Plants of the tropical semi-desert. Semi-desert temperate zones

And in and and between and the desert in the tropical zone.

Semi-deserts are formed in conditions. What they all have in common is a long and hot warm period ( average temperature 20-25°C, and in the tropics up to 30°C), strong evaporation, which is 3-5 times the amount (100-300 mm per year), weak surface, inland waters poorly developed, many dry riverbeds, vegetation is not closed.

Despite common features, inherent in all semi-deserts, they also have many differences.

1. Temperate semi-deserts They stretch in a wide strip (up to 500 km) from the western part of the Caspian lowland, through, to the Eastern. In the Northern and semi-deserts, they occur in shorter, broken sections in the interior and foothills. They differ from semi-deserts located in tropical and subtropical zones in cold winters (up to -20°C). here they are light chestnut, which brings them closer to the steppe, and brown desert, often saline. If you move south through the semi-deserts of the temperate zone, you will notice that the features of the steppes are fading away and the features of the deserts are intensifying. There are also steppe feather grasses and fescue, but among them you can already notice wormwood and solyanka. Animals include saigas and turtles; snakes and lizards are more common.

2. Semi-deserts of the subtropical zone.

They are mainly located in the transitional part from deserts to mountain steppes in the form altitude zone in the inland parts and Andes of America, in western Asia, and especially widely in. The soils here are gravelly, gray-brown and gray soils. Cereals and various types shrubs, a wide variety of cacti. From the animal world, snakes and lizards predominate.

These are deserted savannas. They outline deserts, both inland and oceanic - in Africa and, in South America, the north of the Atacama and the northwest of the Brazilian Plateau, in Asia and in Australia.

The soils here are thin, red-brown. The temperature in tropical semi-deserts does not drop below +10°C even in the coldest months, and in summer it rises to 35°C. It rains extremely rarely here. Precipitation is no more than 200 mm per year. If there is a lack of moisture, the bark is very thin. water in tropical deserts They lie very deep and may be partially salinized.

Only plants that can tolerate overheating and dehydration can live in such conditions. They have a deep branched root system, small narrow leaves or spines; Some plants have leaves that are pubescent or covered with a waxy coating, which protects them from sunlight. These include tree grasses, agaves, cacti, and sand acacias.

Natural semi-desert zones are areas of land characterized by a dry climate with significant daily temperature changes and low average annual precipitation (about 150 mm/year). These territories are arid and characterized by poor surface water flow, which determines the increased salt content in the soil. Often, reservoirs and rivers in these areas tend to dry out; during periods of drought, their bottom is covered with a layer of salts. In the semi-desert zone, vegetation mainly consists of grasses and low-growing shrubs.

Characteristics of the natural semi-desert zone.

Semi-deserts are located on all continents except Antarctica, in three climatic zones: temperate, tropical and subtropical. They are dominated by landscapes formed as a result of the action of strong winds; steppe terrain alternates with stone mounds or hills.

Another map showing the location of semi-deserts.

Temperate semi-deserts in Eurasia stretch from the Caspian lowland to the borders of China. IN North America this zone is located in the foothills of the Rocky Mountains and in the lowlands of the Great Basin. In South America it is located east of the Andes. Climate characterized by low winter temperatures (up to -25 degrees) and high summer temperatures (up to +30 degrees). Soils brown and light chestnut (Fig. 1 (1)), with a low humus content (about 2%), their composition includes gypsum and large number salts, in some places solonetzes and solonchaks are widespread (Fig. 1 (2, 3)), which can be used for agricultural use only under the condition of artificial irrigation and reduction of salinity.

The tropical semi-desert zone in Africa combines the features of savannah and desert, located south of the Sahara, in the humid zone of the Namibian desert, in the northeast of the Kalahari Desert, it is also located in India and Pakistan, on the Arabian Peninsula, in South America on the Brazilian Plateau, in Australia. Maximum summer temperatures reach 50 degrees, minimum winter temperatures– about 12-15 degrees above zero. Red-brown clay soils, subject to erosion processes, predominate. (Fig.2)

The subtropical semi-desert zone is found in the mountains of North and South America, Australia, the Iranian Plateau, North and South Africa. The climate is moderate continental with summer temperatures up to 25 degrees, winters are characterized by temperatures dropping to 0 degrees with fairly rare snowfalls. The soils are scanty gray-brown and gray earth with inclusions of crushed stone. (Fig.3)

Salt lake.

Semi-desert in the foothills of the Colorado Plateau.

Deserts and semi-deserts are characterized by harsh weather conditions and unique natural phenomena. Here you can find animals and plants that practically do not use water, moving hills - dunes, evidence of the existence of ancient civilizations.

Deserts are considered natural areas with arid climate. However, not all of them are characterized by hot weather and plenty of sunshine; there are areas that are recognized as the coldest on planet Earth. Semi-deserts represent an average landscape between desert, steppe or savanna and are formed in arid (dry) climates on all continents, excluding Antarctica.

How are they formed

Predisposing factors for the emergence of deserts and semi-deserts are individual for each of them and include territorial location (continental or oceanic), features of the atmosphere and land structure, uneven distribution of heat and moisture.

The reasons that caused the formation of such natural zones are high indicators solar radiation and radiation, little or no precipitation.

Cold deserts appear for other reasons. In the Arctic and Antarctica, snow mainly falls on the coast, up to internal regions clouds with precipitation practically do not reach. In this case, the annual norm may fall out at one time. As a result, snow deposits form over hundreds of years.

The relief in hot desert zones is varied. They are open to the wind, gusts of which carry small stones and sand, creating wave-like sediments.

They are called dunes, their common type is dunes, the height of which reaches 30 meters. Ridge dunes grow up to 100 meters and have a length of up to 100 meters.

Where are they: location on the map

Deserts and semi-deserts are located in the tropical, subtropical and temperate zones. Natural areas on planet Earth are presented on a map with names.

Mira

IN northern latitudes Deserts and semi-deserts of the subtropical and temperate zones are located. At the same time, there are also tropical ones - in Mexico, on the Arabian Peninsula, the southwestern United States, and the Indo-Gangetic Lowland.

Arabian Peninsula

United States of America

In Eurasia, desert zones are located in the Caspian lowland, on the Central Asian and South Kazakh plains, Central Asia, Western Asian highlands.

IN southern hemisphere natural areas are less common. This includes a list of names: Namib in the Republic of Namibia, desert zones of Peru and Venezuela, Gibson, Atacama, Victoria, Kalahari, Patagonia, Gran Chaco, Great Sandy, Karoo in South-West Africa, Simpson.

Namib and Kalahari

Venezuela

Victoria, Gibson, Great Sandy, Simpson deserts

Patagonia

Gran Chaco

One of the largest deserts in the world, the Rub al-Khali, occupies a third Arabian Peninsula. Tourists visiting Dubai often choose safari excursions to hot places.

The vast deserts of Israel are represented on the map - these are the Judean and Negev.

Polar natural zones are located in the periglacial regions of Eurasia, on the islands of the Canadian archipelago, in northern Greenland.

Greenland

The desert areas of Asia, Africa, Australia are located at a level of 200-600 meters above sea level, in Central Africa and North America - 1000 meters. Borders between deserts and mountains are common. They impede the progress of cyclones. Most of the precipitation falls only on one side of the mountainous area; on the other it is absent or present in small quantities.

Sources of information about how many deserts there are on earth put the number at 51, with 49 being real (not icy).

Russia

The country occupies a vast area with different types of climate, so the answer to the question of whether there are deserts in Russia is affirmative. There are not only hot zones, but also cold ones. On the territory of Russia, deserts and semi-deserts are distributed from the Caspian lowland to China, in the east of Kalmykia and in the southern part Astrakhan region. On the left bank of the Volga, deserts and semi-deserts stretch to Kazakhstan. The Arctic zone is located in the region of the northern islands.

As you can see in the picture, semi-deserts are located in the northern part and are characterized by a steppe landscape. To the south, the climate becomes arid and the vegetation thins out. The desert zone begins.

The largest desert in Russia and Europe is called Ryn-Sands, located in the Caspian region.

Species

Depending on the type of soil and soil, there are types of deserts:

• Sand and sand-crushed stone- are formed on loose sediments of ancient alluvial plains. In different territories they are called differently: in Africa - ergs, in Central Asia- kumami, in Arabia - nefudami. At the same time, sands do not occupy the largest part of the desert zone. For example, in the Sahara they make up only 10%.

  

  Sandy deserts

  

  Sand-gravel deserts

• Rocky (hamads), gypsum, gravelly, gravelly-pebble- their location on mountain ranges, hills, low mountains, and so on. The formation of a hard surface is due to the physical weathering of material from rock cracks, which fills depressions. This species is the most common - in the Sahara it covers 70% of the territory.

  

• Salt marshes. Characterized by a high concentration of salts. The territories are covered with crust or quagmire, capable of sucking in a person or animal.

  

• Clayey- the surface of the territory is a clay layer characterized by low mobility and low water properties(dry quickly and do not allow moisture to penetrate under the clay).

  

• Loess- are formed in areas of accumulation of dusty, porous particles. They are characterized by heterogeneous terrain, the presence of a network of potholes and ravines.

  

• Arctic- distinguish between snowy and snowless (dry). The former occupy 99% of the area of ​​Arctic deserts.

  

  Arctic snowy deserts

  

  Arctic snowless deserts

Depending on the nature of precipitation, deserts are distinguished:

The driest desert is Atacama

Atacama is located on the west coast of South America in Chile. The coastal desert is located at the foot of the mountains, covering it with ridges from the rain, cold sea ​​waters wash the hot shores.

The Atacama is considered the driest natural zone, with an average rainfall of 1 millimeter per year. In some areas, rain occurs once every few decades. There was no significant precipitation from 1570 to 1971. Some weather stations in the desert area have never recorded rain.

It happened there in 2010 anomalous phenomenon- snow fell, covering several cities with snowdrifts.

In Atacama there is the famous eleven-meter sculpture “Hand of the Desert”, depicting a human palm, which protrudes three-quarters out of the sand. It symbolizes loneliness, grief, injustice, helplessness.

Atacama is famous for a mysterious discovery - a humanoid mummy discovered in 2003 in the village of La Noria. Its size is 15 centimeters, instead of the usual 12 ribs there are only 9, the skull has a pronounced elongated shape. For external resemblance with an alien creature, she was called the “humanoid Atacama”.

However, scientists in their reports after research are inclined to the earthly origin of the mummy girl. She probably suffered from the disease progeria (rapid aging) and died either in the womb or after birth. There is a version that she lived for 7 years - this is due to the age of the skeleton.

In the desert on Mount Cerro Unica there is the largest anthropomorphic geoglyph - a drawing 86 meters long, whose age is about 9 thousand years. He is called "Tarapaca", the Giant. The creators are unknown; the entire image can be viewed from an airplane.

The largest hot desert is the Sahara

The natural area is located on the territory of 10 countries: Algeria, Egypt, Morocco, Libya, Mali, Niger, Mauritania, Chad, Sudan.

Its definition of “Queen of the Deserts” is due to its huge territory (9,065,000 square kilometers). Many areas of the zone are uninhabited; settlements are observed only near reliable sources of water and vegetation.

Sahara is full of secrets and mysteries.

It is known for mirages that confuse travelers on the right path and doomed to death. People imagine oases, lakes and even entire cities, but it is impossible to get closer to them - they move away until they disappear completely.

The version that explains the phenomenon calls a mirage a kind of lens that visually brings objects closer that are actually much further away.

Made for tourists special cards indicating the places where phantom images are likely to appear.

In the Sahara on the territory of Mauritania, astronauts discovered an amazing object - a ring with a diameter of 50 kilometers, called the “Eye of Africa” or “Richat Structure”.

Its age is estimated at 500-600 million years, its origin is unknown.

The largest cold desert is the Antarctic

In terms of area, it is recognized as the leader among all desert places, even ahead of the Sahara. According to Wikipedia, the area of ​​the polar zone is 13,828,430 square kilometers. Located on the island and mainland land of Antarctica.

In winter, the air temperature drops to -70 degrees, in summer the typical level is from -30 to -50 (not higher than -20). On the coast of the Antarctic Peninsula, it is possible that in summer the indicators may rise to 10-12 degrees.

Precipitation is in the form of snow, its amount varies from 30 mm to 1000 mm per year. Strong winds, storms and blizzards are typical. Nature is poor, vegetable and fauna poor and monotonous.

The most popular desert is the Mojave

Located in the southwestern United States of America, most of the territory is uninhabited.

However, the desert is popular with tourists; the large cities of Lancaster, St. George, Henderson and, of course, gambling Las Vegas are located here.

Famous museums national parks, nature reserves in the Mojave. Among them, Death Valley stands out. This national park, where the bizarre shapes of salt flats, canyons, sand dunes, and valleys are presented.

Even an experienced tourist finds it difficult to navigate such diversity. Poisonous snakes, spiders, scorpions, coyotes will not let you lose your vigilance.

Description of desert places

Natural areas are characterized by a variety of landscapes and climates. Despite harsh conditions, deserts and semi-deserts are home to adapted species of animals, plants, and insects.

People also inhabit hot zones, farm, and find ways to interact with nature. However, in vast areas, due to the harsh environmental conditions, there is no life, and existence there becomes impossible for almost all organisms.

Soil

In desert zones, poor development of soils is noted, in which water-soluble salts predominate over organic components. Vegetation cover makes up less than 50% of the surface or is completely absent.

Gray-brown soil is characteristic of high plains.

In deserts and semi-deserts, salt marshes with a 1% concentration of easily soluble salts are often found.

Groundwater is predominantly mineralized. When they reach the surface, the soils are located in its upper layer, forming salinity.

The soil in subtropical deserts and semi-deserts is orange and brick-red. Such soils are called red soils and yellow soils.

In northern Africa, South and North America, gray soils are found in deserts.

Climate

The climate in deserts and semi-deserts depends on its location. It is arid, hot, the air is poorly humidified, and practically does not protect the soil from solar radiation.

The average temperature is +52 degrees, the maximum is +58. Excessive heating is associated with the lack of clouds and, accordingly, protection from direct sunlight. For the same reason, at night the temperature decreases markedly, since heat is not retained in the atmosphere.

Daily amplitudes in the deserts of the tropical zone are up to 40 degrees, in the temperate zone - up to 20. The latter are characterized by significant seasonal fluctuations. There are hot summers with temperatures in the range of +50 degrees and harsh winters, in which the thermometer drops to -50, while the snow cover is small.

In hot deserts, rain is rare, but sometimes there are heavy downpours, during which water does not soak into the soil. It flows into dry channels called wadis.

A characteristic feature of deserts is strong winds with a speed of 15-20 meters per second, sometimes more.

They transport material located on the surface, forming sandy and dust storms.

The desert zones of Russia are characterized by a sharply continental climate: dry and harsh with strong daily and seasonal temperature changes. In summer the level reaches more than +40 degrees, in winter it drops to -30.

Evaporation of precipitation exceeds the amount of precipitation; it is mainly observed in spring and summer.

Characterized by strong winds, dust storms and dry winds.

IN arctic deserts there are no transition seasons. The polar night lasts 90 days, winter sets in with temperatures down to -60 degrees. Then summer comes with the polar day. It does not last long, and the temperature is within +3 degrees. The snow cover is constant, winter comes in 1 night.

Animal world

Living organisms living in deserts and semi-deserts have managed to adapt to harsh conditions.

When exposed to cold or heat, they hide in burrows and feed on insects and underground parts of plants.

jungle cat

Carnivorous animals of desert zones include fennec foxes, jungle cats, pumas, and coyotes.

In the semi-desert you can meet a tiger.

Some representatives of the animal world have a developed thermoregulation system. They can withstand fluid loss of up to a third of their own body weight (camel, gecko), and certain types of invertebrates - up to two-thirds of their weight.

North America and Asia are inhabited by a large number of reptiles: lizards, snakes, and insects, including poisonous ones.

The large mammal saiga is also considered an inhabitant of hot natural zones.

In the Chihuahuan Desert, located on the border of Texas, New Mexico and the Mexican states, pronghorn are often found feeding on all plants, including poisonous ones.

In the hot natural zone of Danakil, where the air temperature can rise to +60 degrees, wild donkeys, Grevy's zebra, and Somali gazelle live, feeding on sparse vegetation.

Wild donkey

In the deserts and semi-deserts of Russia there are sand hares, hedgehogs, kulans, goitered gazelles, snakes, jerboas, ground squirrels, mice, and voles.

sand hare

Among the predators there are steppe fox, ferret, wolf.

Steppe fox

Spiders also live in natural areas: karakurt and tarantula. Birds include the steppe eagle, white-winged lark, white heron, and so on.

steppe eagle

In the polar deserts the fauna is sparse. Its representatives feed on seafood and vegetation. Polar bears, muskox, arctic fox, seals, walruses live here, reindeer, hares.

Polar bear and walruses

Reindeer

Among the birds, eiders, gulls, terns, penguins and so on stand out.

Penguins

Plants

In deserts and semi-deserts, the flora is not rich and includes prickly cacti, date palm, hard-leaved grasses, acacia, saxaul, psammophyte shrubs, ephedra, soap tree, and edible lichen.

Date palm

Psammophyte shrubs

Sandy natural areas are characterized by oases - “islands” with rich vegetation and reservoirs.

In Russian deserts and semi-deserts there are white and black wormwood, fescue, Sarepta feather grass, and viviparous bluegrass. The soil is not fertile.

Feather grass of Sarepta

Semi-deserts serve as pastures for livestock from April to November.

During some periods, natural areas bloom, filling with rich vegetation. For example, the Kyzylkum desert (“red sands”), which belongs to Uzbekistan, Kazakhstan, and partly Turkmenistan, blooms in the spring with a bright carpet of flowers and herbs.

Subsequently, they disappear under the rays of the scorching summer sun.

In the Taklamakan Desert in western China, most of the territory is completely devoid of vegetation cover; only in rare areas of groundwater do thickets of tamarisk and reeds appear; camel thorn, saxaul, and poplar grow along the river valleys.

camel thorn

IN arctic desert vegetation is practically absent. In summer, the surface of the earth is covered with moss and lichens, there are sedges and cereals, polar poppy, saxifrage, buttercup, and so on.

Locals

People living in hot natural areas are forced to adapt to environmental conditions. IN economic activity highlight pastoralism.

Agriculture is used only in the valleys of large rivers; irrigation is used.

Oil and gas are produced in many natural areas. This is especially true in Asia.

In the deserts and semi-deserts of Russia, irrigated agriculture is practiced in the floodplains and deltas of large rivers (Volga, Syrdarya, Amu Darya). A large number of wells and wells have been created for watering livestock and places for their wintering.

The most severe conditions for economic activity are observed in rocky and gravelly deserts, here agriculture practically absent.

When there is a shortage of water, local residents develop various ways for its extraction. For example, in the driest Atacama Desert, natives use “fog eliminators” - human-sized cylinders - to collect moisture. The fog condenses on the walls of the vessel, made of nylon threads, and flows into the barrel. With its help it is possible to collect up to 18 liters of water per day.

The nomadic inhabitants of Arabia, the Near and Middle East are called Bedouins.

Their culture is based on the invention of the tent and the domestication and breeding of camels. A Bedouin and his family roam on a camel, which carries a portable home and utensils.

Reserves

Human intervention is recognized as the main threat to deserts and their inhabitants. In addition to hunting for rare and endangered species of animals and birds, natural resources - oil and gas - are extracted in these areas.

Technological progress increases the need for them, which leads to an increase in field development. Mining pollutes nearby areas, causing an environmental disaster.

Anthropogenic impact in the Arctic is contributing to the melting of ice, reducing the area of ​​cold deserts. Her disappearance will cause death large number representatives of the flora and fauna of the natural area.

In Russia and all over the world, environmental work is being carried out, national parks and reserves are being created.

Deserts of the world

The bulk of the world's deserts lie on platforms and occupy very ancient land areas.

Asian, African and Australian deserts are located above sea level at an altitude of 200 to 600 m.

Deserts Central Africa and North America lie at an altitude of 1000 m.

Some deserts border mountains, while others are surrounded by mountains. Mountains are an obstacle to the passage of cyclones, so precipitation will only fall on one side of the mountains, and on the other there will be little or no precipitation.

The reason for the formation of deserts is the uneven distribution of heat and moisture, as well as the geographical zonation of the planet.

Temperature and atmospheric pressure create special conditions for the circulation of atmospheric air masses and the formation of winds. It is the nature of the general atmospheric circulation And geographical conditions areas create a certain climatic situation, due to which a desert zone is formed in both the northern and southern hemispheres.

There are different types deserts depending on natural areas and surface type.

Deserts are:

• sandy;
• rocky;
• clayey;
• salt marshes.

Without Antarctica, the planet's deserts occupy 11% of the land surface, or more than 16.5 million square meters. km. They are distributed in the temperate zone of the Northern Hemisphere, as well as in the Southern Hemisphere within the tropical and subtropical zones.

From the point of view of moisture, some deserts have not received precipitation for decades, and deserts in extra-arid regions receive less than 50 mm per year.

Aeolian landforms are widespread in deserts, while the erosional type of relief formation is weakened.

Deserts are mostly drainless, but sometimes they can be crossed by transit rivers, for example, Amu Darya, Nile, Syr Darya, Yellow River, etc.

Drying rivers - in Africa these are wadis, and in Australia - creeks and lakes that change their size and shape, for example, Eyre, Chad, Lop Nor.

Desert soils are poorly developed, and groundwater often mineralized.

Vegetation cover is very sparse, and in very arid deserts it is completely absent.

In those places where there is groundwater, oases with dense vegetation and ponds appear in deserts.

Snowy deserts formed in the Arctic Circle.

Such things can happen in deserts amazing phenomena, which are not found in other natural areas.

Among these phenomena is “dry fog”, which occurs during calm weather, but the air is filled with dust and visibility completely disappears.

At very high temperature The phenomenon of “dry rain” may occur - precipitation evaporates before reaching the surface of the earth.

Note 2

Tonnes of moving sand can produce high-pitched, melodious sounds with a metallic tint, and they are called “singing sands.” You can also hear in the desert both the “sound of the sun” and the “whisper of the stars.”

Stones bursting at 40-degree heat are capable of making a special sound, and at a temperature of -70...-80 degrees, water vapor turns into ice crystals, which collide with each other and begin to rustle.

Definition 1

Thus, the desert is a special natural area having an almost flat surface with sparse or almost absent flora and specific fauna

Semi-deserts of the world

Semi-desert or otherwise deserted steppe is formed under dry climate conditions.

They have specific vegetation and soil cover and are characterized by the absence of woody vegetation.

As a rule, they combine elements of steppe and desert landscapes well.

In the north, the semi-desert is limited by the steppe and desert in the south.

The semi-deserts of the temperate zone go from the west from the Caspian lowland to the east of Asia to the eastern border of China, which is approximately 10 thousand km.

Subtropical semi-deserts are quite widespread on the slopes of plateaus, plateaus and highlands, for example, the Anatolian Plateau, the Iranian Plateau, the foothills of the Andes, the valleys of the Rocky Mountains, etc.

Tropical semi-deserts occupy large areas, especially in Africa, for example, the Sahel zone in West Africa is located south of the Sahara and has the appearance of a deserted savannah.

Russian semi-deserts occupy a small area. This Caspian lowland, which is a transition zone between steppes and deserts. In addition, it is the most northwestern edge of the vast Eurasian deserts.

The Caspian lowland receives the largest amount of total solar radiation per year on the territory of the Russian Plain.

The climate of the semi-desert is continental, which distinguishes it from the steppes. The high summer temperature is pronounced here +22…+25 degrees, and cold winter with a little snow.

January temperatures range from -12...-16 degrees. The winter period is characterized by strong winds, low snow cover and soil that freezes up to half a meter deep. On short spring accounts for the largest amount of precipitation, the annual amount of which is 300 mm with evaporation of 800 mm.

Desert and semi-desert climate

Deserts and semi-deserts of the world occupy several climatic zones - the temperate zone of the Northern Hemisphere, the subtropical and tropical zone of the Northern and Southern Hemispheres, the polar zone where ice deserts form.

The predominant climate is continental with very hot summers and cold winters.

Precipitation, as a rule, is a very rare occurrence for deserts - from once a month to once every few years.

Small amounts of precipitation do not reach the surface of the earth and evaporate immediately into the air.

In tropical and subtropical deserts, the average temperature during the day ranges from +50 degrees during the day to 0 degrees at night. In the Arctic deserts up to -40 degrees.

The maximum temperature, for example, in the Sahara was +58 degrees.

In tropical deserts, daily amplitudes are 30-40 degrees; in temperate deserts, they are about 20 degrees.

During the day, the desert air is also dry - during the day from 5 to 20%, and at night from 20 to 60%.

The driest deserts are those of South America. Low air humidity in deserts does not protect the surface from solar radiation.

In the deserts of the Atlantic and Pacific coasts, as well as the Persian Gulf, the climate is more favorable, because air humidity due to the proximity of water increases to 80-90%, and daily temperature fluctuations decrease. In such deserts there is sometimes even dew and fog.

Temperate deserts are characterized by seasonal fluctuations - warm and even hot summers and harsh winters with frosts down to -50 degrees. There is little snow cover.

A characteristic phenomenon for all deserts is constantly blowing winds. Their speed can reach 15-20 m/s. Their formation is caused by strong heating of the surface and the resulting convective air currents, as well as the terrain, which is why sand and dust storms are frequent in deserts.

The winds have their own names - in the Sahara it is sirocco, in the deserts of Libya and Arabia - gabli and khamsin, in Australia - brickfielder, and in Central Asia - Afghan.

The queen of deserts is the largest among the hot ones - the Sahara, located in North Africa.

For most of the year, the Sahara is influenced by the northeast trade wind. The Atlas Mountains are a barrier to the penetration of humid Mediterranean air into the Sahara.

The July temperature is +35 degrees in the central part, but in many places it is +50 degrees. At night, the thermometer drops to +10...+15 degrees.

Daily temperatures are high and amount to 30 degrees, and on the soil surface they reach 70 degrees.

According to the precipitation regime, three zones are distinguished - northern, central, southern.

In the north, precipitation falls no more than 200 mm in winter. In the central zone, precipitation occurs sporadically, and its average value does not exceed 20 mm. Within 2-3 years they may not fall out at all. But sometimes there are heavy rainfalls in such areas, causing severe flooding.

The Sahara changes its aridity from west to east. The Atlantic coast is dry because the Canary Cold Current, which runs along the western coasts, cools the air and there is often fog.

Due to the condensation of water vapor, precipitation increases slightly at the tops of mountain ranges and in the highlands. The Sahara is characterized by a high degree of evaporation.

“The further you move south, the scarcer the grass cover becomes. The steppe gradually turns into a huge belt of deserts, which 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 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 dry rainwater beds. a few grasses and ugly shrubs grow in the gullies. Low thorny bushes stubbornly struggle with the deadly sand that accumulates around them and threatens to cover these small sandy mounds, from which thorny branches protrude. 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. not seen or heard; only the heavy, rapid breathing of the camels and the rustle of their wide paws are 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 location. 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, having unequal width in different places. 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 northern border 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 tropical savannas, thorny shrub steppes, thorny woodlands and communities of tropical grasses, and towards the poles - areas with a winter wet period, which are characterized by communities of hard-leaved plants and subtropical winter-green steppes. Moderately hot, but cold in winter, deserts and semi-deserts (desert steppes) border mainly on steppes, which are also cold in winter.

Let's name the most important subtropical deserts and semi-deserts northern hemisphere: North African-Arabian deserts (of which the Sahara alone occupies an area only slightly inferior to the area of ​​​​all of Europe), deserts of the Iranian-Pakistani-Indian region (Dasht-e-Lut and Thar), as well as deserts and semi-deserts of the southwest of North and Central America (desert Sonora). In the southern hemisphere: the Chilean-Peruvian coastal desert of South America, the coastal Namib desert, the Kalahari desert and Karoo semi-desert in southwest Africa, and the 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 areas adjacent to the Aral Sea to Lake Balkhash), the Central Asian desert region of Mongolia and northern China (the Gobi, Taklamakan, Beishan, Alashan, Ordos and Tsaidam deserts), as well as the cold high-mountain deserts of Tibet (cold deserts). In North America cold in winter semi-desert is located in the highland region Big Pool 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 on 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 common. 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 they are extremely rare and irregular, to arid areas with in summer rains and winter drought or, conversely, 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, in the Great Basin of the American Rocky Mountains, in Patagonia closed by the Andes, as well as in 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 clayey 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 separate water-filled soil capillaries, so only the most dry top layer, 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 on clayey habitats, while on flat sandy ones there are herbaceous or shrubby plants. 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 dry valleys sandy deserts exist favorable conditions for plant development, 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 openness of the vegetation cover, the insignificant participation of plants in soil-forming processes (for example, humus does not appear) and almost complete absence 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, certain area There are far fewer plants than in areas 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 root systems more strongly. Plants with widely grown root systems often live here, and the roots occupy many times large area than 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 and reproductive organs above the ground surface 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 the necessary amount of water even 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 a lack of moisture and strong wind erosion, soil salinity is of great importance. As a result of intense evaporation of water, the soils of both periodically and constantly moistened habitats accumulate readily 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.