraceae).

The Ordos Desert

The Ordos Desert occupies a large plateau varying in altitude from 3,609-4,921 ft (1,1001,500 m) in the northwest of the great curve of the Yellow River. The Great Wall of China passes the southern edge of this plateau, separating the barchans in the Ordos Desert from the steppes of the Shaanxi loess plateau. The desert occupies an area of about 95,000 [km.sup.2]. Most of the plateau is covered by sands and dunes.

The climate is extremely continental, with a cold winter and a hot summer. The average temperature in January is 8[degrees]F (-13.5[degrees]C), with an absolute minimum of -6[degrees]F (-21[degrees]C) and an absolute maximum in summer of 108[degrees]F (42[degrees]C). The average annual precipitation varies between 6-12 in (150-300 mm). Monsoons from the southeast bring abundant rains from June to September.

In the eastern part of the desert--thanks to the monsoons--are a considerable number of streams that flow into the Yellow River. The river flows through the loess plains and bears a huge amount of silt derived from the loess areas it drains; this silt colors the river yellow and gives it its name. The plateau's climate increases in aridity from southeast to northwest, and logically the vegetation also changes--from true desert to meadow vegetation in the depressions with moist soils. In the true desert, with less than 20-25% plant cover, the dominant vegetation is the halophyte Reaumuria soongorica (Tamaricaceae), caragana (Caragana tragacanthoides, Leguminosae), Zygophyllum xanthoxylon (Zygophyllaceae), Salsola laricifolia (Chenopodiaceae), and S. passerina. The depressions with moist soils are dominated by grasses such as Agrostis alba, Arthraxon hispidus, and others.

The Tsaidam Basin

The Tsaidam (or Qaidam) Basin, a depression in the high mountains, is surrounded by high ranges in the northeast of the Plateau of Tibet. The depression is at altitudes of 8,530-10,171 ft (2,600-3,100 m); it is surrounded to the north by the Altun Shan Range and the Nan Range and to the south by the Korum Range, and it covers an area of 1,050 [km.sup.2]. The northern part is an endorheic basin, consisting of a partly clay and partly pebble-sand plain with some scattered hills. The central part is a low, highly saline plain (tsaidam is the name given in this part of central Asia to wet saline depressions), with some large solonchaks that lack vegetation. The climate is determined by the great altitude: the summer is relatively cool, with an absolute maximum temperature of 86[degrees]F (30[degrees]C). The winter is cold, with an average temperature in January of 14[degrees] to 5[degrees]F (-10[degrees] to -15[degrees]C), and an absolute minimum of -25[degrees]F (-31.7[degrees]C). The average annual precipitation varies from 2-10 in (50-250 mm).

The Tsaidam Basin receives abundant water from the rivers flowing down from the surrounding mountains. The plant cover is poor, occurring mainly on the stony areas in the surrounding mountains and their piedmonts, as well as on sandy areas with barchan dunes, where the plant cover does not exceed 5-7%. Only in the eastern part, in the areas of desert with most water, is the vegetation more abundant on the sandy soils, where it is represented by communities of wormwood (Artemisia arenaria, A. xerophytica, Asteraceae) and saltworts (Salsola laricifolia, S. paulsenii), with a ground cover that may reach 70-80%.

The Gobi Desert and other gobis

Gobi-type deserts occupy a very large area of central Asia. They are landscapes that are flat or gently undulating, with a pebbly surface, gypsum-rich soils, and a poor and highly scattered vegetation. They usually lack surface water but often have rich underground water resources. In the west are the K'a-shun, Dzungaria, and the Trans-Altai gobis, and in the central part is the Mongolian Gobi (the Gobi Desert). As a whole, they run 1,087 mi (1,750 km) from west to east, with a width of 373 mi (600 km), between 42[degrees]-47[degrees]N and 98[degrees]-118[degrees]E.

The most typical and best known is the Mongolian Gobi, the Gobi Desert. This desert is a plateau at an average elevation of 3,281 ft (1,000 m), though it varies from 2,297-4,921 ft (700-1,500 m). The relief is totally flat or slightly rolling, and it is dissected by many dry valleys. The soil surface is covered by pebbles, and sandy areas only occupy very small areas. Saline and gypsic soils are widespread.

Although the Gobi Desert is not very far from the Pacific Ocean (the easternmost regions are less than 620 mi [1,000 km] from the China Sea), the mountain ranges that border it to the east prevent the arrival of oceanic masses of moist air. As a result, the average annual rainfall is very low--2-8 in (50-200 mm)--depending on the area. On the other hand, underground water is abundant and low in minerals; it occurs at shallow depths and supplies small lakes and springs.

The Gobi Desert is one of the coldest in central Asia. The winters are intensely cold, dry, and cloudless. The minimum temperature in winter may be as low as -40[degrees]F (-40[degrees]C). The summer is also extremely hot, with an absolute maximum of 113[degrees]F (45[degrees]C). Snow cover may reach 40-80% in relatively large patches but does not form hard layers because the strong sunshine causes it to evaporate without melting. The spring is dry, and the vegetation only starts growing in the summer, when the first rains arrive.

In the central and western parts of the Gobi Desert, the plant cover is very sparse. Only in the dry valleys of temporary watercourses, where there are some pools of water, do any plants grow. The eastern region of the Gobi Desert is a subdesert with richer vegetation, merging into steppe to the north and east. On the stony soils of the arid central and western regions, the main vegetation is Haloxylon ammodendron, a low shrub 5-7 ft (1.52 m) tall. The plant layer is so scarce that in the lowest areas, there may only be one or two plants per 100 m2. The only plants that grow are the celebrated tar (Nitraria sphaerocarpa, Chenopodiaceae); (Nanophyton erinaceum, Chenopodiaceae), a spiny plant with a cushion growth form; and Ephedra przewalskii (Ephedraceae). In the wetter eastern region, Salsola passerina (Chenopodi-aceae) and Anabasis brevifolia (Chenopodiaceae) are more frequent. Other plants in this area include Reau-muria soongorica (Tamaricaceae); Zygophyllum xanthoxylon (Zygophyllaceae); Convolvulus gortschakovii (Convolvulaceae); Prunus [=Amygdalus] pedunculata; and grasses like Stipa gobica and Cleistogenes spp., Artemisia scoparia (Asteraceae), and other plants typical of the steppe.

3.2 Cold deserts and subdeserts of Inner Asia and Kazakhstan

The desert and subdesert zone of Middle Asia and Kazakhstan is a compact mass. It covers a large area from the shores of the Caspian Sea in the west to the base of the Dzungaria Alatau Range, the Shan Range, and the Pamir-Alai to the east and southeast. The true desert's northern boundary can be said to follow the 48th parallel (48[degrees]N), while the southern edge coincides with the piedmonts of the Kopet-Dag and the Paropamisus mountains.

The middle Asian desert space

The deserts of Middle Asia and Kazakhstan are characterized by a harsh continental climate, with dry summers and cold winters. The atmospheric humidity reaches them mainly from the Mediterranean and Atlantic air masses, not from the eastern monsoons as in some deserts of central Asia. Climatically, it is possible to distinguish two zones: the northern (Aralo-Caspian) and the southern (Irano-Turanian), both of which receive little rain. The rain in the northern deserts is distributed more equally over the course of the year, but in the southern ones it usually rains in the spring and summer. The frontier between these two zones is approximately the 45[degrees]N parallel. In the southern deserts the summer is hotter than in the tropical deserts because the days are longer. The average temperature in July is 79[degrees]-90[degrees]F (26[degrees]-32[degrees]C), while in the tropical deserts it is usually 75[degrees]82[degrees]F (24[degrees]-28[degrees]C). In the northern deserts of Kazakhstan, the summer heat is not so intense, but the winter is longer and harsher and the snow cover lasts 3-4 months, unlike the fleeting snows of the southern deserts.

The river network of these deserts is very limited. The water of the many rivers that flow down from the mountains is used for irrigation, and part filters into soil. Some rivers such as the Chu, the Sarysu, the Zeravshan, the Murgab, and the Tedzhen dry out and form dry deltas. Only two of the largest rivers--the Amu Dar'ya and the Syr Dar'ya--cross the desert, flowing for more than 620 mi (1,000 km) into the Aral Sea. Currently, however, the Amu Dar'ya is suffering the same fate as most of the other rivers in these deserts: because most of its water is used for irrigation, the river is almost dry when it reaches the Aral Sea; this has caused a catastrophic fall in the water level of the formerly rich and deep Aral Sea. Two other important rivers in Kazakhstan--the Ile and the Karatal, which have their headwaters in the Alatau Range--cross the desert and flow into the great Lake Balkhash.

The Karakum, Kyzyl Kum, and other sand deserts

The Middle Asian deserts are divided into highly diverse desert landscapes, the result of the great heterogeneity of parent materials on which the soils have formed. The sandy deserts occupying 40% of the area occur on the loose sands of former alluvial plains.

The Karakum Desert (karakum means black sands in Turkoman and Kazakh) is the most southerly sand desert of middle Asia, and the largest one as well. It covers 350,000 [km.sup.2] of an immense plain running from the Caspian Sea to the Amu Dar'ya at an altitude of 328-1,640 ft (100-500 m). The desert area contains many dry rivers beds. The largest of all is the dry bed of the Uzboy. The Uzboy is a former fluvial canal of the Amu Dar'ya that flows 311 mi (500 km) into the Caspian Sea. In the southeast of the Karakum there is also the bed of an old fluvial canal, the Uzboy Kelifsky, now largely occupied by the Karakum Canal. In the highest area of the southern Karakum are delta sand deposits of the rivers Tedzhen and Murgab, which flow down from the mountains and end in dry beds in the sands of the Karakum. The desert relief is dominated by lines of dune between which there are clayey areas, especially takyrs. Much of the sand is fixed by the vegetation, and only 5% of the substrate has mobile sands. The underground waters have a high mineral content, but in the sites where the waters of the rivers Amu Dar'ya, Murgab, and Tedzhen filter into the sand, and in those that are close to the Kopet-Dag, the underground water is only slightly saline.

The Karakum's climate is extremely arid, with a very hot and dry summer. The average annual temperature in the region around the Repetek Natural Reserve (38[degrees]14'N-68[degrees]11'E) is 62[degrees]F (16.6[degrees]C); the average temperature in the month of July is 90[degrees]F (32[degrees]C), and the absolute maximum temperatures in summer reaches 122[degrees]F (50[degrees]C; see pp. 293-294). The winter is relatively mild, with moderate frosts and an average temperature in January of 33[degrees]F (0.8[degrees]C), without persistent snow. The absolute minimum temperature in winter is 31[degrees]F (-35[degrees]C). The rainfall occurs in the winter and especially in the spring. The average annual rainfall is 3-4 in (70-100 mm).

The winter and spring distribution of the rainfall in the Karakum means the vegetation shows high productivity in the spring, mainly due to ephemeral plants. This makes the Karakum quite different from other sand deserts. In the spring, almost the entire soil surface is covered with a green carpet dominated by the sedge Carex physodes and members of the Brassicaceae, Leguminosae, and Chenopodiaceae. Haloxylon persicum and H. aphyllum grow on the sands covered by vegetation. The former may reach a height of 10-13 ft (3-4 m) and have a trunk diameter of 14 in (35 cm). The most characteristic shrubs are Ammodendron connolyi (Leguminosae) and about 30 different species of Calligonum (Polygonaceae).

The Kyzyl Kum (in the Uzbekh and Kazakh languages, kyzyl kum means red sands) is very similar to the Karakum. It is a large plain covering about 300,000 [km.sup.2], with some depressions and a number of isolated raised areas; it is located between the rivers Amu Dar'ya and Syr Dar'ya. The most characteristic feature of the Kyzyl Kum Desert is the presence in the central zone and in the southwest of low isolated mountains, which are the remains of ancient mountain ranges and the western extension of the current mountain chains of the Pamir-Alai. The plains of most of the Kyzyl Kum have a surface of sandstone and clay that has weathered into loose sand and pebbles, but most of the desert is covered by bands of sand. The climate is slightly cooler than the Karakum, with an average annual temperature of 56[degrees]F (13.4[degrees]C). The average monthly temperature for July is 86[degrees]F (30[degrees]C) and for January is 25[degrees]F (-4.1[degrees]C); the absolute minimum in winter is 44[degrees]F (-42[degrees]C). The average annual rainfall is 5-8 in (130-200 mm), greater than in the Karakum Desert, and the vegetation is similar.

The Karakum and the Kyzyl Kum are the largest sand deserts in Middle Asia, but on the edges of these areas there are other smaller areas of sand; these areas are scattered over a region that contains different parent materials. Most of them are to the north, within the frontiers of Kazakhstan. The most important of these sandy desert enclaves are 1) the Muyun Kum hillocks and dunes, between the Karatua range (to the south) and the lower reaches of the Chu River (to the north), 2) the Aral Karakum desert, to the northeast of the Aral Sea, 3) the Greater and Lesser Barsuki deserts, northwest and north of the Aral Sea, respectively, and 4) the 50,000 [km.sup.2] of the Volgo-Ural sands of the northern shoreline of the Caspian Sea (the Rynkum Desert). They are all subject to a climate that is less rigorous than that of the Karakum or the Kyzyl Kum, and the average annual rainfall increases toward the north and toward the west, due to the greater influence of the Atlantic cyclones.

The Betpak-Dala Steppe, or the Steppe of Hunger, and other stone and gypsum deserts

Stone-gypsum deserts are also important formations in the cold deserts of middle Asia and Kazakhstan, covering 22% of this area. The largest are in the great Ustyurt Plateau and the broad plain of Kazakhstan, the Betpak-Dala. The Ustyurt Plateau covers about 200,000 [km.sup.2] and lies between the Caspian Sea and the Aral Sea; it is surrounded on all sides by vertical cliffs up to 623 ft (190 m) high. The Betpak-Dala is in the central part of Kazakhstan (between latitudes 44[degrees]-46[degrees]N and longitudes 67[degrees]-72[degrees]E) and covers 75,000 [km.sup.2].

The soils of these deserts are characterized by the presence of shallow accumulations of gypsum--normally at about 10 in (25 cm), and no deeper than 31-35 in (80-90 cm)--containing 50% gypsum. In the Ustyurt, there is pure gypsum under a loose gypsum horizon. The soil surface is covered by rubble and pebbles. Underground waters are generally at great depth and inaccessible to the vegetation. The average annual rainfall, both in the Ustyurt Plateau and the Betpak-Dala/Steppe of Hunger, is very low, at about 3 in (70 mm). Gypsum absorbs and stores the moisture very well, so the plants can grow actively even during the arid periods. Even so, the plant cover in these deserts is extraordinarily poor, often only covering 0.1% of the ground. The dominant vegetation are saltworts (Salsola arbuscula and S. laricifolia, Chenopo-diaceae), known as soliancas in Russian.

The solonchak and takyr deserts

In addition to the rubble-gypsum stony soils of the Steppe of Hunger and the Ustyurt, there are other types of soils, especially solonchaks. Salty-clay soil deserts and subdeserts are very common on the coastlines of the Caspian (Kaidak, Mertvi Kultuk, and others), and also in the north and northwest of the Ciscaspian lowlands. This area only emerged from the waters of the Caspian Sea recently (10,000-40,000 years ago), and though the current position of the saline water table is relatively deep (23 ft [7 m] or more), the soils of these subdeserts are characterized by their high salinity. This occurred because the water table was much closer to the surface when the Caspian Sea was retreating, and salt accumulated in the soil.

Takyrs are mainly found in the form of patches among other types of desert landscapes. They are very widespread in the foothills of the Kopet-Dag and the Karakum. Takyrs are found mainly on the former deltas of rivers (Amu Dar'ya, Syr Dar'ya, Murgab, Tedzhen, etc.) in the low parts of the piedmont plains and in the flat depressions of the sand deserts of the Karakum and the Kyzyl Kum.

The loess deserts

Middle Asia is also home to the loess and clay-loess deserts, called ephemeral deserts because the vegetation is dominated by ephemeral plants. These deserts are located mainly on the piedmont plains of middle Asia--for example, in the foothills of the Kopet-Dag. The underground water is found at great depth; the sierozem soils are very well drained and therefore are fertile and not saline. In spring, when rainfall peaks (in March and April, rain often falls every 4-5 days), these soils are thoroughly wetted and a very rich vegetation grows, consisting almost entirely of ephemeral plants. When the vegetation is densest, the desert looks like a green meadow. From the end of May onward, however, plant life comes to a total halt. The most notable plants of these deserts are the typical ephemeral and ephemeroid species such as the grass Poa bulbosa and the sedge Carex hostii.

3.3 The cold deserts and subdeserts of southwestern Asia

The Iranian Plateau and Anatolian Plateau are quite far to the south and make contact with the hot deserts in the same region. These hot and cold deserts only really differ in their altitude and their rainfall regime. The plateaus of central Anatolia, between 37[degrees]N and 41[degrees]N, are almost entirely at an altitude of over 2,953 ft (900 m) and receive their rainfall in winter and spring. The Iranian plateaus (that stretch west into Afghanistan and to Baluchistan) extend farther to the south (from 27[degrees]N to 38[degrees]N) but are even higher (an average altitude of 3,281 ft [1,000 m]). The sparse rainfall they receive also falls in the winter, as the mountain ranges running parallel to the coast of the Persian Gulf and the Gulf of Oman prevent the arrival of the monsoons.

The deserts and subdeserts of the Iranian and Afghan-Baluchistan plateaus

The Zagros Mountains to the west and southwest, the Elburz Mountains and the northern Khorasan (on the frontier with Turkmenistan) to the north, and the dividing line with the Afghano-Baluchi Plateau to the east, are--broadly speaking--the limits of the large Iranian Plateau. The Afghano-Baluchi Plateau extends to the east of this dividing line that runs from the center of the Khorasan to the Sharhad, and to the foothills of the mountains of central and eastern Afghanistan and Pakistani Baluchistan.

Except for some mountainous areas that receive greater rainfall, most of these plateaus are extremely arid, with an average annual rainfall of less than 8 in (200 mm), concentrated in the winter months or in winter and spring. They are divided into a series of endorheic saline playas (called kavir), equivalent to the takyrs of middle Asia, separated by ridges rising a little above the plain with extensive stony pediments (dasht). Dune fields and erosion surface are the other major landscape types.

The two main deserts of the region are the Kavir Desert, which covers almost the entire northern half of the Iranian Plateau, and the Lut Desert, which occupies most of the southern half. The Margow Desert in southwest Afghanistan and the Kharan Desert in northwestern Baluchistan (on the frontier between Iran and Pakistan) are the most important deserts in the Afghano-Baluchi Plateau. The dominant vegetation consists of thyme scrub and areas of Stipa and other grasses, with some nongraminoid herbs that various authors have incorrectly identified as a steppe (as they have done with other desert formations of the arid periphery of the Mediterranean Basin, from the Ebro Depression in Spain to North Africa in the Pannonian region). Depending on the type of soil (sandy, saline, stony), the vegetation types found are those typical of the deserts of middle Asia: tamarisks in the beds of the temporary watercourses, Haloxylon spp. in the areas of sand, and glassworts (Salicornia) and other halophytes on the saline soils.

The subdeserts of Anatolia

The center of Anatolia is occupied by a large central basin, surrounded by high mountains. It has a harsh continental climate with long cold winters and a very dry summer. The rains do not exceed 14 in (350 mm) and fall in spring rather than winter. (In winter, precipitation falls as snow on the surrounding mountains.) The climatic conditions are thus intermediate between those of the deserts of middle Asia and those of the Mediterranean, and this is shown by the vegetation. The dominant vegetation is open grassland and scrub, and when they are degraded by overgrazing or trampling, they are replaced by scattered clumps of the grass Poa bulbosa and artemisias, mainly Artemisia fragans.

3.4 The cold deserts and subdeserts of North America

Cold deserts and subdeserts occupy a much smaller area in the Americas than in the Eurasian landmass, and they are less isolated from the oceanic moist air masses than the cold deserts in Eurasia, which have more continental climates.

The North American cold desert space

In North America, the cold deserts are all in the southwest of the continent. Centered on the Great Basin depression, though extending beyond it, this vast area of 1,060,000 [km.sup.2] runs from 35[degrees]N to 47[degrees]N and includes, in addition to the Great Basin itself, a significant part of the Columbia Plateau, which runs northeast to the southwest of Washington State on the plains of the Snake River (in southern Idaho). It spreads across the Colorado Plateau from southwest Wyoming to northern Arizona and New Mexico and the slopes of the Rocky Mountains. This region is separated from the Pacific Ocean by the Sierra Nevada and the Cascade Range, which impede the arrival of moist air masses from the Pacific. Thus, in the Great Basin and the adjacent areas, there is much less cyclonic activity related to air from the Pacific--it only increases in the plains to the east. Anticyclones are predominant over the Great Basin deserts, which provoke active eastward movement of the dry subtropical air accompanied by strong winds and often by dust storms that accentuate the dry conditions.

The deserts of the Great Basin

These deserts have an arid continental climate, with large seasonal and daily temperature fluctuations. The entire area lies between low or medium-high mountains, and for this reason the temperatures are lower than those corresponding to this latitude. The average annual rainfall is 4-8 in (100-200 mm), reaching values of 11-12 in (280-300 mm) toward the east and in the highlands. The summer is usually hot and cloudless, and the winter is cold and misty. The average temperature in July is 68[degrees]-77[degrees]F (20[degrees]-25[degrees]C), with an absolute maximum of 106[degrees]F (41[degrees]C); the average temperature in January is between 25[degrees] and 32[degrees]F (-4[degrees] and 0[degrees]C), with an absolute minimum of 7[degrees]F (-14[degrees]C). Precipitation varies very little from one season to another, but most (up to 60%) falls in the cold season. Snow falls frequently from October to April; in winter, up to 28 in (70 cm) may accumulate in some places. Even so, persistent snow cover only forms in the northern regions--the regions that are cold in winter and cool in summer. The Great Basin's relief is very heterogeneous. The plateau is at an altitude of about 3,937 ft (1,200 m) and has many low mountain ranges (though their highest peaks may exceed 9,843 ft [3,000 m]) that are 50-75 mi (80-120 km) long and 6-15 mi (10-24 km) wide. These mountains extend northward and alternate with large depressions or basins that overlap with one another at an altitude of 3,937-4,921 ft (1,200-1,500 m). Apart from this, at the ends of these chains--at altitudes between 4,921-6,562 ft (1,500-2,000 m)--there are smaller depressions, called bolsones, that form small endorheic basins fed by intermittent watercourses. Often, in the center of these bolsones, there are depressions with solonchaks that turn into temporary lakes after the rains.

Most of the Great Basin area is an immense endorheic basin. Its most important river, the Humboldt, does not flow into the ocean. Only some areas on the edge of the Colorado Basin (to the east) and the Snake-Columbia River System (to the north) drain into the Pacific Ocean. Most of these deserts' watercourses are temporary torrents that flow normally in winter and into basins with no outlet. Artesian wells on the edges of the depressions are important sources of irrigation water. Another characteristic feature is the presence of ancient lake depressions in the lower parts of the Great Basin. The largest of these dry lake basins are the clay deposits of the former Lake Bonneville and Lake Lahontan. In the Pleistocene, these depressions filled with water during the interglacial periods. Lake Bonneville covered an area of 51,000 [km.sup.2] and was as deep as 984 ft (300 m). Lake Utah, Lake Sevier, and the Great Salt Lake (which is only on average 13 ft [4 m] deep) are now isolated lakes, but they all used to form part of Lake Bonneville. Lake Lahontan's basin has been reduced to the small Lake Pyramid, Lake Winnemucca, Lake Honey, Lake Walker, and several others, many of which dry up regularly.

The area's geomorphological complexity means that the soils vary greatly. The fact that they are endorheic basins has led to the formation of vast areas of saline soils that often cover the depressions. One of them is the Great Salt Lake Desert, covering an area of 315,000 [km.sup.2]. The surface of this desert is covered by a solonchak crust up to 3 ft (1 m) thick, consisting of the chlorides of calcium, magnesium, and sodium, together with sodium sulfate (Glauber's salt), gypsum, and other salts. The increasing dryness and the large daily and seasonal variations in temperature cause intense mechanical weathering. This gives rise to the accumulation of huge amounts of debris, which, in turn, forms alluvial fans that dam the beds of mountain streams, turning them into chains of small basins. This combination of factors makes mudslides very common.

The vegetation is rather poor in terms of the number of species, with only 142 plant genera having been recorded. Grasses show the greatest diversity, but there are also abundant annual herbs and shrubs. The desert landscape is dominated by communities of sagebrush (Artemisia tridentata [Asteraceae] and other species of Artemisia), shrubs more than 3 ft (1 m) tall with a vigorous root system, and spiny saltbush Atriplex confertifolia (Chenopodiaceae) of the Great Basin. Monotonous vegetation consisting of these plants covers large areas, stretching for tens of kilometers. The areas covered by Atriplex confertifolia increase at lower elevations and on more saline soils. The highest areas with skeletal soils are completely dominated by communities of Artemisia tridentata. Different combinations of factors may lead to the growth of any of 20 different species of sagebrush and about a dozen species of saltbush. Slightly saline soils favor Artemisia spinescens. In the eastern region of the Great Basin, the highly salt-resistant Atriplex nuttalli and A. corrugata grow on highly saline soils. Plants of other genera may also be abundant. Slightly saline soils support abundant clumps of the chenopod Krascheninnikovia [= Eurotia = Ceratoides] ceratoides. The moist saline depressions and the dry beds of rivers are often covered by the greasewood (Sarcobatus vermiculatus, Chenopodi-aceae), another shrubby chenopod that only grows 5 ft (1.5 m) tall but has a root system 7 ft (2 m) deep. Halophytes like red saltwort (Salicornia rubra) and Utah glasswort (Sarcocornia utahensis) grow in the moist solonchaks. The peripheral areas of solonchaks are the habitat of the grasses Distichlis spicata and Sporobolus airoides. Stony soils in the southern part of the desert may well support pure stands of black brush (Coleogyne ramosissimus, Rosaceae), a small evergreen shrub reaching a height of only 24 in (60 cm).

The vegetation bursts into life in spring (March and April), when the snow melts and the first spring rains arrive. Then, the desert plateaus are covered by a thick carpet of annuals such as sand verbena (Abronia spp., Nyctaginaceae), pepperwort (Lepidium spp., Brassicaceae), legumes of the genera Lupinus and Astragalus, grasses of the genera Bromus and Vulpia, as well as the chenopods Halogeton glomeratus and Bassia hyssopifolia. The lowlands, which receive less rainfall, have a sparse plant cover. The plant cover is also sparse in the southern parts of the desert. From June to September, plant growth practically halts, restarting only after the autumn rains in October.

On the northern and eastern limit, the desert shrubs are replaced by grasses--the vegetation typical of the prairies. At elevations of more than 6,562 ft (2,000 m), the typical desert sagebrush community tends to be replaced by open xerophytic pine-juniper forests dominated by two species of pine with edible seeds (Pinus edulis and P. monophylla) and two junipers (Juniperus utahensis and J. scopulorum). Above this limit, these open woodlands turn into genuine pine forests of Pinus ponderosa, with a dense steppe-type grass layer; at even greater altitudes, there are mixed coniferous forests, with Douglas fir (Pseudotsuga menziesii) and Colorado fir or American white fir (Abies concolor). Above the tree line (10,827-11,811 ft [3,300-3,600 m]) are alpine shortgrass meadows.

3.5 The cold deserts and subdeserts of South America

In South America, the cold deserts and subdeserts run from 40[degrees]-55[degrees]S, covering an area of about 400,000 [km.sup.2], corresponding basically to the vast area of Patagonia.

The South American cold desert space

The only area of South America that can be classified as a cold desert is Patagonia, on the Atlantic side of the southern Andes. This is, however, a very atypical cold desert. It is dry despite being right next to the ocean, and it is not hundreds or even thousands of kilometers from the sea like all the other cold deserts. It is cold, but without being extremely cold--the absolute minimum temperature recorded is 0[degrees]F (-18[degrees]C)--although frosts are frequent and occur almost throughout the entire year. Patagonia is, in fact, the only cold coastal desert in the world and is subject to continuous cold dry winds gusting down from the peaks of the Andes. In effect, although Patagonia is considered a cold desert, the temperature does not fluctuate nearly as much as in the similar deserts of North America and Asia, since it is exposed to a greater maritime influence. Perhaps the most important of these influences is the strong wind from the west, although the winds retain very little moisture after crossing the Andes and are completely dry when they reach the Patagonian Desert. Increasing land use for raising sheep in the twentieth century has favored wind erosion and a considerable expansion of the desert area.

The deserts of Patagonia

Patagonia's deserts are in southern Argentina between the foothills of the Andes and the Atlantic Ocean, extending somewhat into Chile in the region of the Strait of Magellan. On the foothills of the Andes, the deserts extend to higher elevations and lower latitudes (roughly to 35[degrees]S) in a narrowing band between the monte to the east and sub-Antarctic forest and other formations to the west. In Rio Negro province and in the northwestern tip of Chubut province, the highlands of the inland plateaus are Patagonian, while the lowlands (except for a small Patagonian enclave in the Valdes Peninsula) are still monte. Yet in the Magallanes region and in Tierra del Fuego, the Patagonian Desert is limited to the lowlands below 1,640-1,969 ft (500-600 m). From the Andes to the Atlantic and from north to south are the districts known as Payunia, Occidental, Chubut, Santa Cruz, and the Golfo de San Jorge zone.

The district known as Payunia is a narrow strip with blurred limits; it follows the foothills of the Andes in much of the Argentinean provinces of Neuquen and the south of Mendozat , between the monte to the east and the puna to the west. It has not been studied much but appears to be dominated typically by tussock grasses; its southern border with the Occidental (western) region is unclear.

The western district, like the Payunia, is a zone of transition to the Andean vegetation in a strip about 62 mi (100 km) wide between the foothills of the Andes and the Patagonian plateaus. It extends from Neuquen province to the northwest corner of Santa Cruz province, with some extensions farther to the south along the northern coast of Lake Argentino. The plant cover is poor, rarely exceeding 60%, and dominated by grasses, especially coirones (Stipa, Festuca and Poa), accompanied by three shrubs that often grow as spiny cushions: Mulinum spinosum (Apiaceae), Adesmia campestris (Leguminosae), and Senecio flaginoides (Asteraceae). The Chubut district, farther south, is the most typically desertlike landscape in Patagonia, especially in the plains and at elevations greater than 1,312 ft (400 m). The plant cover, which is as poor as in the western district or poorer, is dominated by rounded clumps of quilembai (Chuquiraga avellanedae, Asteraceae); this plant can reach a height of 2 ft (0.5 m) and has yellow inflorescences and small sharp-tipped coriaceous leaves that protect it from herbivores. The clumps of coirones (Stipa speciosa, S. humilis, Poa ligularis)--mainly the coiron amargo (S. speciosa) and the small ground-hugging clumps of colapiche (Nassauvia glomerulosa, Asteraceae), whose adpressed leaves give its branches the appearance of the scale-covered tail of an armadillo--occupy the spaces between the quilembais. There are also isolated patches of taller shrubs, which can be razed by sheep, that may contain algorrobo Prosopis denudans (Leguminosae), Lycium ameghinoi (Solanaceae), and Berberis cuneata (Berberidaceae). In the lowlands of the valley of the Rio Chico and in the Sarmiento Basin, the vegetation is similar, but with halophilous species such as the chenopods Atriplex lampa (Chenopodiaceae) and A. sagittifolium and the alkali-heath Frankenia patagonica (Frankeniaceae).

The Santa Cruz district, often considered together with the Chubut as part of a large central district, is the largest of all. It runs from the southern tip of the Patagonian Desert and in some places runs from the Atlantic coastline to the Andes. The most frequent plants are dwarf subdesert shrubs, but the ground cover rarely exceeds 40%. The dominant species, as already mentioned, is colapiche, together with Acantholippia seriphioides (Verbenaceae), Chuquira-ga aurea (Asteraceae), the umbellifer Mulinum microphyllum and several coiron grasses (Stipa speciosa, S. chrysophylla). The infrequent scrublands are only found on the lowest plateaus near the Atlantic Coast and consist of mata negra (Verbena stipens, Verbe-naceae) and Lepidophyllum cupressiforme (Astera-ceae).

The smallest district of the Patagonian Desert is around the Gulf of San Jorge; this district has the greatest plant cover. The desert is bounded on the coast by three plateaus (Meseta de Montemayor , the Pampa del Castillo, and the divide between the San Jorge Gulf and the Rio Deseado) and touches the southern tip of the monte. Almost two-thirds of the plant cover consists of just two shrubby species--Trevoa patagonica (Rhamnaceae) and Colliguaja integerrima (Euphorbiaceae)--and the remainder consists of grasses (Stipa spp.) growing under the branches of the Trevoa.

163 The typical appearance of a cold desert is shown here in the landscape in Parya Canyon in the Vermillion-Cliffs Wilderness, Arizona (United States), an area dominated by sagebrush (Artemisia tridentata). Low precipitation and high evaporation, which rapidly dry the soil, means that in these zones (as in the hot deserts), the availability of water is the factor limiting animal and plant life. Yet the dramatic seasonal variations in temperature, with frosts in winter, introduce a distinctive factor absent from the tropical desert areas; this is why they are referred to as cold deserts. These ecosystems form at mid-latitudes in the interior of continents, far from the buffering action of the oceans (that attenuates the differences between the seasons and between night and day); furthermore, they are situated in the rain shadow of mountain ranges, which stops the clouds. The mean average annual precipitations do not exceed 20 in (500 mm), so the climate of the cold deserts can be classified as arid or semiarid.

[Photo: Carr Clifton / Minden Pictures]

164 A cloudburst in the Gobi Desert. The average annual rainfall in the Gobi, in Mongolia, is 2-8 in (50-200 mm; slightly more in the northeast of the region), and, though irregular, it falls mainly in the summer. The Gobi is characterized by its extremely continental climate and is almost untouched by the summer monsoons. (These are due to solar warming of the interior of the continent and would bring moisture-bearing clouds, but the Himalayas intercept them and cause them to shed all their moisture.) Furthermore, the winds that blow in the Gobi during the summer arrive from the Arctic Ocean, so they are also very dry. All in all, these factors mean that atmospheric humidity is very low, and the sunshine acts directly on the soil, causing it to heat up rapidly during the daytime and cool down rapidly at night, with temperature changes that may be as great as 122[degrees]F (50[degrees]C) over a single day. Due to the dryness of the soils, any water arriving from the surrounding mountains is quickly lost, and most of the rivers flowing toward the Gobi are seasonal, flowing only during the rainy season.

[Photo: Konstantin Rogovin]

165 Temperature and rainfall diagrams for a subdesert region of central Kazakhstan and the Repetek Desert, showing the temperature (in red) and the precipitations (in blue), illustrate the differences between a typical subdesert climate and a desert climate. In addition to the higher rainfall and lower temperatures, the rainfall curve for the Kazakhstan Desert is usually above the temperature curve, so that the moist periods (in blue) are much more abundant than the dry ones (in yellow) The opposite happens in the Repetek Desert; the temperature curve is above the precipitations curve, and the dry periods last for most of the year. It should also be pointed out that frosts are rare in the Repetek Desert, while in the Kazakhstan Desert the minimum temperatures are below 32[degrees]F (0[degrees]C) for the winter months (from November to March). The data available cover a period (eight years) long enough to be highly statistically significant, though they correspond to different periods (1954-1961 for the Kazakhstan Desert, and 1965-1972 for the Repetek Desert).

[Drawing: Jordi Corbera, bas-ed on Goodall, 1983]

166 A winters' day with fog and snow lying around the rocky pillars of Monu-ment Valley, in Arizona and Utah (United States). This is not the best-known aspect of this famous desert landscape, where many westerns have been filmed, but winter cold spells and snow are not uncommon. This area used to enjoy a wetter climate, but in the midtwelfth century, it started to get drier. The Anasazi Indians, who had until then lived in the area by cultivating the arid ground with complex irrigation systems, were obliged to abandon them and head south. In the sixteenth century, the Navajo Indians arrived with their herds of goats and sheep, and the red rocks of Monument Valley were their home until the arrival of whites.

[Photo: Ed Darack / Planet Earth Pictures]

167 The characteristic polygonal structure of the takyrs, seen here in the Aydingkol Depression (492 ft [150 m] below sea level) in the Tarim endorheic basin (Xinjiang, China), between the Takla Makan and the A-la Shan deserts. Takyrs only form in endorheic plains or depressions where there are fine sedimented materials. When relatively intense rain falls, water accumulates in these zones and the soils are temporarily flooded and remain waterlogged. Before the water evaporates, algal communities thrive, followed by lichens and mosses. Phanerogams are completely absent, though they may grow sporadically in soils with characteristics similar to those of takyrs. Though these soils may be flooded once every year, the water hardly infiltrates into soil, so it evaporates very quickly. Only plants that can support almost total dehydration, poikilohydric plants, find this a suitable place to grow. Even so, the seeds of ephemerals plants can also germinate successfully in this environment.

[Photo: Ramon Folch / ERF]

168 In northern China, a huge plain of loess has filled a series of basins that two million years ago were occupied by lakes in which red clays were deposited. The loess was deposited on top of these red clays, forming accumulations known as yuan that may be 328-656 ft (100-200 m) deep. The particles forming loess sediments are deposited slowly enough for iron-bearing particles to be laid down in alignment with the magnetic pole. Measuring the magnetic alignment of the sediments forming the yuan shows a change of polarity about 730,000 years ago, and another about 2.4 million years ago, at the point where the deposition of red clays starts. This accumulation has not been continuous; it was interrupted by several periods of soil formation, always in cold dry periods. The tendency of the climate to be cool and dry has continued to the present day and was definitively confirmed by the uplifting of the Tibetan Plateau, which blocks the arrival of the moisture-bearing monsoon clouds and has caused the desertification of large corridors to the north of Tibet. The particles forming the loess are derived from the breakdown of the rocks in the mountains that surround the deserts.

[Photo: J.H. Kauffman / ISRIC / Wageningen]

169 Clay soils covered by gravel cover large areas in the desert regions in southwest Urumqi in Xinjiang (China). Desert pavements of this type--they normally run around the edges of large and more or less rocky plains--vary greatly in their composition, which depends partly on the type of sedimentary rocks on which they occur. They are found in both desert zones and semidesert ones where the vegetation is mainly annual plants that vary greatly in their seed production from one year to the next.

[Photo: J.H. Kauffman / ISRIC]

170 Greyzems or sierozems are nonsaline soils that are very characteristic of the peripheral zones of the cold Asiatic deserts. They form on piedmont plains where the water table is deep, and, as this profile of a Greyzem in China shows, they are well structured and gray in color. They are very permeable soils that allow the growth of a plant cover consisting largely of plants with deep roots; if suitably irrigated, they are fertile agricultural soils that give good yields of cereals, fruit, and vegetables.

[Photo: J.H. Kauffman / ISRIC]

171 The cold deserts of central Asia cover nearly three million [km.sup.2] and have cold winters with hot dry summers, as shown by these temperature and rainfall diagrams for four widely separated locations: Khotan (in the south of the Takla Makan Desert), Urumqi (near the Dzungaria Basin), Dalandzad-gad (in the A-la Shan Desert), and Golmud (in the Tsaidam Basin). The northern part of this vast region receives rain more or less regularly over the course of the year, while in the southern area, where the temperatures are higher, most rain falls in the spring and summer. The precipitations also decline from east to west, from the Ordos Desert to the Lob Nor Depression, as the eastern region receives some rain from the monsoons originating in the China Sea.

[Drawing: IDEM, from several sources]

172 Chains of barchans, separated by corridors known as bahirs, are present throughout the Takla Makan Desert, in Xinjiang, China. (See photo of Mazar Tagh.) They are especially abundant in the eastern zone, in the area known as the Qargan Desert. This is the kingdom of shamo, the local name for this fine light sand. The dunes can reach heights of 328-656 ft (100-200 m), and they show a great diversity of shapes, depending on the winds, which in this area are complex. In spring, when the warming of the earth creates rising currents of air, the winds arriving from the northeast are very violent and can give rise to immense dust storms that fill the air with fine sand to a height of 16,40419,685 ft (5,000-6,000 m).

[Photo: S. Bachelier / Explorer]

173 Alluvial fan in the arid Flaming Mountains, a mountain system surrounding the Turpan Depression, between the Takla Makan and the Bei Shan deserts (Xinjiang, China). The Turpan Depression is a zone with numerous oases that support a large agricultural community. Mountain chains surround the depression: to the northeast lies the Mongolian Altai, to the west is the western end of the Tian Shan Mountains, and to the south is the Altun Range. These mountain ranges enable the survival of the oases scattered through the Turpan; during the spring melt, temporary watercourses form that leave characteristic erosion forms when they dry up, like the one shown in the photo.

[Photo: AGE Fotostock]

174 Erosion severely affects the surface of the gobis of southern Mongolia and northern China. The action of the wind leaves the rock bare and favors the expansion of the desert, as the sand spreads farther and farther, rendering more and more land used for agriculture and stockraising totally useless. There are two types of gobi: those formed by denudation, where the bedrock is broken down by the action of the wind, and those formed by accumulation, mostly transported by water and largely consisting of gravel. The fact that the vegetation is scarce and low accentuates both the effect of the wind and the accumulation.

[Photo: Konstantin Rogovin]

175 The cold desert and subdesert areas of inner Asia and Kazakhstan include stonecovered areas, plains of loess, and vast areas of sand such as the Kyzyl Kum (red sands) in Uzbekistan and the Karakum (black sands) in Turkmenistan, the two largest deserts in the region. The other smaller deserts include the Muyun Kum Desert in southern Kazakhstan, and the Barsuki Desert, north of the Aral Sea. These four deserts are at very high latitudes, and because there are no mountain ranges to isolate them from the Arctic regions, they have extremely cold winters and parching summers. In winter, the mountaintops are covered in snow and the strong winds cause sand and salt storms. Rainfall is very scarce in the region, and the riverbeds may remain totally dry for periods of centuries. In the plateau of the Afghano-Iranian regions, there are two further desert areas, one of them to the north, consisting of the Dasht-e-Kavir (salt desert), and a southern area consisting of the Dasht-e-Lut and the Dasht-i-Margo. The Dasht-e-Lut is a completely inhospitable and uninhabited salt wasteland, while the second is covered by sands and stones and contains greater biological diversity. Both have a similar climate, with little rainfall and an interannual temperature range that may be as great a 144-162[degrees]F (80-90[degrees]C).

[Drawing: IDEM, based on several sources]

176 The Repetek area on the border between Turk-menistan and Uzbekistan occupies the southeast of the large Karakum Desert and forms a complex ecological unit. About 80% of its area is covered by dunes with some vegetation, 5-10% is totally barren or has very little vegetation, and the rest of the area is covered by takyrs and saline flats that form islands in the depressions among the sand. Sand dunes, which occupy much of the Karakum, may reach great size: they may be many kilometers long and are usually 10100 times longer than they are broad or tall. The sand they are formed from was originally deposited by the river when it burst its banks and changed its course over the plains of Turkmenistan.

[Photo: Rodger Jackman / Oxford Scientific Films]

177 In many of the subdeserts of the Great Basin, a significant depth of snow accumulates during the winter months, as shown by this photo of the snow-covered landscape of the Arches National Park in Utah (United States), whose strange windcarved rock formations are also featured in figure 248. Despite these snowfalls, the winters in the Arches Park are not very harsh, and when the snow layer melts in the spring, the landscape becomes a riot of color, especially during the months of April and May. The mild and pleasant temperatures of spring give way to the hot dry summer typical of the deserts and subdeserts of North America.

[Photo: Larry Minden / Minden Pictures]

178 The desert and subdesert areas of North Amer-ica. These areas lie between the barrier formed by the Sierra Nevada and the Cascade Range and comprise the Great Basin Desert, the Great Salt Lake Desert, and the Great Sand Desert. The entire area is a series of depressions separated by relatively high mountain ranges, with shallow lakes here and there (the Great Salt Lake, for example), some of them permanent and others temporary. The climate is cool and dry, with low rainfall distributed very evenly over the course of the year, and low winter temperatures, due to the fact that the region is at a high altitude. To the south, the deserts of the Great Basin merge into the Mojave Desert, which is hotter and drier and marks the transition with the hot Sonora and Chihuahua deserts (see figure 38).

[Drawing: IDEM, based on several sources]

179 The landscapes of the deserts of Patagonia are in general very monotonous. There are two clearly distinct areas: the Atlantic side and the Pacific side. The Atlantic side has a dry arid climate and is an extension of the Argentinean pampa; it consists of plateaus with large grasslands. In this area in the Argentinean sector of Patag-onia, the desert is spreading from west to east. The lowland sector of Los Glaciares National Park, shown in the photo, clearly shows this landscape uniformity, partly caused by the dominant winds sweeping the area from the Pacific to the Atlantic. In the summer, these winds from the pampa raise huge dust storms. Nearly all the rainfall is in the Andean region and, in the summer, is caused by depressions originating over the Pacific. After this, the dryness of the air gives rise to increasingly poor vegetation, which deteriorates from the grasslands in the eastern zone in the Andes to the deserts on the Atlantic coastline. Despite this general aridity, grasslands grow on soils formed by sandy clays and loess areas, providing grazing for sheep and even for cattle.

[Photo: Xavier Ferrer & Adolf de Sostoa]

180 The cold deserts and subdeserts of South Ameri-ca basically occupy the area of Patagonia in southern Argentina. Their aridity results from their location in the rain shadow of the most southerly foothills of the Andes, which stop moisture-bearing winds from the west. The precipitations are mainly in the summer, from October to March, and the average annual temperatures are very low; together with the persistent winds from the west that severely erode the soil, they prevent the development of a dense plant cover. Climatic conditions are not uniform over the entire region, among other reasons because they range in altitude from sea level on the Atlantic coast to 3,281 ft (1,000 m).

[Drawing: IDEM, based on several sources]
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Publication:Encyclopedia of the Biosphere
Date:Apr 1, 2000
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