Four different deserts are distinguished in North America based upon their distinctive plant associations and climatic regimes. Three are hot deserts; one is a cold desert.
The hot deserts.
All three of these deserts are characterized by the presence of creosotebush (Larrea divaricata). It occurs in combination with a variety of other woody shrubs, succulents, and ephemerals representative of the respective desert.
The Mohave Desert falls under the Mediterranean regime of winter precipitation. The Joshua tree (Yucca brevifolia) is an indicator species and other yuccas are frequent. The suite of ephemerals respond to winter rains and carpet the desert floor in early spring. A variety of chollas, barrel and prickly pear type cacti occur.
The Chihuahuan Desert falls under the monsoonal regime of the continental interior, wherein summer low pressure systems draw moisture in from the Gulf of Mexico. This desert is dominated by shrubs, both evergreen and deciduous. Indicator species include tarbush (Flourensia ternua), whitethorn acacia (Acacia constrictor), and creosotebush. Small cacti occur; leaf succulent agaves (Agave spp.) and yuccas are prevalent. Plants bloom in response to the summer rains.
The Sonoran Desert, lying between the Mohave and the Chihuahuan, experiences a double maximum in annual precipitation. Some plants (including many of the ephemerals) bloom in spring in response to winter rains; others bloom in late summer in response to summer rains. This desert is characterized by columnar cacti (Cereus spp.) such as sahuaro, organ pipe, and cardon, but contains the greatest diversity of other forms of cactus, including barrels (Echinocereus), chollas (Opuntia spp.), and prickly pear (Opuntia spp.). Other types of succulents are also abundant.
The fog desert of Baja California is usually considered a part of the Sonoran even though it is distinctly different in composition and characterized by nearly endemic species such as the boojum tree (Idris columnaris) and elephant tree (Bursera microphylla). In the driest parts terrestrial and epiphytic lichens extract their moisture directly from the fog, while vascular taxa take the form of rosettes that intercept the fog and funnel dew toward the plants’ roots.
The cold desert.
The Great Basin Desert, found on the high Colorado Plateau of northen Arizona and in the Great Basin of Nevada and Utah, experiences a long, cold winter and a significant proportion of its precipitation falls as snow. Sagebrush (Artemesia tridentata) is the indicator species of this desert. Sagebrush is often in association with the yellow-blossomed rabbitbrush (Chrysothamnus nauseosus). Succulents are represented primarily by prickly pears. This desert is one of the several communities comprising Merriam’s Upper Sonoran Life Zone.