You don't have to go to Africa or Australia to visit the desert. Deserts and semi-deserts are also found on Russian territory. The most depressed part of the Caspian lowland is occupied by deserts, where flat surfaces alternate with sandy deposits. The climate here is sharply continental: very hot and dry summers, cold winters with little snow. Apart from the Volga and Akhtuba, there are no other sources of water here. There are several oases in the deltas of these rivers.
The strip of semi-deserts of Russia is located in the southeast of the European part of the country, starting in the area of the left bank of the Volga and reaching the foothills of the Caucasus Mountains. This is the western part of the Caspian region and the Ergeni hill. The climate here is also sharply continental and dry. Water arteries of the semi-desert zone are the Volga and Sarpinsky lakes.
In deserts and semi-deserts, an insignificant amount of precipitation falls - up to 350 millimeters per year. The soils here are mainly sandy and desert-steppe.
The word "desert" suggests that there is no life here. But it is not so.
Climate of deserts and semi-deserts of Russia
The climatic conditions of deserts and semi-deserts influenced the formation of special flora and fauna. The vegetation in this area is arranged in a mosaic pattern. Perennial grasses - ephemeroids - have spread predominantly in semi-deserts. Ephemera still grow here, life cycle which is two to three months. In general, the plants are small, but have a powerful root system. In the semi-desert region, black wormwood and saltwort, bulbous bluegrass and two-spike conifer, camel thorn and fescue grow. Closer to the Caspian Sea, the semi-desert turns into a desert, where vegetation becomes less and less common. Sometimes you can see elmius, wormwood or hairworm here.
Ecological problems of deserts and semi-deserts of Russia
If we talk about environmental problems deserts and semi-deserts of Russia, then the very intervention of man in the nature of this area is a danger. The process of desertification itself—an extreme degree of soil erosion—leads to significant changes, especially under the influence of anthropogenic factors. Another problem in the deserts and semi-deserts of Russia is poaching and extermination of animals and plants in large quantities. And since some people live here rare species, human activities cause serious damage to nature. Therefore, it is necessary to protect and preserve the landscapes of the country’s deserts and semi-deserts, since this is the wealth of our planet.
Deserts and semi-deserts are waterless, dry areas of the planet where no more than 25 cm of precipitation falls per year. The most important factor in their formation is wind. However, it is not observed in all deserts hot weather, some of them, on the contrary, are considered the coldest regions of the Earth. Representatives of flora and fauna have adapted to the harsh conditions of these areas in different ways.
How do deserts and semi-deserts arise?
There are many reasons why deserts arise. For example, there is little precipitation in the city because it is located at the foot of the mountains, which cover it from rain with their ridges.
Ice deserts formed for other reasons. In Antarctica and the Arctic, the bulk of the snow falls on the coast; snow clouds practically do not reach the interior regions. Precipitation levels generally vary greatly; one snowfall, for example, can result in a year's worth of precipitation. Such snow deposits form over hundreds of years.
Hot deserts have a wide variety of topography. Only some of them are completely covered with sand. The surface of most is strewn with pebbles, stones and other various rocks. Deserts are almost completely open to weathering. Strong gusts of wind pick up fragments of small stones and hit them against the rocks.
In sandy deserts, wind moves sand across an area, creating wave-like deposits called dunes. The most common type of dunes is dunes. Sometimes their height can reach 30 meters. Ridge dunes can be up to 100 meters high and extend for 100 km.
Temperature
The climate of deserts and semi-deserts is quite diverse. In some regions, daytime temperatures can reach 52 o C. This phenomenon is due to the absence of clouds in the atmosphere, thus nothing saves the surface from direct sunlight. At night, the temperature drops significantly, which is again explained by the absence of clouds that can trap the heat emitted by the surface.
In hot deserts, rain is a rare occurrence, but sometimes heavy downpours occur here. After rain, water is not absorbed into the ground, but quickly flows from the surface, washing away particles of soil and stones into dry channels called wadis.
Location of deserts and semi-deserts
On the continents, which are located in northern latitudes, there are deserts and semi-deserts of the subtropical and sometimes tropical ones - in the Indo-Gangetic Lowland, in Arabia, in Mexico, in the southwestern United States. In Eurasia, extratropical desert areas are located in the Central Asian and South Kazakh plains, in the Central Asian basin and in the Western Asian highlands. Central Asian desert formations are characterized by a sharply continental climate.
In the southern hemisphere, deserts and semi-deserts are less common. Here are located such desert and semi-desert formations as the Namib, Atacama, desert formations on the coast of Peru and Venezuela, Victoria, Kalahari, Gibson Desert, Simpson, Gran Chaco, Patagonia, Great Sandy Desert and Karoo semi-desert in southwestern Africa.
Polar deserts are located on the mainland islands of the periglacial regions of Eurasia, on the islands of the Canadian archipelago, in northern Greenland.
Animals
Over the many years of existence in such areas, animals of deserts and semi-deserts have managed to adapt to harsh climatic conditions. They hide from cold and heat in underground burrows and feed mainly on underground parts of plants. Among the fauna there are many species of carnivores: fennec foxes, pumas, coyotes and even tigers. The climate of deserts and semi-deserts has contributed to the fact that many animals have an excellent thermoregulation system. Some desert inhabitants can withstand fluid loss of up to a third of their weight (for example, geckos, camels), and among invertebrates there are species that are capable of losing water up to two-thirds of their weight.
In North America and Asia there are a lot of reptiles, especially lizards. Snakes are also quite common: ephas, various Poisonous snakes, boas. Among the large animals there are saiga, kulans, camels, pronghorn, which recently disappeared (it can still be found in captivity).
Animals of the desert and semi-desert of Russia are a wide variety of unique representatives of the fauna. The desert regions of the country are inhabited by sand hares, hedgehogs, kulan, jaiman, and poisonous snakes. In the deserts that are located in Russia, you can also find 2 types of spiders - karakurt and tarantula.
They live in polar deserts polar bear, musk ox, arctic fox and some species of birds.
Vegetation
If we talk about vegetation, then in deserts and semi-deserts there are various cacti, hard-leaved grasses, psammophyte shrubs, ephedra, acacias, saxauls, soap palm, edible lichen and others.
Deserts and semi-deserts: soil
The soil, as a rule, is poorly developed; its composition is dominated by water-soluble salts. Among them, ancient alluvial and loess-like deposits predominate, which are reworked by winds. Gray-brown soil is typical for elevated flat areas. Deserts are also characterized by salt marshes, that is, soils that contain about 1% of easily soluble salts. In addition to deserts, salt marshes are also found in steppes and semi-deserts. Groundwater, which contains salts, when reaching the soil surface is deposited in its upper layer, resulting in soil salinization.
Completely different characteristics are characteristic of such climatic zones as subtropical deserts and semi-deserts. The soil in these regions has a specific orange and brick-red color. Due to its shades, it received the corresponding names - red soils and yellow soils. In the subtropical zone in northern Africa and in South and North America there are deserts where gray soils have formed. In some tropical desert formations, red-yellow soils have developed.
Natural and semi-deserts are a huge variety of landscapes, climatic conditions, flora and fauna. Despite the harsh and cruel nature of the deserts, these regions have become home to many species of plants and animals.
natural zone forest-steppe tundra
The semi-desert zone enters the Russian Plain only in the southeast, occupying the Ergeni Upland and the northern half of the Caspian Lowland. Its southern border west of the Volga runs at a distance of about 150 km from the coast of the Caspian Sea; in the Volga-Ural interfluve it moves even further from the sea and passes here along the line: Lake Baskunchak - Lake Aralsor - the mouths of the Small and Bolshoi Uzeni - the Ural River south of Kalmykov.
The location in the southeast of the Russian Plain in the depths of the Eurasian continent determines the sharply continental, dry climate of this zone. Summer in semi-deserts is sultry and sunny. average temperature July reaches 23--25°, in the city of Novouzensk during the warm period there are 85 days with dry winds. Winter is as cold as on the Kola Peninsula: the average January temperature is --7--8° in the southwest of the zone and --13--14° in its northeast. Snow cover is thin - from 10 to 30 cm. Total, annual amount atmospheric precipitation 300--200 mm; this is three to four times less than the evaporation value. For example, in the city of Novouzensk the annual precipitation is 250 mm, and evaporation is 910 mm.
The surface flow in the semi-desert is negligible, so its own river network is not developed. Groundwater is saline and most of it is not suitable for drinking.
In addition to climate, the geological and geomorphological features of the territory have the strongest impact on the landscape of the zone - low absolute height, flatness, weak erosional dissection, the presence of saline bedrock and Quaternary rocks. There are few ravines and gullies in the zone. Instead of these erosional forms, closed basin-depression forms are widespread - steppe depressions, estuaries, soras, etc. Their genesis is different - from suffusion-subsidence to karst and tectonic (some estuaries).
The continental climate, flat terrain and saline soils contribute to the accumulation of salts in semi-desert soils, including easily soluble ones. Solonetz soils are as characteristic of semi-deserts as light chestnut soils, which are zonal here. Lack of moisture and soil salinity lead to a patchy, clump-like distribution of vegetation. The abundance of depression-depression forms causes extraordinary diversity and complexity of plant and soil cover. With a lack of moisture, even the most insignificant depressions - 10 - 20 cm deep - lead to sudden changes in soil and vegetation. We can say that a semi-desert is a zone of complexes in which a grassy steppe in depressions, a wormwood-hodgepodge desert on solonetzes, and a fescue-chamomile semi-desert proper on light chestnut soils are closely intertwined.
In the fauna of semi-deserts, an outstanding role belongs to rodents. Among them, in terms of abundance and impact on the landscape, ground squirrels stand out, represented here by two species - the small ground squirrel, which lives on loamy plains, and the yellow ground squirrel, which inhabits the sands. The occurrence of gophers is very high. In some places, up to 740-750 gopher burrows can be counted on one hectare. The discharges of gophers create the tubercular microrelief characteristic of the Caspian region, which further enhances the complexity of the soil and vegetation cover.
In addition to gophers, common rodents in the semi-desert are jerboas, gerbils, voles, steppe pieds, and mice. The saiga antelope, which previously inhabited the steppe and forest-steppe zones of the Russian Plain, is found within the zone. Here and there a wild boar is found in the reed thickets of river valleys. The most common predators are the wolf, corsac fox, and steppe polecat.
The composition of birds (steppe eagle, harrier, larks), reptiles and insects is also quite diverse.
Most of the semi-desert zone is used as pasture. Estuary and irrigated agriculture is developed in some places.
The southern third of the Caspian lowland belongs to the desert zone. Due to the small size of the territory and the uniformity of geological and geomorphological conditions, the desert zone on the Russian Plain belongs to one landscape provinces--provinces sandy and clay-salt deserts of the Caspian region. The features of dryness and continental climate, characteristic of the southeast of the Russian Plain, reach their maximum in the desert zone. The annual amount of precipitation in deserts is less than 200 mm. In the city of Astrakhan, on average, 170 mm of precipitation falls per year, with an evaporation rate of 936 mm. The winter is exceptionally light-snowy, even at the end of it the snow cover does not reach 10 cm. For this reason, the Caspian desert, especially west of the Volga (Black Lands), where the winter is warmer, is a good winter pasture.
Surface flow in deserts is so insignificant (less than 0.5 l/sec) that not a single local river crosses the territory of the province.
Geologically, the territory of the Caspian desert is very young; Its coastal parts turned into dry land quite recently. Unlike the semi-desert, the desert zone in the Quaternary was flooded by all three transgressions of the Caspian Sea, including the Khazar one. Almost the entire territory of the province lies below sea level.
Huge areas in the desert are occupied by sands of marine (late Khvalynian Sea) and alluvial-deltaic origin. The area of the Volga-Ural sands alone is about 50 thousand km3.
On the coast of the Caspian Sea and especially near the Volga delta and to the west of it, Baer mounds are found. More precisely, these are low (6-20 m) and long (from several hundred meters to 5-6 km) sandy ridges, mainly in a latitudinal direction. First described by Academician K. M. Baer, the mounds then more than once served as the object of special study. A variety of hypotheses have been put forward regarding their genesis - aeolian, tectonic, water-erosion, water-accumulative and many others. Most likely, their formation should be associated with the accumulation and movement of sediments by the waters of ancient sea basins retreating to the south. Later, some of the mounds underwent aeolian reworking. Brown desert-steppe soils appear in the soil cover of deserts, and salt marshes stretch in a wide strip along the shores of the Caspian Sea. Vegetation is closely dependent on soils. On saline clayey soils, wormwood-solyanka groups are present. The vegetation of sandy deserts, characterized by shallow fresh groundwater, looks more diverse. It is formed by grass-wormwood groups with the participation of bluegrass (Poa bulbosa), Siberian wheatgrass (Agropyrum sibiricum), twig grass, and milkweed. In the north-west of the Volga-Ural sands in the Urda sands, rich in fresh water, small groves of poplar and aspen have survived, and orchards and melons are planted.
Deserts are used as pastures and hayfields. In the wide Volga-Akhtuba floodplain, horticulture, vegetable gardening and melon growing are developed. The area of floodplain land used for agriculture is still small and can be successfully increased many times over.
“The further you move south, the scarcer the grass cover becomes. The steppe gradually turns into a huge desert belt that stretches across the whole of Central Asia from west to east. You repeatedly cross low mountain ranges, and between them again lie endless expanses of flat sandy and rocky deserts , where for days on end you will not see an antelope or any other animal. This area looks waterless and bare, covered with stones and rubble, and in places with sand or loess-like loams. Only on the slopes of the hills, along the edges of salt marshes and sand dunes and along the dry riverbeds of rainwater gullies grow a few grains and ugly shrubs. Low thorny bushes stubbornly struggle with the deadly sand that accumulates around them and threatens to fill them in. These small sandy mounds, from which thorny branches protrude, are like giant hedgehogs with splayed needles.
Behind the last eastern mountain spurs of the Gobi Altai, a sandy desert dominates. Only gnarled, as if dead, saxaul trunks stick out from the soil here and there." *
* (Unfortunately, this part of the quote could not be identified.)
“All day long you walk among an endless sea of sand: dune after dune, like giant waves, rise before the eyes of a tired traveler, revealing short, yellow horizons. Even having climbed to a higher peak, you see nothing - all sand, sand and sand. Animal life too. neither seen nor heard, only the heavy, rapid breathing of camels and the rustle of their wide paws can be heard. giant snake a camel caravan meanders along the sands, now rising to the ridges of dunes, now plunging between their capricious slopes..." *
* (Kozlov P.K. Mongolia and Kam, part 1, 1905, p. 126.)
The descriptions given above belong to the famous Russian explorer of Central Asia P.K. Kozlov, who crossed the Gobi Desert at the end of the last century. But the Gobi is only one region of a desert zone that covers the entire globe.
Geographical position. The semi-desert and desert zone is well represented mainly in the northern hemisphere, where it extends between 15 and 50° N. w. in the form of a belt, in different places having unequal width. The zone occupies more than a quarter of the entire land surface of the Earth. There are subtropical hot deserts and semi-deserts and moderately hot but cold winters. The first ones reach 30 - 35 °C. and Yu. w. Their northern border coincides with the northern border of date palm cultivation. Deserts and, first of all, semi-deserts - vast transitional areas to real deserts - are gradually replaced by a wide variety of plant communities. Towards the equator from subtropical deserts and semi-deserts there are communities of tropical savannas, thorny bush steppes, thorny woodlands and communities of tropical grasses, and towards the poles there are areas with a wet winter period, which are characterized by communities of hard-leaved plants and subtropical winter-green steppes. Moderately hot, but cold in winter time Desert and semi-desert years (desert steppes) border mainly on steppes, which are also cold in winter.
Let us name the most important subtropical deserts and semi-deserts of the northern hemisphere: the North African-Arabian deserts (of which the Sahara alone occupies an area only slightly smaller than the area of the whole of Europe), the deserts of the Iranian-Pakistani-Indian region (Dasht-Lut and Thar), as well as the deserts and semi-deserts of the south -western North and Central America (Sonoran Desert). In the southern hemisphere: Chilean-Peruvian coastal desert South America, coastal Namib desert, Kalahari desert and Karoo semi-desert in southwest Africa, as well as semi-deserts of Central and South Australia. The cold winter arid regions of the mid-latitudes of Asia include the Central Asian deserts and semi-deserts of the Iran-Turan desert region (Northern Iran, the Aral-Caspian lowland with the Karakum and Kyzylkum deserts), the Kazakhstan-Dzungarian semi-desert region with the Hungry Steppe (Kazakhstan from the lower Volga and further east through the territories adjacent to the Aral Sea to Lake Balkhash), the Central Asian desert region of Mongolia and northern China(Gobi, Taklamakan, Beishan, Alashan, Ordos and Tsaidam deserts), as well as the cold high-altitude deserts of Tibet (cold deserts). In North America, the cold-winter semi-desert is found in the Great Basin highlands region between the Rocky Mountains and the Sierra Nevada range. Finally, in the southern hemisphere, the cold winter semi-desert is located in Argentina; this is a vast Patagonian shrub semi-desert, occupying areas poor in sediment (to the west of them are the Andes).
Climate and soils. Deserts and semi-deserts are characteristic of arid regions globe. The latter differ from other regions in the insignificant amount of precipitation and strong evaporation of moisture: here the amount of moisture evaporating from the open water surface during the year exceeds the annual amount of precipitation falling over the same area. In areas with an arid climate, due to the predominance of upward flow of soil water, soil salinization (saline soils) often occurs. Drainless lakes and beds of temporary (drying up) watercourses are also often found. In deserts and semi-deserts you can also find quite large rivers, but their sources are located outside arid zones. They often flow into closed lakes. Consequently, semi-deserts and deserts are self-draining territories that do not have surface water flow.
Their climate is very different. First of all, as we have already said, temperature conditions they are divided into hot subtropical and moderately hot, but with cold winters, as well as cold high-mountain deserts and semi-deserts. In terms of the amount of precipitation they receive, they are also very different: from extremely arid areas, where there is no rain at all or it is extremely rare and irregular, to arid areas with summer rains and winter droughts or, conversely, with rainy winters and dry summers; There are areas with two short wet periods and those whose humidification occurs almost exclusively with fogs.
The existence of areas poor in precipitation, especially characteristic of the subtropical zone, is explained by the presence in them of more or less permanent areas of high atmospheric pressure; downdrafts of air masses dissipate clouds, and therefore drying occurs. Dry trade winds blow all year round. This subtropical belt of dry areas, dependent mainly on air circulation, is located in western parts continents (Baja California and western South America, the Sahara and southwestern Africa, as well as Southwestern Australia) behind high, sediment-retaining mountains, extends far into temperate latitudes, for example, into the Great Basin of the American Rockies, into Patagonia, closed by the Andes, and also to desert and semi-desert Central Asian regions surrounded by high mountain ranges.
The sky over deserts and semi-deserts is almost always cloudless, which leads to extremely sharp temperature fluctuations during the day. Thus, at noon, the ground air layers can warm up to 60°C and above, and at night the temperature can drop to several degrees above zero; daily temperature fluctuations of 40-50°C are not uncommon here.
In determining the nature of the vegetation cover of areas where there is very little precipitation, along with the macroclimate, the microclimate, which depends on the characteristics of the terrain, also plays a significant role. Differences in relief, as well as soils and soil-forming rocks, determine a significant diversity of plant communities of the same desert or semi-desert. And since water here turns out to be a factor limiting the development of vegetation, in this zone, along with the relief features, the ability of the soil to retain moisture better or worse is no less important.
The water regime of some soils in arid regions is directly opposite to the regime of the same soils in humid regions of mid-latitudes, where there is a lot of precipitation. There, the wettest soils are clay soils, which have the greatest ability to retain water (film water), and the driest soils are sandy and rocky soils. In arid regions, scanty precipitation never moistens the soil to its entire depth and does not saturate it with water; therefore, in clay and loess soils, immediately after rains, only the surface layers become wet. Subsequent intense evaporation of moisture contributes to the rapid drying of the upper soil layer and the appearance of cracks in it as a result of shrinkage, while sandy soils, into which water easily seeps, accumulate a lot of soil moisture. Large pores between soil particles are separated by water-filled soil capillaries, so only the top layer dries out, while the bulk of rainwater remains inside the soil. In rocky soils rainwater seeps into cracks filled with fine soil particles, where evaporation is minimal and conditions for moisture conservation are favorable. It should hardly be surprising that in arid regions there are almost no plants in clayey habitats, while on flat sandy areas there are herbaceous or shrubby plant communities, and even trees often grow in rocky habitats. That is why in deserts and semi-deserts, even where there is very little precipitation, plants still develop in many places, but there is no closed vegetation cover there. In the dry valleys of sandy deserts, there are favorable conditions for the development of plants, since groundwater is at a relatively shallow depth. Here, optimal conditions for plant growth exist at the places where they reach the surface. fresh water, that is, near sources; such places are called oases.
In general, in regions poor in precipitation, almost no soil formation occurs. The reasons for this are wind erosion, which manifests itself very strongly due to the open vegetation cover, the insignificant participation of plants in soil-forming processes (for example, humus does not appear) and the almost complete absence of soil organisms. Since moistening is only periodic and, moreover, short-term, water also hardly contributes to soil formation. Consequently, the properties of such soils are almost entirely determined by their grain size (the granulometric composition of the solid substrate). Certain geological rocks and their derivatives, in the formation of which physical weathering predominates (due to the lack of water, chemical and biological processes play a subordinate role), determine the types of deserts - sandy, pebble, rocky, clayey and loess (the latter are often saline deserts ).
Forms of plant growth. In all arid regions of the globe, a pattern can be traced: in the direction from the steppes and savannas surrounding semi-deserts and deserts, towards the centers of deserts, the vegetation cover becomes increasingly sparse. Its density is proportional to the decrease in precipitation. Where there is a lack of moisture, there are much fewer plants in a certain area than in places with more moisture. In deserts, vegetation develops better in habitats with a favorable water regime for plants, such as the base of slopes, valleys and lowlands. But if in semi-deserts plants are scattered relatively evenly over the soil surface, then in deserts there are large areas where there are no plants at all.
Plants in arid regions have various adaptations that allow them to provide themselves with water; they are able to make maximum use of available water and conserve it, reducing the rate of evaporation (transpiration). By reducing the leaf surface, desert plants develop more root systems. Plants with widely grown root systems often live here, and the roots occupy many times a larger area than the above-ground organs. Thanks to this, they are able to quickly absorb rain moisture from large areas. Other plants, especially sandy desert shrubs, on the contrary, form roots (or widely branched root systems) that go many meters deep: this allows them to use groundwater. The most striking example is the Juzgun clan (Calligonum) from the buckwheat family; These shrubs, distributed from the Sahara to the Gobi Desert, have roots reaching a depth of 30 m. Finally, there are plants with large leaves spread above the ground, which, despite the insignificant air humidity, are able to absorb morning dew.
In addition to adaptations that ensure water absorption, desert plants have another feature: they are able to tolerate even long-term drought. Desert plants can be divided into several environmental groups. The first group includes the so-called annual ephemerals. These are short-lived plants; they develop from seeds immediately after rainfall and often complete the entire development cycle up to the formation of seeds within a few days. At this time, a rare phenomenon occurs - the desert blooms, which can be seen in the picture below. The seeds of these plants remain viable during long periods of drought (so-called latency).
The group of ephemeral geophytes includes perennial plants with typical underground storage organs (tubers and bulbs). They develop leaves above the ground and reproductive organs only for a short time immediately after rainfall. These plants survive drought, which can last for years, in the form of underground storage organs that are dormant.
The third group includes desert plants that can exist with periodic moisture (they are called poikilohydric); these are mainly lower plants, such as some blue-green algae and lichens, as well as mosses, a few species of mosses (Selaginella) and ferns and even a very few flowering plants. All of them are able to tolerate drought in a dormant state, being severely dehydrated. After the rains they turn green, grow and reproduce for a while, and then dry out again.
A widely represented group of desert plants is xerophytes. Their above-ground organs remain alive during dry periods. In deserts and semi-deserts, xerophytes are represented mainly by rigid-leaved shrubs (sclerophyllous xerophytes), which, thanks to their highly branched and deeply penetrating root systems, receive required amount water and during drought. To reduce moisture evaporation, their leaves are densely pubescent or greatly reduced. In extreme cases, assimilation is carried out by shoots that look like leafless rods or thorns. To limit water evaporation, some of these plants shed leaves and even entire branches during dry periods. When there is a lack of moisture, their stomatal fissures close. Typical examples of such xerophytic plants of deserts and semi-deserts are representatives of the genera (Tamarix) from the comb family (Tamaricaceae), juzgun (Calligonum) from the buckwheat family (Polygonaceae), parfolia (Zygophyllum) from the family Zygophyllaceae, and in addition, many species from the families Ephedraceae and Capparidaceae.
Finally, a group of succulents should be mentioned. True (non-halophilic) succulents contain a supply of water in their leaves, branches, trunks, or underground organs that is replenished during rainfall. During dry periods, the evaporation of moisture both into the atmosphere and into the soil is extremely limited. At the same time, metabolic processes and, as a result, plant growth slow down greatly. Typical representatives of true succulents: cacti (family Cactaceae) of the American semi-deserts, as well as morphologically similar plants from other families (euphorbias, crassulaceae, species of the genera Senecio and Aloe, etc.), especially often found in the South African Karoo semi-desert.
In deserts, and especially sandy ones, under the influence of wind, intensive movement of particles of the substrate on which plants live occurs. In order for perennial plants to exist in such conditions, special adaptations are required. Like our grasses that take up residence on the dunes, these plants must also resist dormancy; therefore their shoots grow quickly. These shrubs and grasses should rise above the drifting sands deposited around them.
For plants in arid regions, along with lack of moisture and strong wind erosion great importance has soil salinity. As a result of intense evaporation of water, the soils of both periodically and constantly moistened habitats accumulate easily soluble salts. This primarily applies to areas where groundwater lies close to the surface and there is an upward flow of moisture in the soil, to lowland habitats where puddles form for a while after rains, as well as to drainless lakes of deserts. Thus, all desert and semi-desert habitats that are best supplied with water are threatened by salinization (salinization) of soils. The same picture is observed in areas with artificial irrigation. Many arid regions are characterized by vast areas of saline and solonchak soils located in the lowlands. Most often they contain sodium chloride and magnesium chloride, as well as calcium sulfate (gypsum). But the latter is poorly soluble in water and therefore is of secondary importance during soil salinization. On saline soils, halophytic plant communities typical for such places develop. In order to survive, halophytes must adapt to relatively high salt content in soils. This is facilitated by the inherent salt tolerance of halophytes in their cytoplasm, which is associated with the entry of salts into the cell sap and their accumulation in it. Thus, a high content of sodium chloride is found in the cell sap of halophytes growing on soils containing chlorides. Chlorides cause swelling of the cytoplasm, which leads to an increase in cell volume (hypertrophy). This is precisely what explains the fleshiness (succulent nature) of the plants of this group. Halophytes growing on alkaline soils also show succulence, while halophytes growing on soils containing sulfates do not show succulence, since the protoplasm contracts under the influence of sulfates. The content of soluble salts in halophytes reaches 35% of the weight of plant dry matter.
That's it in a nutshell general characteristics living conditions of plant organisms in deserts and semi-deserts, as well as specific adaptations of plants to these conditions. Let us move on to a description of the main deserts and semi-deserts of the Earth and the conditions of existence of plant communities characteristic of them.
Semi-desert temperate zones natural land areas in the temperate zones of the Northern and Southern Hemispheres with a predominance of semi-desert landscapes. The largest area is occupied in the interior of Eurasia, where they extend (approximately 10 thousand km). km) from the Caspian lowland in the north to the eastern edge of the Ordos plateau in the east; the width of the strip of semi-deserts, within which plains predominate, reaches in some places 500 km. In North America P. z. u. p. are located in a meridionally elongated strip of the foothills of the Rocky Mountains and basins of the Great Basin, where they alternate mosaically with desert landscapes. IN Southern Hemisphere distributed in the south of South America (to the east of the Andes, in Patagonia). Climate P. z. u. The Northern Hemisphere is arid, continental, with cold winters and long, hot and dry summers. The radiation balance is about 5 Mj/m 2 or 120 kcal/cm 2 per year, evaporation is several times higher than the annual amount of precipitation (usually 200-300 mm). The average temperature in July is 22-25 °C, in January up to -20 °C. Winters usually have little snow, with strong winds. In the Southern Hemisphere (Patagonia) the climate is less continental. In summer the air temperature is 15-20 °C, in winter - about 1 °C. The Andes retain most of the moisture brought by the dominant western regions. winds, so precipitation falls only 100-150 mm(in some places - up to 250) per year. Surface drainage is poorly developed, many rivers dry up in the summer, and they are usually full of water only in the spring, during the melting of seasonal snows. Significant areas are generally devoid of surface runoff. Numerous brackish and salt lakes. There has been a constant moisture deficit in soils since the middle of the growing season. Light chestnut and brown soils predominate, often in combination with solonetzes; along the depressions of the relief, solonchaks and meadow-saline soils are common. The soils are characterized by complexity and low humus content (1.5-3%); soils are often characterized by a high content of gypsum, carbonates, and the manifestation of solonetzization processes. They are often suitable for agriculture, but require irrigation and, in some places, elimination of salinity and resettlement. The vegetation is xerophilic and often has a complex character. In the semi-deserts of temperate latitudes of the Northern Hemisphere, grass-wormwood communities predominate with a significant participation of ephemerals and ephemeroids. On sandy soils Tree and shrub vegetation is common (elf, birch, pine, dzhuzgun, sand acacia). In the Southern Hemisphere, semi-desert vegetation is sparse, predominantly semi-shrub with the participation of grasses and succulents. Desert and steppe species of animals predominate. Semi-deserts of temperate latitudes are usually good pastures for year-round grazing. M. P. Petrov, Yu. K. Efremov.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
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