There follows a concise description of how the terrestrial biome is made.

By biome we intend the whole ecosystems of a particular geographical area of the planet, defined according to the type of dominant vegetation found there. The conformation of the terrestrial biomes depends on the climate, the latitude, the altitude, the seasonal cycles, the abundance of the precipitations and the temperature. They vary from the poles to the equator according to the climatic bands: from the tundra of the high latitudes, characterized by low vegetation consisting of moss and lichens, passing to the taiga, represented by vast forests of conifers; then there are the areas of moderate deciduous forest, dominated by broadleaf trees such as birch trees, oaks, beech trees and maples, the grasslands, the deserts and, finally, the tropical rain forest. 

THE RAIN FOREST

This is a type of forest that extends in those regions close to the equator. The term was coined at the end of the XIX century by the German botanist Alfred Schimper. The rain forest is generally constituted by many layers of vegetation. At ground level there are a lot of varieties of plants present, among which are various kinds of epiphytes (such as orchids and bromeliads). There is then a layer of evergreen vegetation, which can be between 30 to 50 m high, dominated by some species of trees that grow much higher than this height. Another characteristic is the presence of the lianas, plants which have a very long stem whose diameter can be over 20 cm, that grow supporting themselves on the other plants, constituting a sort of conjunction between the ground level and the upper canopy of the forest. The word "jungle" derives from the Hindu jangal and in India it is used for pointing out the densest and most impenetrable forests. The widest equatorial forests are found in the basins of the Rio in the Amazon and in Congo, as well as in Southeast Asia. They contain the greatest number of different living species on our planet: It is thought, for example, that millions of insects not yet classified from a scientific point of view are to be found there.

SAVANNAH

This is a type of vegetation which is found in tropical regions, characterized by grasslands on which are scattered bushes and thin trees of various dimensions. It is characterised by the particular conditions of the soil, periodic fires (caused by lightning or by human intervention) or by the climatic conditions. The savannas caused by the climate, such as those of west and southwest Africa, develop in regions characterized by an arid climate, with annual precipitations between 100 and 400 mm; such savannas vary from open canopy forests, with a modest presence of grasses, to real savannas, in which, instead, grasses are dominant and the trees are very thin. With precipitations of only 100 mm a year, only the grasses can bear the arid season; the vegetation expands when the precipitations increase, until it forms a canopy which shadows the grasses when 400 mm a year are reached. In the regions with high precipitations, such as those of East Africa, the vegetation of the savannah is artificially checked through periodic fires that allow the invasion of trees and bushes to stop and favour the growth of new grass. These savannas are wide pastures for great herbivores such as gnus and zebras, which consume the vegetation that has survived the fires; the growth of the forest can, instead, be limited by elephants. The savannas caused by the conditions of the soil include the llanos of Venezuela and the campos cerrados of Brazil; their soil is covered by a hard crust, formed by iron oxides. The grasses grow above the crust, while the trees grow there only where the roots, following cracks in the crust, can reach the deeper underground waters.

STEPPE

The steppe is a biome characterized by grassy vegetation. This environment is generally formed in the tropical, subtropical and moderate regions, in which precipitations are scarce. The steppe is therefore poor in water, except in the short rainy season. The relatively low grassy vegetation and the formation of xerofiles that characterize the steppe are present in regions that have scarce precipitations and an alternation of warm and arid summers with cold and rainy winters. The areas where grassy expanses whose predominant vegetation is formed by bushes and gramineae can assume different denominations according to the continents in which they are found: we can speak therefore of South American pampas, Mediterranean gariga, Eurasian steppe, South African veldt, Hungarian puszta and so on. Beside the grassland steppes of northern Europe, of south western Siberia and of southern Canada (with little rain and formed primarily of evergreen bushes with small leaves) are found the pre-desert steppes of Australia, Africa and the areas neighbouring the desert areas of California and Arizona (with a warm semiarid climate, shrubs and bushes). The fauna of the steppe includes mainly animals which are able to adapt themselves to the difficult climatic conditions and use to their full advantage the land characteristics of the steppes, humid in the lower reaches and rich in humus on the surface. Therefore, among the animals of the steppes are found, among others, numerous arthropods (spiders and insects), reptiles (snakes, sorrels), terrestrial birds (galliform), mammals (great herbivores, such as bovines and equidae, and small carnivorous rodents).

GRASSLAND

The grassland develops in the areas, set at mid-latitude, in which there is not enough humidity to form forests but not so arid in the dry season to be prevent the growth of vegetation. The climate to which this form of vegetation is tied is characterized by cold winters and hot, dry summers, with scarce and irregular precipitations. Among the characteristic of the grasslands are the frequent fires and pasture for livestock or wild animals, two factors which are also determined by the predominantly grassy vegetation of this biome. In fact, graminaceae have the ability to grow much quicker than woody plants after a fire or after grazing by a herd of herbivores. The graminaceae are the prevailing vegetable family of the grasslands, generally mixed with other grassy plants, such as, in the humid regions, numerous kinds of wild flowers. Trees and bushes grow only potentially along the banks of bodies of water or in the areas of transition between humid grassland and moderate forest. The vast extensions of grasslands in the northern hemisphere are situated in North America (the so-called Great Plains) and in Russia. In the United States of America the climate becomes increasingly arid towards the west: it goes from the luxuriant grasslands of the east, characterized by vast expanses of grassy plants, to the wide and arid plains of the west, where the vegetation is constituted by low scattered bushes. In Asia the situation is the opposite: it goes from the grasslands of eastern Europe, which are similar to those of the east of North America, to the dry steppes of Kazakhstan and Mongolia, that resemble instead those of the west of the United States. The land above sea level found in the moderate band of the southern hemisphere are much less wider than those situated in the northern hemisphere, and accordingly there are few vast extensions of grassland in this area. The most important are the pampas of Argentina and Uruguay. Unlike the greater part of the steppes and grasslands, which are generally formed in the inner part of the continents, the pampas extend along the coastal areas of the Atlantic ocean. The factors that favour the growth of the grassy plants in these regions are the semiarid climate together with the flat territory, characterized by little drainage. Other grasslands and steppes of the south hemisphere are found in southeast Australia, in the basin of the river Murray, in some zones of the New Zealand and in South Africa, in the High Veldt (northern Karroo).

TUNDRA

 The arctic tundra of Eurasia and northern America is a particular biome. A series of characteristics particular to the polar environment of the northern hemisphere is responsible for its formation. One of these characteristics is the so-called permafrost, that is, the frozen soil, that can be a hundred meters deep. Another particularity of the arctic tundra is the scarce energy to be had from sunlight that, during the winter months, remains below the horizon, and which is also scarce in summer, when it never sets, because it rises very high in the sky. Finally, another factor that contributes to the formation of the biome of the tundra is its apparently paradoxical condition of cold and humid desert: it is a desert, because the annual precipitations are decidedly scarce, and humid because only a minimum part of humidity evaporates, given the low temperatures. The arctic tundra extends  on a vast flat or undulated territory, without any trees and with the soil saturated with water during the summer months. In fact, when the temperatures rises, the snow melts and water  tends to pool in shallow depressions in the soil, where only the upper layer of the soil thaws, and the underlying permafrost prevents  drainage, provoking the formation of ponds and marshes. A large part of the vegetation that develops near these puddles of water is constituted by moss, bog grass and rushes. On higher and drier ground grow lichens, evergreen dwarf bushes, deciduous willows, birch trees and a lot of grassy species with a so-called  "carpet-like" growth, an adaptation that allows them to resist the strong winds of the tundra and  limits the dispersion of heat. During the long summer days the ponds of the tundra are populated by swarms of insects, attracting great flocks of birds that migrate to the Arctic to look for food and reproduce. The vitality of the tundra is  almost entirely in stark contrast with the polar deserts which contain no life. These are formed in some valleys which have become arid due to strong summer winds that blow from the interior of the continent. Even more deserted are the expanses of perennial ice that cover a large big part of  southern Greenland. In the southern hemisphere, vast expanses of perennial ice cover the greater part of Antarctica, where the regions not affected by this phenomenon are characterized by the polar desert environment. Only some narrow areas, on the borders of the Antarctic continent,  allow the growth of some species of plants such as lichens and moss.

TAIGA

This is a biome prevalently characterized by conifer forests  that stretch out in the northern hemisphere and occupy the northern regions of Europe, Asia and northern America. The taiga,  delimited to the north by the biome of the tundra, covers the sub-arctic regions of the planet. The continental cold climate of these areas, characterized by long, rigorous winters and brief dry summers, makes the vegetation of the taiga less diversified in comparison to that of other biomes. The arboreal species that are more diffused are therefore  pines, firs (above all red and Siberian firs) and the larches, whose leaves in the form of needles  and  pyramidal crowns  allow them to bear the intense snowfalls; there are also, however, forests of birch trees, poplars and alders. In the brushwood are prevalently found  heathers and mosses. The taiga hosts numerous animals, among which big mammals such as the bear, the elk, the reindeer and the wolf; but also animals of more modest size such as the lynx, the wolverine and the sable. Among the birds,  crossbills and nutcrackers are common in the taiga and usually frequent the forests of conifers.

DESERT

Areas that are characterized by scarce and very irregular precipitations (annual average lower than 250 mms) and therefore poor or completely devoid of vegetation. Because of the particular phenomena that characterize them (among which the process of rapid evaporation, strong winds and temperature range), the deserts are extremely inhospitable environments; inhabited by a scarce and very specialized fauna, they are unsuitable to a permanent settlement by man. Present at all  latitudes, the deserts cover 50 million km2, or, rather, around a third of the emerged land. The seasonal differences of temperature let us  distinguish between hot deserts, in which the summers are very hot and the winters warm; cold deserts,  also characterised by very hot summers, torrid even, but with harsh winters; and polar deserts, where the temperatures are always very low.

CONIFER FORESTS 

Conifer forests of the northern hemisphere: they are found in the sub-arctic regions  and at lower latitudes, in mountainous areas at high altitudes (for instance in the Alps). The forests situated at high latitudes are characterized by the presence of the spruce and the fir; pines, larches and tsugas are instead the dominant plants of the southern latitudes. The conifer forests generally occupy regions that in  past epochs were affected by the phenomenon of  glaciation.

DECIDUOUS FOREST 

The forests composed by deciduous plants or caducous trees, that is to say characterized by a seasonal cycle that foresees the loss of all the leaves at the beginning of the cold season and the renovation of the foliage at the beginning of the warm season, are diffused in the humid regions of the moderate belt, in which a hot season alternates with cold winters and the precipitations, of both rain and snow,  are present during the whole year. Deciduous trees, bushes and woody climbers, both at the tropics and at  mid-latitudes, lose their leaves during the period of drought, which corresponds to winter in the moderate zone. In the regions with very harsh winters, in which the soil freezes, the roots of the trees are not able to absorb water. In spring, the soil of the forest is covered with flowers, thanks to the light of the sun that succeeds in penetrating between the branches of trees and bushes, before these are once more covered by leaves which prevent the heating of the soil. At the end of the long season in which  the growth and the reproduction of the plants occurs, the  change in the colour of their leaves signals that they are preparing for winter. During the cold season, the forest  has a bare and desolate aspect, interrupted only by the presence of some evergreen species. In the coldest regions, proceeding towards the forests of conifers of the North, the landscape is dominated by forests of transition, in which deciduous and evergreen species co-exist. The moderate deciduous forests  are primarily diffused in the northern hemisphere, in which three principal belts can be distinguished. In Europe, the area of deciduous and mixed forests extends from the British Islands to France and the whole of central and eastern Europe, up to the Ural mountains; in eastern Asia, it is diffused in the far east of Russia, in Manchuria, Korea and Japan; in northern America, it occupies a large part of the area between the Great Lakes, the Atlantic ocean and the gulf of Mexico to the south. Although separated by thousands of kilometres, these deciduous forests are very similar, not only in their aspect, but also for the kinds of plants that constitute it : birch trees, hornbeams, alders, beech trees, oaks, chestnut trees, lime trees, elms,

walnut-trees, maples and ashes. The long colonization of these woody regions by man, especially in Eurasia, has reduced these forests to highly limited areas; in some areas of western Europe, deforestation has favoured the development of vast treeless heaths.

MIXED FOREST 

In eastern United States a mixed forest, dominated by deciduous plants to the north and by various kinds of yellow pines to the southeast, has been mostly uprooted or cut down. In New Zealand the western coast of the southern island  includes some of the vastest extensions of native mixed forest and furnishes the largest part of  exported lumber such as the kauri, the rimu, the kahikatea and the totara. In Russia a mixed forest, composed by conifers and broadleaf trees, occupies the central area of the eastern Russian plain, from St. Petersburg in the north, up to the border with the Ukraine to the south. Here the dominant species are oaks, birch trees, maples and the white hornbeams. 

Guido Bissanti