North America has four major deserts: Great Basin, Mohave, Chihuahuan and Sonoran. All but the Sonoran Desert have cold winters. Freezing temperatures are even more limiting to plant life than is aridity, so colder deserts are poorer in both species and life forms, especially succulents.
The four North American deserts
The Great Basin Desert (plate 10) is both the highest-elevation and northernmost of the four and has very cold winters. The seasonal distribution of precipitation varies with latitude, but temperatures limit the growing season to the summer. Vegetation is dominated by a few species of low, small-leafed shrubs; there are almost no trees or succulents and not many annuals. The indicator plant (the most common or conspicuous one used to identify an area) is big sagebrush (Artemisia tridentata), which often grows in nearly pure stands over huge vistas. (Such cold shrub/deserts in the "Old World" are called steppes.)
The Mohave Desert (plate 11) is characterized largely by its winter rainy season. Hard freezes are common but not as severe as in the Great Basin Desert. The perennial vegetation is composed mostly of low shrubs; annuals carpet the ground in wet years. There are many species of these two life forms, but few succulents and trees grow there. The only common tree species is the characteristic joshua tree (Yucca brevifolia), an arborescent (treelike) yucca that forms extensive woodlands above 3000 feet (900 m) elevation.
Though the Chihuahuan Desert (plate 12) is the southernmost, it lies at a fairly high elevation and is not protected by any barrier from arctic air masses, so hard winter freezes are common. Its vegetation consists of many species of low shrubs, leaf succulents, and small cacti. Trees are rare. Rainfall is predominantly in the summer, but in the northern end there is occasionally enough winter rain to support massive blooms of spring annuals. The Chihuahuan Desert is unexpectedly rich in species despite the winter cold.
