In the semiarid and arid West, the natural vegetation is grass and shrubs. Over a large part of California, this takes the form of a fire-adapted scrub community called chaparral or Mediterranean scrub, in which evergreen, often spiny shrubs form dense thickets. The climate, with rainy, mild winters and hot, dry summers, is like that around the Mediterranean Sea, where a similar kind of vegetation, called maquis, has evolved. However, chaparral and maquis vegetation are not closely related genetically. Humans have greatly altered the chaparral through overgrazing of livestock and other disturbances.
The North American deserts, which are located between the Rocky Mountains and the Sierra Nevada, cover less than 5 percent of the continent. Shrubs are the predominant vegetation, although there are many species of annuals. Desert plants, commonly cacti and other succulents, are sparsely distributed.
In Southern California, Arizona, New Mexico, West Texas, and northwestern Mexico, there are three distinct deserts. The Sonoran Desert stretches from Southern California to western Arizona and south into Mexico. A characteristic plant of the Sonoran is the giant saguaro cactus. To the east of the Sonoran, in West Texas and New Mexico, is the Chihuahuan Desert, where a common plant is the agave, or century plant. North of the Sonoran Desert, in southeastern California, southern Nevada, and northwestern Arizona, is the Mojave Desert, where the Joshua tree, a tree-like lily, is a well-known plant. It can reach 50 feet (15 meters) in height. Creosote bush is common in all three deserts. To the north, the Mojave Desert grades into the Great Basin Desert, which is a cold desert, large and bleak. The dominant plant of the Great Basin Desert is big sagebrush. Plant diversity there is lower than in the hot American deserts.
Deserts dominated by grasses rather than shrubs once occurred at high elevations near the Sonoran and Chihuahuan Deserts. Much of this area has been overtaken by desert scrub, including creosote bush, mesquite, and tarbush. Cattle grazing may have been a factor in this change. Desert is very fragile; even one pass with a heavy vehicle causes lasting damage.
See also: Boreal Coniferous Forest, Eastern Deciduous Forest, Forests, Central Grasslands, Tundra, Coastal Vegetation
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