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Nature Gallery (Eco Regions)

Sub-Topical Deserts and Semi-Deserts

Cells of atmospheric high pressure maintain hot, dry air over those parts of the Earth just outside the tropics. Rainfall in this zone is scanty and irregular, and is far outpaced by the forces of evaporation. Winters here are not much cooler than the very hot summers, although daily fluctuations in temperature from day to night can be great. Under these conditions, the deserts and semi-deserts of the subtropics have developed.

Desert plants must cope with high heat, intense sunlight, and prolonged drought. Some species sprout, bloom, set seed, and die during the brief period after the rains fall. These are the ephemeral wildflowers, which survive until the next rains as dry seeds. Desert perennial herbs endure the period of drought just as perennials overwinter in a temperate zone garden (underground tubers, bulbs, corms, and rhizomes). Many woody plants in the desert conserve water with small, hard leaves that close their pores during the dry season. Others are deciduous, shedding their thin leaves during the drought and regrowing them in the rainy season. Some species have green stems and can photosynthesize even when in leafless condition. An important group of desert plants-the aloes, agaves, euphorbias, cacti, and other succulents-store water in swollen stems, leaves, or roots.

Subtropical deserts and semi-deserts vary considerably in their aspect, from comparatively lush woodlands and scrub to apparently barren sand dunes and salt flats. In North America, the Mojave, Sonoran, and Chihuahuan deserts of the southwestern United States and northern Mexico support a diverse flora that includes spectacular giant yuccas and cacti. Adjacent to these deserts are semi-desert savannahs, with carpets of grass and stands of mesquite and other woody plants. The Atacama and Sechura deserts along the Pacific coast of Chile and Peru are a response to the offshore Humboldt Current, whose cold waters do not readily evaporate, thus depriving the subtropical coast almost completely of rain. What vegetation there is derives most of its moisture from the fogs that form when the cold current chills the warm air above and causes its moisture to condense. East of the Andes, less rigorous conditions permit the development of the semi-desert shrublands of the Argentine monte. Thornscrub and succulents of another semi-desert, the caatinga, cover most of the arid northeast in Brazil.

In Africa, the extremely arid Namib develops along a strip of the Atlantic coast of southern Africa that is washed by the cold Benguela Current, which exercises the same effect as does the Humboldt Current off South America. The Namib adjoins the Kalahari, which shares the characteristics of semi-desert and dry tropical woodland. To the south lies the Karoo, a semi-desert of succulents and scrubs that contains many unique species. In the north of Africa, the vast Sahara stretches from the Atlantic Ocean to the Red Sea. In its southern reaches, it intergrades with the Sahel, a region transitional between semi-desert and savannah. A casualty of the worldwide problem of desertification, the Sahel is being degraded by a combination of drought, overgrazing by livestock, and firewood collecting.

Subtropical deserts continue eastwards from Africa across the peninsula of Arabia to the Thar and Sind deserts of Pakistan and northwestern India. The final great desert region of the eastern hemisphere is in Australia, where saltbush, a species of acacia called mulga, spinifex grass, and other drought-adapted plants cover much of the landscape in the centre and west.