Natural History of the
Southwest Fieldcourse

Desert Regions of the Western USA

In broad terms, there are 4 desert and semidesert regions in the western United States. The Intermountain Desert encompasses several regions between the Sierra Nevada and Cascade Mountains in the west and the Rocky Mountains in the east. This desert region is sometimes subdivided into smaller units (e.g., Great Basin Desert and Painted Desert). Because of its northerly location, this desert region receives most of its limited precipitation during the winter as storms move in from the Pacific Ocean. Characteristic vegetation includes big sagebrush (Artemisia tridentata), saltbush (Atriplex), greasewood (Sarcobatus vermiculatus), and rabbitbrush (Chrysothamnus), but there are few cacti and other succulents. We visit the Great Basin Desert on the return trip for the fieldcourse through the Pacific Northwest, and we spend one night at Great Basin National Park in Nevada (near the border with Utah).

Great Basin Desert, Nevada
Photograph by Jenn Nylund, August 1997

The Mojave Desert is in southeastern California and adjacent areas of southwestern Nevada and northwestern Arizona; it includes Death Valley. The driest of the North American deserts, most of the limited precipitation in the Mojave Desert falls during the winter. In some ways, the Mojave Desert is a transition between the Intermountain Desert to the north and the Sonoran Desert to the south. Some species from the Intermountain Desert occur at higher elevations in the Mojave Desert, and some species from the Sonoran Desert occur at lower elevations. However, perhaps 25% of the species in the Mojave Desert are endemic. Although other species, such as the widespread creosote bush (Larrea tridentata) and cacti (Opuntia), are abundant, the species most often associated with the Mojave Desert is the Joshua tree (Yucca brevifolia). Relict populations of desert palms (California fan palms; Washingtonia filifera) also occur near springs.

Mojave Desert, Southern California
Photographs by Bob Nicholson

The Sonoran Desert has the greatest species richness of the North American deserts and is typically subdivided on the basis of vegetation. Most of this desert occurs within Mexico. On our trip, we visit the subdivision known as the Arizona Upland in southeastern Arizona. This region receives precipitation during the winter and during the summer "monsoons". Succulents are abundant and diverse; they include saguaro (Carnegiea gigantea), chollas and prickly pears (Opuntia), and various barrel cacti. Creosote bush (Larrea tridentata), palo verde (Cercidium) and other woody legumes, ocotillo (Fouquieria splendens), agaves (Agave), sotols (Dasylirion), and yuccas (Yucca) are also conspicuous.

Sonoran Desert at Arizona-Sonora Desert Museum, Tucson, Arizona
Photograph by Mark Eberle, March 1999

The easternmost desert in North America is the Chihuahuan Desert, most of which occurs in Mexico. We visit the northern portion of the Chihuahuan Desert in southern New Mexico and West Texas. Most precipitation in this desert falls during the summer. Characteristic species of plants include mesquite (Prosopis), creosote bush (Larrea tridentata), ocotillo (Fouquieria splendens), and chollas and prickly pears (Opuntia), but woody lilies (Agave, Dasylirion, and Yucca) are conspicuous, particularly lechugilla (Agave lechugilla) and palmilla (Yucca elata; also known as soaptree yucca).

Chihuahuan Desert, Carlsbad Caverns NP, New Mexico, and Guadalupe Mountains NP, Texas
Photographs by William Stark, March 2000

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