Values, Alterations and Degradation of the Natural Landscape 89
winter, whereas summer is hot and rainless. Those ecosystems are variable.
Frequent fires are an environmental factor to which they are adapted by
their nature.
5.2.2 grassland
In grasslands, trees are absent or they grow sparsely. Rainfall presents
intermediate values, between those of woodlands and those of deserts,
i.e. 250–600 mm in the temperate zone. Large grasslands appear in the interior of North America, of Europe and Asia, in Central and South Africa,
in the southern part of South America, and in Australia. Probably many of
Russia’s steppes as well as tropical savannahs were once forest zones where
the action of fires combined with intensive grazing from herbivores have
prevented the return of the forest.
Mammals and birds that can be found in grasslands exhibit great variety
(e.g. bison, antelope, wild horse, zebra, rhinoceros, lion, ostrich). Vegetation
mainly consists of short-lived plants and a large amount of organic matter
is continuously added to the ground. The transformation of forest litter
to humus is fast but the decomposition of humus to inorganic materials is
much slower. Thus the grasslands’ soil may contain 5–10 times more humus
than the forest soil. This richness has led to the development of profitable
agricultural exploitations. The subsoil part of the plants forms, together
with the soil, a protective carpet that prevents soil erosion caused by the
wind and runoffs. If the protective carpet is destroyed by ploughing, rapid
erosion may follow.
5.2.3 tundra
Beyond the taiga, the treeless plains of tundra develop in a zone around
the Polar Regions. In those areas, where the average yearly temperature
is below –5°C, the ground defrosts in the summer only at a depth of less
than 1 meter. Atmospheric precipitations are less than 250 mm/year but
due to low evaporation the limiting factor usually is not the water but temperature. There is ecological similarity between the tundra and mountain
regions with high altitude, above the tree line, the so-called alpine zones.
Tundra’s productivity is not very low because a large number of species
have developed admirable adaptations in order to survive in the cold and
take advantage of the short period when the temperature is higher. Primary
producers are mainly lichens, mosses and weeds. Reindeer, wolves and
polar bears are some of the mammals that live there. A variety of migratory
birds nest in the summer and feed on insects and freshwater invertebrates
that exhibit a great seasonal growth.
winter, whereas summer is hot and rainless. Those ecosystems are variable.
Frequent fires are an environmental factor to which they are adapted by
their nature.
5.2.2 grassland
In grasslands, trees are absent or they grow sparsely. Rainfall presents
intermediate values, between those of woodlands and those of deserts,
i.e. 250–600 mm in the temperate zone. Large grasslands appear in the interior of North America, of Europe and Asia, in Central and South Africa,
in the southern part of South America, and in Australia. Probably many of
Russia’s steppes as well as tropical savannahs were once forest zones where
the action of fires combined with intensive grazing from herbivores have
prevented the return of the forest.
Mammals and birds that can be found in grasslands exhibit great variety
(e.g. bison, antelope, wild horse, zebra, rhinoceros, lion, ostrich). Vegetation
mainly consists of short-lived plants and a large amount of organic matter
is continuously added to the ground. The transformation of forest litter
to humus is fast but the decomposition of humus to inorganic materials is
much slower. Thus the grasslands’ soil may contain 5–10 times more humus
than the forest soil. This richness has led to the development of profitable
agricultural exploitations. The subsoil part of the plants forms, together
with the soil, a protective carpet that prevents soil erosion caused by the
wind and runoffs. If the protective carpet is destroyed by ploughing, rapid
erosion may follow.
5.2.3 tundra
Beyond the taiga, the treeless plains of tundra develop in a zone around
the Polar Regions. In those areas, where the average yearly temperature
is below –5°C, the ground defrosts in the summer only at a depth of less
than 1 meter. Atmospheric precipitations are less than 250 mm/year but
due to low evaporation the limiting factor usually is not the water but temperature. There is ecological similarity between the tundra and mountain
regions with high altitude, above the tree line, the so-called alpine zones.
Tundra’s productivity is not very low because a large number of species
have developed admirable adaptations in order to survive in the cold and
take advantage of the short period when the temperature is higher. Primary
producers are mainly lichens, mosses and weeds. Reindeer, wolves and
polar bears are some of the mammals that live there. A variety of migratory
birds nest in the summer and feed on insects and freshwater invertebrates
that exhibit a great seasonal growth.