THE DESERTS OF NORTH AMERICA

The North American deserts are highly accessible, well-researched and very diverse, so they provide a good basis for understanding desert ecology. These deserts are found in a broad band running down the western side of the USA and into Mexico. They lie in a large basin between the Rocky Mountains to the east and the Sierra Nevada to the west. They receive relatively little water - typically less than 25 cm (10 inches) per year - because most of the precipitation falls on the higher mountain ranges and not in the lower-lying desert regions (which are in the "rain shadow" of the mountains).

Approximate boundaries of the four main deserts of North America

These North American deserts are grouped into four major types - the Great Basin Desert, Sonoran Desert, Chihuahuan Desert and Mojave Desert - depending on their characteristic physical features (rainfall, topography, soil types) and characteristic vegetation and associated animal communities.

• The Great Basin Desert is the largest desert area of North America. It is also the most northerly, covering most of Nevada (Ne), the western third of Utah (U) and parts of Idaho (Id) and Oregon (Or). It is a cold desert because of its northerly location and its relatively high altitude - most of the land lies above 1200 metres (4000 ft), but in the 'rain shadow' of the higher mountain ranges. Much of the precipitation occurs as winter snowfall, but not all of this melts into the ground because some of it evaporates in spring. The vegetation tends to be very uniform over large areas of this desert. It is dominated by various types of sagebrush, or by saltbush where the soil has a high salt concentration. In fact, the soils often have a high salt content (sodium and calcium ions) caused by evaporation of water in the hot summer months, and no vegetation can grow in the saltiest regions.
• The Mojave Desert occurs further south and covers the southern part of Nevada and part of California (Ca) but elements of it extend into Arizona (Az) where it blends into the Sonoran Desert. Again, it is classed as a cold desert because of the low winter temperatures. The precipitation occurs in winter, usually as rain but sometimes as snow at the higher elevations. The features of the Mohave Desert are difficult to define because of the marked variation in topography, soils and climate. For example, the northern section is composed of low-growing shrubs, similar to those of the Great Basin Desert, whereas the southern section blends into the Sonoran Desert, with extensive tracts of creosote bush. The Mohave Desert includes Death Valley - the lowest (below sea level) and driest of all desert regions, where there may be no rain for several years. The Majove Desert contains some highly characteristic plants and animals - most notably the joshua tree at higher elevations.
• The Sonoran Desert covers the southern part of Arizona and part of California, but extends south into the mainland of Mexico and into the extended isthmus of the state of Baja California (BC) in Mexico. The Sonoran Desert is a hot desert and, unlike all the other desert regions of North America, it receives both winter and summer rains. This pattern of rainfall is caused by the seasonal shifts of major storm tracks across the USA. The Sonoran Desert receives winter rainfall from moisture-laden air carried on winds from the Pacific Ocean, and summer rainfall from air carried northwestwards from the Gulf of Mexico. As a consequence, parts of the Sonoran Desert can support unusually lush vegetation, including several trees and sub-trees, and some very large cacti such as the saguaro and cardon. The Baja California peninsula of Mexico is also included in the Sonoran Desert, but the west-facing slopes of this peninsula receive moisture-laden air from the Pacific Ocean and have some uniquely lush vegetation, including epiphytic plants that gain their moisture from the sea mists.
• The Chihuahuan Desert occupies the extreme west of Texas (Tx) and part of New Mexico (NM), but the largest part of this desert occurs in mainland Mexico. This desert region receives summer rains from the Gulf of Mexico - typically about 20-30 cm per year. Over much of this desert the soils are derived from calcareous rocks and thus have relatively high pH. The Chihuahuan Desert also lies at relatively high elevation (typically about 1200 metres, or 4000 ft) and thus has cool winters with periodic frosts, but the summers are hot. The combination of relatively high rainfall, calcareous soils and cool winter temperatures favours the growth of grasses, yuccas and agaves. There are many small cacti, but few of the larger cacti associated with the Sonoran Desert.

Whilst some of the best desert areas of North America are now protected from development by being designated as State Parks, National Parks or National Monuments, vast areas of land dominated by creosote bush and other less spectacular vegetation are now been used for irrigated agriculture, drawing water from major rivers. When irrigated, these desert can be phenomenally productive because of their year-round warmth and solar intensity.

Irrigated agriculture (right) compared with the natural vegetation (left and foreground)