Desert Biome, Arctic and Alpine Tundra
The desert biome of the world covers more than one-third of its land area. The deserts stand out as unique regions, fascinating for survival techniques. Some desert plants, called ephemerals, wait years for a rainfall event. At that time their seeds quickly germinate, and the plants develop flower, and produce new seeds, which then rest again until the next rainfall event. The seeds of some xerophytetic species open only when fractured by tumbling, churning action of ash floods cascading down a desert arroyo, and of course such an event produces the moisture that germinating seed needs.
Perennial desert plants employ other adaptive features to cope with the desert, such as long, deep tap roots (e.g., the mesquite), succulence (i.e. thick, water-holding tissue such as that of cacti and saguarq cactus in Arizona desert of U.S.A.) spreading root systems to maximise water availability, waxy coatings and ne hairs on leaves to retard water loss, leaf less conditions during dry periods, re-effective surfaces to reduce leaf temperatures, and tissue that taste bad to discourage herbivores.
The deserts of the world are classified into (i) hot deserts, principally tropical and subtropical, and (ii) cold deserts, principally in mid-latitude.
Earthï¿½s warm deserts and semi-desert biomes are caused by the presence of dry air and low precipitation from subtropical high pressure cells. The true deserts are under the influence of the descending, drying, and stable air of high pressure system from 8 to 12 months of the year. These areas are very dry, like the Atacama Desert of northern Chile, where only a minute amount of rain has ever been recorded (a 30 year annual average of only 0.05 cm). A few of the subtropical deserts like Atacama (Chile), Western Sahara, Namibia, Arizona (USA) and Great Victoria Desert (Australia) are along the sea coast and are influenced by cold offshore ocean currents. As a result, these are true deserts. These deserts experience summer fog that mists the plant and animal population with moisture needed (Fig. 2.1). Rub-alKhali (Arabian Peninsula) Dast-e-Kavir (Iran) Dast-e-Lut (Iran) and are the other important hot deserts.
The cold deserts and semi-desert biomes tend to occur at higher latitudes where seasonal shifting of the subtropical high pressure is of some influence less than six months of the year. Specifically, the interior locations are dry because of their distance from moisture sources (oceans) or their location in rain-shadow areas on the leeward side of mountain ranges such as the Himalayas, Andes and Rockies. The combination of interior location and rain shadow positioning produces the cold deserts of the Great Basin of Western North America, Patagonia, Ladakh, Central Asia, Gobi (Mongolia), and Northwestern China (Fig. 2.1). Winter snow occurs in the cold deserts, but snowing is generally light. Summers are hot with temperature ranging between 30oC to 40oC. Night-time lows, even in the summer, can cool 10oC to 20oC from the day-time high temperature. The dryness, clear skies, and sparse vegetation lead to high radiative heat loss resulting into cool evenings and pleasant nights.
Arctic and Alpine Tundra
The biome in the northern most portion of North America, Europe, and Russia, featuring low ground-level herbaceous plants as well as woody plants (Fig. 2.1). Winters in this biome are long and cold; cool summers are brief. Winter is governed by intensely cold continental polar air masses and stable high pressure anticyclones. A growing season of sorts lasts only 60 to 80 days, and even then frosts can occur at any time. Vegetation is fragile in this flat, treeless world; soils are poorly developed periglacial surfaces, which are underlain by permafrost. In the summer months only the surface horizons thaw, thus producing a mucky surface of poor drainage. Roots can penetrate only to the depth of thawed ground, usually about a meter. Tundra vegetation is low, ground-level herbaceous plants such as sedges, mosses, arctic meadow grass, snow lichen, and some woody species like dwarf willow. The cryospheric conditions of the tundra region are not conducive for the growth of vegetation.
Alpine tundra is similar to arctic tundra, but can occur at lower latitudes because it is associated with high elevation. This biome, is usually described as above the timberline, that elevation above which trees cannot grow. Timberline increases in elevation equatorward in both the Hemispheres. Alpine tundra communities occur in the Andes near the equator, the Himalayas, Hindukush, White Mountains of California, the Rockies, the Alps, and Mount Kilimanjaro of equatorial Africa, as well as mountains from the Middle East to Asia. Alpine meadows (Margs) feature grasses and stunted shrubs, such as willow and heaths.
Written by princy