ABSTRACT
Arid ecosystems extend from the tropical to sub-alpine zones and from below sea level to above 3000 m altitude. Due to the extreme variations in geographic and climatic patterns, global arid environment consists of highly diversified and heterogenous ecosystems. According to UNEP (1992) report, 19.6% (25.5 x 106 km2) of the global land area is covered by arid deserts and 17.7% (23.1 x 106 km2) by semi-arid deserts. These figures are continuously changing as a result of man-made processes. The expansion of deserts by human activities, often in combination with adverse climatic conditions, has necessitated an understanding of the biological processes in extreme arid environments. The arid areas experience an annual rainfall of 100-400 mm with a coefficient of variation of 40-70%. Low and erratic distributions of rainfall, coupled with extreme temperature, result in frequent crop failures and affect the agricultural economy considerably in these regions. Further occurrence of frequent droughts lead to wind erosion and creeping sand dunes, covering fertile agricultural lands. Incidence of intense solar radia tion, coupled with high wind velocity, leads to high evapotranspiration rates, which are most detrimental to arid lands.
