Overview

Petroglyph at Cocoraque Butte, Ironwood Forest
National Monument. Source: BLM The Sonoran Desert as currently defined covers approximately 200,000 square miles (520,000 sq. km), including100,000 square miles of land and 100,000 square miles of sea, and comprises 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 latitudeA horse latitude is a subtropical latitude between 30 and 35 degrees both north and south. desert; the rest is rainshadow desert. It is lush in comparison to most other deserts.

Subdivisions

The Sonoran Desert can be divided into seven subdivisions based on vegetation, according to a classification first advanced by Shreve (1951) and shown in the map. These ecoregions have sharp definition of vegetation type as well as geology and climate. The names of these subdivisions are:

• Lower Colorado River Valley
• Arizona Upland
• Plains of Sonora
• Central Gulf Coast
• Vizcaino
• Magdalena
• Foothills of Sonora

Landscape characteristics

Subdivisions of the Sonoran Desert, redrawn from Shreve (1951).

The visually dominant elements of the landscape are two lifeforms that distinguish the Sonoran Desert from the other North American deserts: legume trees and large columnar cacti. This desert also supports many other organisms, encompassing a rich spectrum of some 2000 species of plants, 550 species of vertebrates, and untolled thousands of invertebrate species. Much of the northern portion of the Sonoran Desert is drained by the Gila River.

Climate

The amount and seasonality of rainfall are defining meteorlogical characteristics of the Sonoran Desert. Much of the area has a bi-seasonal rainfall pattern, though even during the rainy seasons most days exhibit partial sunshine. From December to March frontal storms originating in the North Pacific occasionally bring widespread, gentle rain to the northwestern two-thirds. From July to mid-September, the summer monsoon brings surges of wet tropical air and localized deluges in the form of violent thunderstorms to the southeastern two-thirds. So distinct are the characters of the two types of rainfall that Sonoran residents have different Spanish terms for them. The winter rains are equipatas (derived from the Yaqui-Mayo word for rain, quepa), the summer rains are las aguas ("the waters" in Spanish).

Distinguishing characteristics

Saguaro Cacti. Carnegiea gigantea. Source: Mark A. Dimmitt. © 2002 ASDM

The Sonoran Desert prominently differs from the other three deserts of North America in having mild winters. Most of the area rarely experiences frost, and the biota are partly tropical in origin. Many of the perennial plants and animals are derived from ancestors in the tropical thornscrub to the south, their life cycles attuned to the brief summer rainy season. The winter rains, when ample, support great populations of annuals (which make up nearly half of the plant species). Some of the plants and animals are opportunistic, growing or reproducing after significant rainfall in any season.

Ecological boundaries

The Sonoran Desert  It is bounded on the west by the Peninsular Ranges, which divide it from the California chaparral and woodlands to the northwest, and Baja California desert - Vizcaino subregion, central and southeast ecoregions of the Pacific slope. To the north in California and northwest Arizona, the Sonoran Desert transitions to the colder-winter, slightly higher elevation Mojave Desert, Great Basin, and Colorado Plateau deserts. To the east and southeast, the deserts transition to the Temperate coniferous forests of the Arizona Mountains and Sierra Madre Occidental pine-oak forests at higher elevations. Finally, to the south the Sonoran-Sinaloan transition subtropical dry forest is the ecotone from the Sonoran Desert to the Sinaloan dry forests of Sinaloa.

See also

• Subdivisions of the Sonoran Desert

Further reading

• Mark A.Dimmitt. Biomes & Communities of the Sonoran Desert, Arizona-Sonoran Desert Museum. 
• Homer Aschmann. 1959. The Central Desert of Baja California: Demography and Ecology. Iberoamericana No. 42. Berkeley, California
• Shreve, Forrest, 1951, Vegetation of the Sonoran Desert, Washington, D.C.: Carnegie Institute of Washington Publication 591
• Forrest Shreve and Ira L Wiggins. Vegetation and Flora of the Sonoran Desert. Stanford: Stanford University Press, 1964