Trees of humid equatorial forests. Tropical semi-deserts and deserts

In terms of ecological conditions and basic features, the nature of these forests does not differ much from the giles of South Asia, but on the southern continents their area is much larger. They occupy vast areas in the Western Amazon and in the central parts of the Congo Basin, as well as the eastern slopes of the Guiana, Brazilian and East African highlands, the highlands of South Africa, the southern slopes of the North Guinean highlands, and the lower mountain belt on the western slopes of the Northern Andes. On the Australian continent, there are such forests only in the extreme northeast, where they have many common flora elements with the South Asian ones.

The humid equatorial and tropical forests of the southern continents, like all gileas, grow in conditions of constantly high temperatures (average monthly - 24-28 ° С) and excessive moisture... The amount of precipitation, uniform throughout the year, usually exceeds 2000 mm. In most cases, the substrate for such forests is loose weathering crusts of various surface deposits, which were formed for a long time with an abundance of heat and moisture. The conditions for the existence of organisms here do not change much over time, but they are very different in different layers of forest vegetation.

The Gilei of the Amazon is the largest wetland massif in terms of area. equatorial forests on the globe... They have their own name - selvas.

Their floristic composition is diverse: there are up to 4500 species of tree species alone, and the total number of flowering plant species reaches 15 thousand. It is difficult to find two identical plants next to each other. Many endemics. The upper tiers of selvas form a continuous canopy: the tallest trees (seibas, copifers, bertholletias, Mauritius palms, etc.) are slightly inferior in height to various legumes. Epiphytes from bromeliads, aroids, cactus, orchids are extremely rich (about 1/3 of all types of flowering plants), there are epiphytic ferns. Real selvas, which in the Amazon are called ete, occupy more or less drained areas - watersheds and slopes (the local name of these lands is terra-firm). On the flooded, swampy floodplains of the river. The Amazon and its tributaries grow poorer in species composition and relatively stunted forests. They are called varzea - ​​forests on high floodplains, and igapo - on constantly flooded low floodplains. The Igapo Formation resembles coastal mangroves. Plants have respiratory roots and all kinds of props. As in the mantras, rhizophores and avicennias are not uncommon here. An interesting participant in these biocenoses is myrmecophilous (living in symbiosis with ants) cecropia, which grows on the floodplains of the rivers of the Western Amazon (riu-brancus). Palms - raffia - are also abundant here.

On the eastern slopes of the Brazilian and Guiana Highlands and in the lower belts of the Northern Andes, forests grow, in terms of the diversity of the floristic composition and structure, very similar to the Amazonian selvas. The differences are explained by the fact that they grow on well-drained slopes. Only at the foot of the mountains, on a relatively narrow strip of coastal lowlands, conditions practically do not differ from those of the Amazon. On the slopes up to an altitude of 1000-1200 meters, gileas dominate with a dominance of palm trees.

The humid equatorial and tropical forests of Africa resemble the Amazonian varzea: the upper tiers in them are thinned out, and the crowns are closed only in the lower tiers. African gileas are inferior to selva in terms of species diversity.

There are much fewer flowering plants here - about 11 thousand species. 70% of them are trees, and only 30% are other life forms. These forests are characterized by powerful ficus “suffocators”. They settle as epiphytes on the branches of other plants, give aerial roots, gradually reaching, take root, and the plant that originally served them as a support dies due to a lack of moisture and nutrients. There are such “stranglers” in South American forests, but they belong to other families (clusia, kusapoya, etc.) and are inferior to ficuses in power. In the flooded gileas of the Congo Basin, muses (a special genus of Cecropiaceae), mitragines and other endemic genera and plant species with stilted and aerial roots grow.

In Australia, tropical rainforests are very small on the northeast coast.

They differ from the South Asian giles by the complete absence of dipterans, which are widespread in Asia. Treelike ferns are abundant in these forests, and eucalyptus trees dominate the south of the tropics.

Under the humid equatorial and tropical forests of all three continents, red-yellow ferralite soils are formed mainly on the weathering crusts of various rocks.

They are all rich in minerals. However, organic matter decomposes quickly at high temperatures. Decomposition products - mobile mineral forms - are immediately consumed by plants or carried by abundant waters into the lower soil horizons. Residual low-mobile hydroxides of iron, aluminum, manganese are deposited in the illuvial horizon, usually in the form of nodules. Humus does not accumulate. For the vast floodplains of the rivers of the Amazon and Congo basins, swamping processes are characteristic: gleying, the formation of peat horizons.

On the sandy terraces above the floodplain, powerful illuvial-humus podzols with signs of soil moisture are formed. The soils of humid equatorial (tropical) forests are usually acidic and infertile (the humus content in the upper horizons is 2-3%).

The fauna of the tropical rainforest zone is unusually rich and diverse. All layers of the vegetation cover are densely populated, although at first glance the forest may seem uninhabited: animals hide in the crowns of trees, among numerous epiphytes and lianas, in hollows, under fallen trunks and in various other shelters. Their presence is betrayed by loud sounds, which are emitted by many animals with powerful vocal apparatus.

South American selvas are distinguished by a particularly rich fauna.

It is home to a huge number of insect species, most of which are endemic to the mainland. Butterflies and beetles sometimes reach gigantic, many of them are brightly colored. A variety of ants live in all layers of the forest.

A small number of ungulates - tapirs, small mazam deer, baker-pigs - lead a terrestrial way of life. Representatives of endemic families of rodents live here: paka, agouti, capybara, the largest of the rodents. The body length of a capybara reaches 120 cm. All land animals are adapted to life near the water. Of the predators, a bush dog is found in the ground tier.

Most of the representatives of the forest fauna South America live in trees. In general, sloths do not descend to the ground. They lead a "hanging" lifestyle, clinging to branches with all four limbs, equipped with strong claws. In a relaxed state, the muscles support the fingers with claws in a closed position. The animal must contract its muscles to open its claws and move its paw, so sloths can sleep in a hanging position. Some anteaters - tamandua and a small anteater with a prehensile tail - freely climb trees. The same tails, which help to move through the trees, are also found in the marsupial rats-stossums, and porcupines-koendu, and raccoon-kinkajou, and many of the wide-nosed monkeys characteristic of South American gilis. Bats, among which there are bloodsuckers, are widespread. Of the predators, some felines hunt the inhabitants of both terrestrial and arboreal tiers - jaguars, jaguarundis, ocelots, as well as coati (coati) from raccoons, marten-taira.

In the jungle, birds are numerous, usually with bright plumage and loud voices. Parrots are especially varied. Hummingbirds, characteristic of South American forests, feed on the nectar of flowers and participate in their pollination.

From reptiles, large boas stand out - the water anaconda and the land boa constrictor. There are many lizards and poisonous snakes. The rivers are inhabited by caimans and some species of real crocodiles.

Amphibians - frogs are diverse. Many of them live in trees and differ in peculiar ways of reproduction.

Ungulates are more widely represented in the African gilea than in the South American one.

There are forest antelopes, water deer, some types of pigs, buffaloes, hippos, okapi (from the giraffe family). Of predators, leopards, jackals, and civets are widespread, and of rodents, the brush-tailed porcupine and spiny-tails capable of gliding "flight". There are two types of lemurs and many monkeys - baboons, monkeys, mandrills, gwerets (colobus). The African forests are home to great chimpanzees and gorillas. There are many types of birds. Brightly colored bananoids (turaco) are endemic. There are several types of parrots. The ecological niche of hummingbirds is occupied by sunbirds. As in all gileys, there are many species of reptiles, amphibians, insects. The rivers are inhabited by blunt-nosed crocodiles, there is a giant frog - goliath.

The fauna of the tropical forests of Australia, like that of the whole continent, is very distinctive. The forests are inhabited by marsupial mammals, which are mainly arboreal.

These are koala, ossums, tree kangaroo. Platypus (from monotremes) settle along the rivers, spending a lot of time in. A very large number of endemic birds: lyrebirds, birds of paradise, cassowaries, weed chickens. There are various parrots. American hummingbirds and African sunbirds are being replaced by honey suckers. Common snakes and some species of tree frogs. In the rivers there is a horn-toothed fish from the relict detachment of the same name.

The indigenous gili types on all three Southern Tropical continents have been significantly altered by human activities. Only in some remote areas of Africa and South America (in the Western Amazon and in the east of the Congo Basin) areas of virgin forests with undisturbed vegetation cover are still preserved. The composition and structure of forest communities is changing as a result of selective felling of valuable tree species, the exploitation of rubber plants and other wild plants that provide valuable products. Big changes are being made by slash and fire methods of farming, which are still widespread in these areas. If small areas of the forest are destroyed, they quickly become covered with very dense thickets (clearings receive a lot of light and heat). But the recovering forest communities differ significantly from the indigenous type in a different species composition, a low height. Required very long terms so that biocenoses close to the original type appear on the site of disturbed forests. Forests suffer the most from clear-cuttings for plantations, industrial construction and laying transport highways... As a result of the reduction of significant areas of the forest, the heat and moisture circulation of the soil is disturbed, nutrients are washed out, in loose surface deposits not fixed by the roots of plants, processes of solifluction and suffusion are rapidly developing, landslides are formed on the slopes. Thus, the lithogenic base itself is disrupted and even destroyed. natural complexes... This impedes the recovery of forest communities. Biocenoses are dying. The process in some cases can become irreversible. The vulnerability of giles threatens their very existence under intense unregulated exploitation.

Seasonally humid monsoon rainforests

As you go climatic conditions from equatorial to subequatorial with a distinct dry period constantly moist forests are replaced by seasonally wet ones. They are often called monsoon. Deciduous plants appear in these forests, losing their foliage during the dry season. In terms of diversity and phytomass, seasonally humid tropical forests are slightly inferior to gilea. South America is especially rich in them, where they are often combined with typical jungle.

They predominate in the Eastern Amazon, where there is a short dry period, and some plants of tropical rainforests can survive only along rivers in the so-called gallery forests. Plakors, on the other hand, are occupied by seasonally humid forests and in places - tall-grass savannas. The more xerophytic variant is widespread in southeastern Brazil, in the basin of the upper and middle Parana, in combination with woodlands and savannas.

In Africa, seasonally moist forests border the gilles. They are park forests dominated by pandanus and ficuses. On the northern border of the gili zone, seasonally moist forests are apparently of a secondary nature.

Here, the conditions are "boundary" for the existence of tropical rainforests: the lush evergreen vegetation of the gili can survive a short dry period due to microclimatic features, within the community, where there is shading, evaporation is reduced, soils are initially overwetted, etc. The biocenosis in its former form is not restored if the vegetation cover is disturbed by felling and fires. Those plants that have been able to adapt to the changed conditions survive, for example, the oil palm, lofira, kaya, terminalia and some others. They are often represented here as species other than in the giley, having adaptations for experiencing unfavorable periods. Evergreen trees usually occupy the lower tiers in such forests, and deciduous species dominate in the upper ones. By southern border African giles are dominated by sparse light forests, they are called miombo.

In woodlands Northern Australia dominated by eucalyptus. Many have vertical leaflets to reduce evaporation during dry seasons. Dense undergrowth, shrub and grass layers develop under their canopy.

Fauna seasonally wet forests differs little from gilles. Many species have either adapted to experiencing a short dry period or migrate during drought to gallery forests along rivers or to neighboring wetter areas.

Savannah and woodlands

With the increase in the duration of the dry season in areas of the subequatorial climate, more and more areas are occupied by savannas. This type of vegetation is very widespread on the southern continents: 85% of the total world area of ​​the zone is concentrated here. In Africa, savannas and woodlands occupy almost half of the mainland (46%), in South America - more than a third (36%), and in Australia - more than a quarter (26%).

The structure and appearance of the zone's vegetation depend on the duration of the dry season and the total amount of precipitation. But on all three continents, as a rule, these are areas with a predominance of herbaceous vegetation and with groves or free-standing trees of a characteristic xerophytic appearance.

The zone of savannas and woodlands is characterized by high average monthly temperatures (15-32 ° С) and a rather large annual quantity precipitation (up to 2500 mm). However, the amount of precipitation varies greatly within the zone and on the border with deserts drops to 250-300 mm.

Most important factor, which determines all the features of the soil and vegetation cover of the zone, is the change of dry and wet seasons at constant high temperatures air. During humid summers with almost daily downpours, vegetation grows rapidly and multiplies. A leaching regime is established in the soil cover. In the dry season, evaporation significantly exceeds the amount of precipitation. The terrestrial organs of plants die off, the trees shed their foliage - the savannahs acquire the appearance of lifeless, red-brown spaces scorched by the sun. The processes of physical weathering and deflation are intensifying. Soil solutions experience capillary uplift, which contributes to salinization. In the upper horizons, iron oxides are also deposited, which cement soil particles. This results in the formation of soils typical of savannahs with a relatively low humus content, usually enriched with iron oxides, sometimes saline.

The land cover of savannahs depends on the length of the dry season. It ranges from 2-3 to 8-9 months. Such differences determine the intrazonal heterogeneity of the soil and vegetation cover.

Where the dry period is short, tall grass savannas or woodlands dominate. Of the grasses, cereals dominate throughout the zone. Trees have devices to protect them from lack of water.

Under tall-grass savannas, mainly red and yellow soils are formed with a humus content of 2-4%, but sometimes up to 8%, with an acid reaction, enriched with iron and aluminum hydroxides. The parent rock for them is often ancient weathering crusts with laterite shells located at a depth of several tens of centimeters to the very surface. With the increase in the duration of the dry season, savannas acquire an increasingly xeromorphic appearance: the grass cover becomes lower and very thinned, succulents appear among the woody elements, and tree forms are often replaced by shrubs. The soils here are reddish-brown - low-humus (less than 1.5%), with a poorly differentiated profile, enriched with iron oxides and carbonates. Under typical savannas, occupying territories with an average dry period, there are red-brown soils with a humus content of 2-3%, an abundance of ferruginous secretions in the form of films and nodules, usually with a carbonate horizon at a depth of 20-30 cm.

The different types of savannah replace each other gradually, as the duration of the dry and wet seasons changes. There are no sharp boundaries between them. However, with a certain degree of conventionality, three subzones with different plant formations can be distinguished: tall-grass savannas and light forests; typical savannas and dry woodlands; deserted savannas, xerophytic light forests and shrubs. To one degree or another, they are common on all Southern Tropical continents, but on each of them they have their own characteristics and differ in floristic composition.

In Africa, savannas and woodlands occupy the largest territories (more than half of the global area of ​​the zone). Savannah formations successively replace each other from the border of the forest zone to deserts. The boundaries in the northern part of the continent run almost sublatitudinally, in the east and south they have a more complex configuration: in some places different types of savannah replace each other when moving from west to east.

The strip of savannahs, as well as of seasonally humid forests, along the border with the gileae in places, is probably of secondary origin. Light woodlands with a fairly large species diversity are combined here with tall grasses. The trees are evergreen (usually in the lower tiers) and deciduous (in the upper tiers). The grass cover of this type of savannah and woodland is dominated by tall grasses, the main type of which is the impera grass. Areas covered with woody vegetation are similar to miombo or other types of seasonally moist woodlands. Gallery forests along rivers are similar to gileas.

In North, South and East Africa, large areas within the subequatorial and partly tropical belts occupy typical savannahs. These are vast expanses of grasses with separate groups of trees of a characteristic xerophytic appearance. The grass cover contains bearded and aristides, many bulbous and rhizome plants from lily, amaryllis, etc. Typical trees include baobabs with thick trunks and powerful bark, mimosa, mainly acacia with umbrella-shaped crowns. There is a dum palm with a branching trunk and a fan palm. During the dry season, the trees shed their foliage and the terrestrial organs of the grasses dry out and burn out.

Deserted savannas have a sparse grass cover of dense sod grasses with the participation of ephemeroids. Of the trees, acacia and arboreal milkweed are common. Thickets of thorny bushes, often called bush, are widespread here. The savannahs of the Sahel zone along the border with the Sahara, the Somali Peninsula, the east of the Ethiopian Highlands, the Kalahari region, and the veldy of southeastern Africa have such a character.

South American savannas are half the size of African ones. Due to the influence of orography and lithological composition of surface rocks, they are represented by separate isolated massifs. Their boundaries are rather submeridional than sublatitudinal.

Tall grass savannas in South America are common on the plains of Orinoco (here they are called llanos), Mamore, Gran Chaco. Depending on local conditions, they are combined with woodlands and are called campos serrados. The most studied llanos Orinoco. Savannahs are formed here in low flooded areas, not so much due to periodic droughts, but as a result of seasonal waterlogging. Tall grasses: bearded, aristides, as in the savannas of Africa, are the basis of the grass cover of the Low Llanos. They are joined by sedges, cyperus and other species growing in swamps. There are few trees - mostly Mauritius palm, which tolerates flooding well. On the better drained Vysokye Llanos, the dry period is well pronounced. The grass cover consists of lower grasses, bearded, paspalum. Low-growing acacias and chaparro (curatella) grow. Similar formations are common in the so-called campos-serrados of the Brazilian Highlands. Cacti and other succulents play a significant role here. In drier conditions, grassy savannas are formed without the participation of trees - campos limpos. In the western part of the Chaco plains, the monte formations are common with the dominance of acacias and an abundance of succulents: cacti, agaves, milkweed, terrestrial bromeliads and even lianas. Also characteristic are sparse forests of kebracho (quebracho) trees with unusually hard and heavy wood.

The most big variety succulent and "barrel" or "bottle" forms with swollen trunks grows in the arid north-east of the Brazilian Highlands in the so-called caatinga. It is characterized by the absence of grass cover for almost the entire year. Herbs only emerge from bulbs and tubers during a short rainy season. There is a layer of undersized cacti, agaves, caesalpinias and bromeliads. Sometimes they are joined by dwarf palms. Kaatinga changes its appearance unusually quickly In dry weather, on the bare rocky soil, there are trees and shrubs devoid of leaves, thorny cacti, prickly pears, agaves. But once the rain passes, in a few hours the foliage unfolds, ephemeroids germinate, and the caatinga turns green. They say that you can fall asleep in the scorched dry landscape, I wake up in the green world of tall grasses, bushes and trees.

In Australia, the zone of savannas and open woodlands covers an almost closed ring from the north, west and east of desert areas. Tall grass savannas of the north and northeast of the mainland are gradually turning into shrub thickets, and along the border with deserts they acquire especially xerophytic features of deserted savannas. They are called scrubs.

The grass cover of the Australian savannas and woodlands, as well as on other continents, is dominated by cereals with the participation of a special species of bearded vulture - blue grass and endemic temeda genera. Of the tree species, the most common are acacias from mimosa, eucalyptus and other myrtle. Casuarins with reduced leaves and xanthorrhea or herbaceous trees close to liliaceae are characteristic. Both are Australian endemics.

The sparse forests of the northeast are called bigelow scrub. In them, plants from other families, including bottle trees of the endemic genus Brachychiton from Sterculiaceae, are added to acacias, casuarins and eucalyptus trees. In drier areas, savannas with acacias are common - mulga-scrub, and in deserted savannas with shrubs (mally-scrub), shrub eucalyptus prevails.

In the presence of similar features (growing conditions, adaptive properties of plants, external appearance, structure of communities), savannahs and woodlands of the southern tropical continents also have great differences. They are especially pronounced in the floristic composition of communities and in the distribution different types savannahs within each of the continents.

The fauna of the savannah has a clear seasonal mode of life. During the dry season, many herbivorous animals take refuge in shelters and either hibernate or live off supplies made in the summer. Others migrate to neighboring zones or accumulate near water bodies. They also often adapt to a change in the type of food and move from one phytocenosis to another. Many species can travel long distances quickly in search of food and water. Predators follow herbivores.

Adaptations to living conditions in animals of the savannahs of Africa, South America and Australia are similar, but the fauna of each of the continents is very peculiar.

There are few ungulates in the savannas of South America. The ecological niche of antelopes and dukers is occupied only by mazam deer. But there are many rodents, including endemic whiskach and tuko-tuko. Hamsters are characteristic. Nutria lives along the shores of reservoirs. Armadillos and anteaters from edentulous and marsupial possums are widespread. Carnivores are not as diverse as in Africa. They are represented by jaguars, pumas, ocelots from felines, and from canines - by maned wolves, savannah foxes and bush dogs. Noses (coati) from raccoons are endemic.

In Australia, almost all ecological niches are occupied by marsupials. Instead of ungulates, savannahs are inhabited by various kangaroos and wallabies, rodents are replaced by wombats and other herbivorous marsupials. There are also some types of common rodents - mice and rats. The trees are inhabited by koalas, feeding on the leaves of certain types of eucalyptus, and ossums. Predators are few in number. The marsupial wolf and the marsupial devil appear to be completely extinct. There is a marsupial cat and several types of carnivorous marsupial rats. The role of predators is performed mainly by wild dingo dogs and introduced by humans to combat the introduced and highly multiplied fox rabbits. It is believed that the dingo dog entered the mainland together with the ancient man in a semi-domesticated state and became feral.

Desertification of savannas. The xerophytic features of the vegetation cover of savannas increase with the duration of the dry season. Savannahs are gradually replaced by semi-deserts and deserts. The position of the border between them shifts one way or the other, depending on climate fluctuations. If abnormally dry years in some period follow one another, savannah communities degrade - desertification occurs. It also happens the other way around: years with an abnormally high amount of precipitation follow in a row. In this case, the border of the savannah shifts towards the zone of deserts and semi-deserts. The desertification process is more active: savannah communities under conditions unfavorable for plants are preserved for some time due to their own microclimate. The restoration of a destroyed community is only due to improving external conditions, and this is a long and difficult process. Desertification is very actively promoted by human activities. Savannah vegetation degrades and is destroyed as a result of land cultivation, overgrazing, burning grasses in a dry period in order to improve pastures: ash fertilizes the soil, besides, livestock more readily eats young grass. Trees and bushes are cut down as this is a source of firewood. As a result, the process of desertification is spreading to all three Southern Tropical continents.

Tropical semi-deserts and deserts

This zone forms where the band begins, in which precipitation falls irregularly. By average annual number rainfall these territories may not yield to the savannah zone, but the rainy season does not happen here every year.

Deserts and semi-deserts are widespread in Africa and Australia. The area of ​​the zone is especially large in Africa. This continent accounts for more than half of the global area of ​​the zone. Different types of deserts occupy a wide strip on the plains and plateaus of the North and South-West and central part South Africa.

The largest tracts of desert landscapes are located in the north of the African continent. There are huge spaces (about 7 million km 2) from Atlantic Ocean to the Red Sea is occupied by deserts, which are united under the general name of the Sahara. These territories are heterogeneous in terms of relief, composition of surface rocks and vegetation.

The vegetation of the Sahara is close to that of the Arabian deserts. It is characterized by milkweed, perennial thorny shrubs (retam, camel thorn, etc.), wormwood and saltwort on saline soils. As in the Asian deserts, the Jericho rose grows here - an ephemeral that uses periods of brief irregular rains for breeding. Stony placers and rocky outcrops are covered with lichens.

Animal world... In terms of the composition of the animal world, the Sahara is very similar to Western Asia and the Mediterranean and is included with them in the Holarctic zoogeographic region.

It is inhabited by various antelopes, overcoming large spaces in search of water and food (addax, oryx, bubal, mendes, etc.), some species of gazelles, from predators - jackals, hyenas, cheetahs, fennec fox, etc. There are birds - African ostrich , bustard, desert raven, etc. There are many reptiles: snakes, lizards, turtles. In rare permanent reservoirs, crocodiles have been preserved - a legacy of the pluvial eras. Among the numerous insects that can tolerate the loss of moisture and heat, there are pests (for example, locusts) and poisonous ones - scorpions, phalanges.

There are desert territories in South Africa... The coastal ("cold", "wet") Namib Desert occupies the western South African plateaus and plateaus, and in the Kalahari Basin it turns into deserted and dry savannas with endemic flora and fauna.

The area of ​​the zone in Australia is large: 1/5 of all the deserts in the world are located on this small continent. They are common in the West Australian Highlands and in the plains of Central Australia.

Vegetation... Where at least a small amount of precipitation falls, spinifex deserts dominate, occupied by rare bushy holly grasses (from the genera spinifex and triodium) with dense turf. In the driest areas of the center of the continent, large areas are generally devoid of vegetation and are stony placers or mobile sands.

In the West Australian Highlands, stony deserts form on thick ferruginous crusts (a legacy of the humid ages). Their bare surface has a characteristic bright orange color. On the Nullarbor Plain, composed of fractured limestones, the desert extends to the southern coast of the mainland. In some places, you can see rare quinoa bushes and some hodgepodge or thickets of dwarf eucalyptus.

Animal world. In the Australian deserts, as in the entire fauna of Australia, among mammals the leading place belongs to the lowest.

The giant kangaroo can traverse large areas. In the shrub thickets, the echidna from monotremes is found. There are many endemic species of reptiles, lizards are widespread (for example, moloch and frilled).

In South America, the territories occupied by tropical desert biocenoses are negligible. Only by west coast between 5 ° and 30 ° S NS. there is a strip of coastal deserts. Small areas of desert landscapes are found in intermontane depressions (in the Precordillera, in the longitudinal valleys between the Coastal and Western Cordilleras, for example, Atacama).

Natural complexes of deserts are extremely vulnerable. Any impact on their components, including biotic ones, can lead to unpredictable consequences and the disappearance of not only individual plant and animal species, but also entire biocenoses. Intervention in natural processes here must be carefully thought out from a scientific point of view and taking into account the long-standing experience of the population of areas with similar conditions.

Altitudinal zonality

This pattern is most pronounced in the Andes of South America. The structure of altitudinal zonation is different depending on the distribution of heat and moisture in this mountain system, which has a huge length from north to south and large differences in the moisture content of slopes of different exposure. In the Andes, there are all types of zonal landscapes - from humid tropical forests to deserts, both hot and periglacial, which are represented by mountainous, including alpine variants. Some formations are found throughout the mountain system, for example mountain meadows - paramos, others are locally distributed.

The range of altitudinal zonality is most fully expressed in the Northern Andes, especially on the western slopes of high ridges, where the landscapes of the true gili of the lower belt are replaced by mountain giley, in which there are almost no palms, tree ferns, bamboos, etc. dominate. The belt of low-growing evergreen forests and shrubs begins even higher. with an abundance of ferns, lyes, mosses, lianas and epiphytes (nephelogylea - "forest of mists"). At altitudes of more than 3000 meters, a belt of mountain meadows and shrubs of a xerophytic appearance - paramos begins, and higher, in the periglacial zone, rocky placers are sometimes covered with mosses and lichens. Such a zonal spectrum is also found in other Andean regions, but in many parts of the system, the structure of the altitudinal zonality is completely different.

Thus, Southern continents They are distinguished by the dominance of zonal landscapes of humid and variably humid equatorial and tropical forests, savannas and woodlands of various types and tropical deserts. Formations in other zones have a more limited distribution.

Humid equatorial forests

Wet equatorial forests

evergreen forests, mainly in the equatorial, less often in subequatorial belts in the north of South America, in Central America, in Western Equatorial Africa, in the Indo-Malay region. Into the bass. Amazons they got the name helium, selva... Distributed in areas with annual precipitation of more than 1500 mm, relatively evenly distributed over the seasons. A wide variety of tree species is characteristic: from 40 to 170 species are found per hectare. Most trees have straight, columnar trunks, branching only in the upper part. The tallest trees reach heights. 50-60 m, trees avg. tier - 20-30 m, lower - approx. 10 m. Many trees have board-like roots, sometimes rising to a height. 8 m. In swampy forests, trees have stilted roots. Change of foliage different types trees happens in different ways: some shed their leaves gradually throughout the year, others only at certain periods. Opening young leaves initially hang as withered, sharply differing in color, which is characterized by a wide range of colors - from white and pale green to crimson and burgundy. Flowering and fruiting also occur unequally: continuously throughout the year or periodically - once or several times a year. Often on the same tree you can see branches with fruits, flowers and young leaves. Many trees are characterized by caulifloria - the formation of flowers and inflorescences on the trunks and leafless areas of the branches. The dense crowns of trees almost do not allow sunlight to pass through, so there are very few grasses and shrubs under their canopy.

In the equatorial forests there are many vines, mainly with woody stems, less often grassy ones. Their trunks reach dia. 20 cm, and the leaves are raised to the height of the tree crowns. Some vines, for example. rattan palms, resting on tree trunks with short shoots or special outgrowths; others, e.g. vanilla, are fixed by adventitious roots; however, most tropical vines are curly. There are often cases when the trunk of a vine is so strong, and the crown is so closely intertwined with several trees that the tree braided by it does not fall after death.
Epiphytes are very diverse and numerous - plants growing on trunks, branches, and epiphylls - on tree leaves. They do not suck out nutritious juices from the host plant, but use it only as a support for growth. Epiphytes from this. bromeliads accumulate water in the rosettes of leaves. Orchids store nutrients in the thickened areas of the shoots, roots or leaves. Breeding epiphytes, for example. ferns "bird's nest" and "antlers" accumulate soil between the roots, epiphytes-sconces - under the leaves adjacent to the tree trunks. In America, even some types of cacti are epiphytes. Wet equatorial forests have been predatory and continue to be destroyed. By now, their area has already halved and continues to decrease at a rate of 1.25% per year. They are inhabited by St. 2 / 3 of all species of plants and animals of the Earth, many of which perish, even without being discovered and explored by man. In place of the destroyed primeval forest, low-growing and very species-poor forests of fast-growing trees begin to grow. With regular fires and clearings, secondary forests are replaced by savannas or pure thickets of cereals.

Geography. Modern illustrated encyclopedia. - M .: Rosman. Edited by prof. A.P. Gorkina. 2006 .

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