Desert Zones of Subtropical Belts

Desert Zones of Subtropical Belts


natural zones in the subtropical belts of the northern and southern hemispheres in which desert landscapes predominate. Whereas in North Africa these desert zones stretch along the southern Mediterranean coast, in Asia they are not continuous. They occur in parts of southern Middle Asia (south of 40° N. lat.), within the Iranian Plateau, and in patches in the semidesert area of northern Syria and Iraq. The desert zones of the subtropical belts also include the high-altitude “cold” deserts of the Eastern Pamirs and Tibet. In North America, subtropical deserts are found in the southern part of the Great Basin, the Mojave Desert, and the northern part of the Mexican Meseta Central. In Australia, subtropical desert zones occupy the southern parts of the continent, where they are a continuation of the deserts of the tropical belt. In South Africa, the landscapes of the subtropical deserts lie on plateaus and within the intermontane depressions of the Great Karroo. In South America, subtropical desert zones are confined to the eastern slopes and foothills of the Andes, where they alternate with semideserts.

Subtropical desert zones show a great diversity in their topography, which encompasses both the flat coastal plains of the southern Mediterranean and the high mountainous regions of the Pamirs and Tibet. Orographic features are frequently responsible for the existence of subtropical deserts. For example, the subtropical deserts of North and South America are largely caused by orographic barriers that intercept much of the precipitation borne by the westerly winds.

On the plains of the northern hemisphere the climate of such desert zones is marked by hot summers and cool winters. The July temperature normally ranges between 25° and 35°C and the January temperature, between 5° and 15°C. Death Valley in North America, where the temperature may reach 56.7°C, is one of the hottest places on earth. The high-altitude deserts of the Pamirs and Tibet have a cold continental climate. Here, summer temperatures do not exceed 10°-15°C, and temperatures of –15° to –20°C are common in winter. The annual precipitation does not exceed 200 mm, with the most arid deserts receiving less than 50 mm. The evaporation greatly exceeds precipitation. The annual solar radiation is 670-750 kilo-joules per sq cm (160-180 kilocalories per sq cm).

The surface runoff is negligible. Rivers usually contain water only during rains, and they frequently empty into undrained salt lakes.

The vegetation of the hot deserts of Eurasia and North Africa is quite varied. Associations of wormwood (Artemisia) and ephemerals grow on clay soils, psammophytic scrub vegetation occurs on sandy stretches, and halophytes are common in low-lying places. The landscapes of Tibet and the Eastern Pamirs are almost totally devoid of vegetation. Often the plant cover consists solely of sparse growths of low tussocky semishrubs (Fanacetum, Ceratoides, Ephedra), herbacious perennials with woody stem bases (Artemisia, Astragalus), and low-growing turf grasses. Associations of Carex and Kobresia are found in many low-lying areas. The vegetation of the Australian deserts consists of sparse stands of xerophilic shrubs and semishrubs, growing on red and brown stony and gypsum subtropical soils. In South Africa, where the winters are warm, the subtropical deserts support stem and leaf succulents as well as shrubs. Associations of shrub halophytes, including tamarisk, occur on saline soils. The prevalence of succulents (cactus, yucca) is also characteristic of the vegetation of the subtropical deserts of North and South America.

Desert animals have adapted to life in the open spaces. They can go without water for long periods and often have a yellow or yellow-brown adaptive coloration. Jerboas, gerbils, ground squirrels, and antelopes inhabit the Asian deserts, and marmots, lizards, and snakes abound in the high-mountain Asian deserts. Antelopes and hyenas thrive in the African deserts. On other continents the fauna of subtropical deserts is more varied and includes many species that also live in tropical deserts. The desert zones of subtropical belts are used mainly as year-round pastures.