1. Warm deserts
A desert is a land where evaporation exceeds rainfall. No specific amount of rainfall serves as criterion; deserts range from extremely arid to those with sufficient moisture to support a variety of life. Deserts have been classified according to rainfall into semi deserts (rainfall between 150 and 300 to 400 mm per year) true deserts (rainfall below 120 mm per year) and extreme deserts (rainfall below 70 mm per year). Deserts occupy about 26% of the continental area and occur in two distinct belts between 15 degrees and 35 degrees latitude in both the Northern and Southern Hemispheres - the tropic of Cancer and the Tropic of Capricorn. Deserts are a result of several forces. One force that leads to the formation of deserts and the broad climatic regions of the Earth are the movement of air masses. High pressure areas alter the course of rain. Secondly, the influence of mountain ranges causes a rain shadow on the lee side.
All deserts have in common low rainfall, high evaporation (from 7 to 50 times as much as precipitation) and a wide daily range of temperature from hot by day to cool by night. Low humidity allows up to 90% of solar radiation to penetrate the atmosphere. Rain is usually torrential, and runoff instead of penetrating the hard ground resulting into Bad Lands. The unprotected soil erodes easily and further eroded by wind.
Woody stemmed and soft brittle stemmed shrubs are characteristic of desert plants. The shrubs grow with other plants including cacti and succulents. Both plants and animals are adapted to the scarcity of water either by drought evasion or by drought resistance. Plant evaders only flower in the presence of moisture. They persist as seeds during drought periods, ready to sprout, flower and produce seeds when moisture and
temperature are favourable. Drought evading animals may go into a stage of dormancy during the dry season. Nutrient cycling is tight, phosphorous and nitrogen is in short supply.
In spite of the aridity, deserts have not been spared impacts from humans. Only extremely arid deserts have escaped significant disturbance. Desert regions, especially the Middle East have been invaded by the oil industry, radically changing and polluting the desert environment. Irrigation agriculture has turned some areas of desert green, but for how long? Some species in the deserts have been disturbed threatening extinction.
The greatest impact occurs on the semi arid edges of natural deserts of the world, which support some agriculture and grazing. Mismanagement of land has created new deserts. Expanding population and periods on inadequate rainfall encourage encroachment on marginal lands, which are then overgrazed and over cultivated exposing the land to erosion by water and wind. Infrequent rainfall in these areas brings about famine and
further degradation. Eventually the land is destroyed, to a point of no return, - the result is desertification - the creation of new deserts on the periphery of natural deserts. Supplied with water and
managed well, many deserts areas can be converted into productive agricultural land, but poor irrigation practices can lead to salinization.
2. Cold deserts (Tundra)
Tundra means treeless plain (Finnish) and is found in the Northern Hemisphere as a , a frozen plain. At the top of the Northern Hemisphere is the arctic tundra; at lower latitudes is the alpine tundra. The tundra is characterised by low temperatures, a short growing season and low precipitation (cold air carries very little water vapour).
The arctic tundra is a land dotted with lakes and crossed by streams. Where the ground
is low and moist, extensive bogs exist. On high dry areas and places exposed to the wind, vegetation is scanty and scattered and the ground is bare and rock covered. The arctic tundra falls into two broad categories; the tundra desert (100% cover and wet to moist soil), and polar desert (less than 5% cover and dry soil).
Conditions unique to the arctic tundra are a product of at least three interacting forces: permafrost, vegetation and the transfer of heat. Permafrost is the permanently frozen subsurface that may be hundreds of metres deep. It develops where the ground temperatures remain below zero for many years. Its upper layers thaw in summer and refreeze in winter. Because the permafrost is impervious to water, it forces all water to
remain and move above it, resulting in a soggy ground which enables the plants to exist even in the driest part of the Arctic.
Vegetation and its accumulated organic matter protect the permafrost by shading and insulation, which reduce and retard the warming and thawing of the soil in the summer. Any natural or human disturbance can cause the permafrost to melt, and if vegetation is removed, thawing occurs. Permafrost chills the soil, retarding the general growth of both above ground and below ground parts of the plants, limiting the activity of soil
microrganisms and impoverishing the aeration and nutrient content of the soil.
Alternating freezing and thawing of the upper layer of the soil create the unique symmetrically patterned landforms so typical of the tundra. The action of frost pushes material and stones upward and outward from the mass to form a patterned surface. Solifluction is another movement, which forms terraces, through mass movement of super saturated soil.
Alpine tundra has little permafrost, confined mostly to high elevations, but frost induced processes such as solifluction terraces and stone polygons are present. The lack for permafrost results in drier soils, only in alpine wet meadows and bogs of moisture conditions compare to those of the Arctic. Precipitation (snowfall and humidity) is higher in the alpine regions than in the arctic tundra, but steep topography induces a rapid runoff of water.
Structurally the vegetation of the tundra is simple. The number of species is few, the growth is slow, and most of the biomass and functional activity are confined to relatively few groups. Species, which survive, must have the ability to withstand constant disturbance of the soil, buffeting by the wind, and abrasion from wind carried particles of soil and ice. Vegetation is patchy; ground is low covered with cotton grass,
dwarf heath, moss. Well-drained areas support heath grass, herbs, mosses, and lichens. The driest and most exposed sites are usually covered with coarse rocky material and subject to extreme action by frost, can support sparse vegetation. Tropical alpine tundras support tussock grass, small leafed shrubs and heaths. Primary production is low, low temperature, a short growing season ranging from 50 to 60 days in the high arctic to 160 days in the low latitude alpine tundra.
Eskimos lived in harmony with the arctic environment. Western culture has broken down the traditional way of life and weakened the ecological relationship. Winter igloos and summer tents have given way to permanent settlements with wooden houses, dog drawn sleigh have been replaced with snowmobiles and harpoons and spears with rifles. These social and cultural changes have affected the environment negatively. The oil industry with heavy equipment, oil pipelines, oil spills have destroyed the plant cover allowing the permafrost to melt, and resulting in soil subsidence and gulley erosion. Solid waste and sewage pollute streams and surface waters and toxic chemicals drain into the arctic wetlands. Roads have opened up the tundra to recreational developments ski trails and increased human activity damage the soil and vegetation and animal life.
francis1897 answered the question on October 11, 2022 at 07:21