North American Deserts: Environments and Vegetation

Abstract

North American deserts range from subtropical thorn scrub to high-latitude steppe to extreme barren desert. The deserts of North America are significantly smaller than those in the Old World, and generally not arid enough to be considered true deserts; North America is thus largely typified by semideserts (Shmida and Whittaker 1979; West 1988). The main determinant of arid climates in western North America is the presence of local mountain ranges creating rain shadows in the Great Basin and Mojave Deserts (the Sierra Nevada-Cascades and Rockies), the Sonoran Desert (the Peninsular Ranges and Sierra Madre Occidental), and the Chihuahuan Desert (the Sierra Madre Oriental and Sierra Madre Occidental). In contrast, most of the world’s great arid deserts are created by predictable descending high pressure systems at subtropical latitudes (Shmida 1985). Although a multitude of desert definitions proliferate in the literature, the 120–150 mm isohyet of annual precipitation is generally recognized as the boundary between deserts and semideserts (Shmida 1985) and of arid and semiarid climates (Meigs 1953). Based on this classification, only the western extent of North America’s deserts, which lie adjacent to the primary rain shadow of the Sierra Nevada and associated cordillera, can be considered true arid deserts. Shmida (1985) made a further distinction at the 70 mm isohyet between true desert and extreme desert, which corresponds roughly to the boundary between diffuse and contracted vegetation.