Introduction

The Sonoran Desert region and its subdivisons. (Source: Arizona-Sonora Desert Museum)

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 Great Basin Desert

Great Basin Desert. (Source: Arizona-Sonora Desert Museum)

The Great Basin Desert 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. (See Great Basin shrub steppe ecoregion.)

The Mohave Desert

Mohave Desert. (Source: Arizona-Sonora Desert Museum)

The Mohave Desert (Mojave Desert) 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. (See Mojave desert ecoregion.)

The Chihuahuan Desert

Chihuahuan Desert. (Source: Arizona-Sonora Desert Museum)

Though the Chihuahuan Desert 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. (See Chihuahuan desert ecoregion).

The Sonoran Desert

The Sonoran Desert as currently defined covers approximately 200,000 square miles (520,000 sq. km- 100,000 square miles of land and 100,000 square miles of sea) and includes much of the state of Sonora, Mexico, most of the southern half of Arizona, southeastern California, most of the Baja California peninsula, and the islands of the Gulf of California. Its southern third straddles 30° north latitude and is a horse latitude desert; the rest is rain shadow desert. It is lush in comparison to most other deserts. The visually dominant elements of the landscape are two life forms that distinguish the Sonoran Desert from the other North American deserts: legume trees and large columnar cacti. This desert also supports many other life forms, encompassing a rich spectrum of some 2000 species of plants, 550 species of vertebrates, and unknown thousands of invertebrate species. (See Sonoran desert ecoregion.)

Further Reading

• Biomes and Communities of the Sonoran Desert, Arizona-Sonoran Desert Museum.
• Desert Biome, UCMP.