By Edward R. Ricciuti
Although all deserts share certain characteristics, to the ecologist’s eye they can differ significantly from one another. The 440,000 square miles of territory that comprise the Great American Desert of the western United States, for example, encompass four different deserts – the Sonoran, Mohave, Chihuahuan and Great Basin – which are each defined by different plant communities and climates. Arizonans who are desert buffs – a not-inconsiderable multitude – have it made because all of these desert types exist to explore within the state’s borders.
Scientists generally define a desert as an environment receiving less than 10 inches of rain yearly, often much less than that. It is not just a matter of how much precipitation originates aloft. High temperatures can evaporate moisture on the way down, causing most of it to vanish before it hits the ground. Terrain and geography play a role, too. The four American deserts all lie in the leeward rain shadow of mountains, especially the Sierra Nevada and Coast Ranges. As air from the Pacific Ocean moves up the western slopes, it cools and its moisture condenses and falls so that the winds are wrung dry by the time they descend on the east.
The Mohave is the westernmost North American desert. It covers about 25,000 square miles, mostly in California, but extends into Nevada, southwestern Utah and the extreme western part of Arizona. It is the desert that claims Death Valley, at the salty pool at Badwater which at 282 feet below sea level is the lowest location in North America, and one of the hottest places in the world. Year-round, the Mohave mercury can vary greatly, sometimes dipping well below freezing. Its annual rainfall is about 5 inches and its marker plants are the creosote bush, saltbush and the Joshua tree, a huge yucca that towers up to 40 feet above the sandy soil.
The Joshua tree grows only in terrain above 3,000 feet in altitude; despite the depth of Death Valley, the Mohave is largely high desert, between 2,000 feet and 5,000 feet above sea level. It is also a transitional desert, between the even higher, cold Great Basin Desert to the north, and the hotter, lower Sonoran – considered by many as Arizona’s signature desert – to the south.
Covering 130,000 square miles, largely on high plateau above 4,000 feet and extending to 6,500 feet, the Great Basin Desert spans most of Nevada and Utah, fringing into California and Idaho and reaching into central Oregon. Arizona’s share lies largely north and east of Flagstaff, in the extreme northwest and north of the Grand Canyon. As anyone who has coped with winter snows on the canyon’s North Rim knows, it is the coldest, wettest North American desert – with 7 to 12 inches of rain annually. Vast stretches of the Great Basin Desert, with its trademark big sagebrush and blackbrush shrubs, is bland and sere, but in northeastern Arizona, largely between the Little Colorado River and the Hopi Mesas, it blazes with glorious colors. Here, the landscape is daubed with the purples, whites, blues, reds and other colors that tint the shales and sandstones of the so-called Painted Desert. Within its margins lies the famed Petrified Forest and the national park bearing its name.
Stretching over most of southwestern Arizona – and deep into Mexico – the Sonoran is the hottest and, if a desert can be such, the most verdant North American desert. It is comparatively wet; if doused by any more rain than it already gets, it would not qualify as desert. No Arizonan needs to be told how hot this desert gets (at daytime average 90 degrees Fahrenheit, often much more, in summer). Altitude-wise, the true Sonoran Desert lies below 4,500 feet and most is well below that. Plant communities include the palo verde, saguaro cactus, and what is known as the “creosote bush-bur sage” community. There are endless opportunities of deserts to explore in Arizona, among the best being Saguaro National Park outside of Tucson.
More typical of west Texas, southern New Mexico and adjacent Mexico, the Chihuahuan Desert creeps into the southeastern corner of Arizona, mainly Cochise County. Mostly above 3,500 feet and covering almost 140,000 square miles in total, it is higher in altitude than the Sonoran but lower than the Great Basin. Its summer temperature approximates that of the Sonoran, with colder winter temperatures. It is also wet, enough so that some of it approximates grasslands. Large cacti are scarce in this desert. Tarbush, lechuguilla and yuccas are typical plants here.
Desert survival hinges on the ability to get water where it is at a premium and conserving it once gotten. Some desert plants spread wide nets of roots just below the surface. Saguaros have root systems as wide as they are tall, while others, like the mesquite, bore deep to tap water as much as 100 feet below the surface. Even among plants, survival in the battle for water can be a matter of kill or be killed. The next time you are in a desert with creosote bushes, check out the spacing between them. You will find it is relatively uniform, almost as if each bush had been formally planted. There is a dark secret behind this neat arrangement. The roots of the creosote bush produce chemicals that kill competitors threatening to crowd them and steal their water.
As befits organisms with the ultimate green lifestyle, desert plants have evolved a myriad of adaptations for conserving water. During long, dry periods, acacias and the ocotillo shrub shed leaves that would allow water to escape, appearing lifeless; while in reality they shift into low gear to curb moisture use. Some desert plants spend most of their lives as seeds, sprouting into life only when awakened by water. Their time from sprouting to maturity may be a matter of only weeks. The tough, waxy skins of plants like yuccas slow water loss; so does small leaf size. Coats of spines or hair not only protect, but also insulate and hold in moisture, while the pulpy interiors of cacti store water.
Animals face similar water issues. The little kangaroo rat, which frolics about Arizona deserts after dark, is an advertisement for wise water use. The seeds and insects it consumes not only provide immediate moisture, but are broken down within its body to generate the most water possible. Its kidneys concentrate and solidify urine for waterless elimination. Conserving water is literally no sweat for these rodents because they lack glands that produce perspiration. They reduce body heat by exhaling, but their warm breath is cooled as it passes over the lining of their nasal passages, reducing evaporative water loss.
During the heat of the day, a desert lizard may keep cool by facing the sun head on, reducing the amount of body surface exposed to its rays. If you see a lizard positioned this way, you may also notice that its tail is elevated and its body is raised off the ground – additional heat reduction measures. When the midday sun sizzles, and positioning does not suffice, lizards head for cool underground burrows. In the cool early mornings and late afternoons, a lizard may go broadside to the sun, warming up.
Adaptations for survival that have worked for eons may fall flat when human activities disrupt the desert. For all their fierce harshness, deserts are fragile places. Scant water means desert plants grow slowly and take years, even centuries, to recover from serious damage. Once plants and the protection they offer to the thin soil disappear, a vicious circle commences. The soil goes blowing in the wind, leaving behind ground so barren that new plants have difficulty colonizing, resulting in wasteland. According to some calculations, a single motorcycle driving across one mile of desert can dislodge almost a ton of topsoil.
It is no secret in Arizona that when housing and other development spreads in uncontrolled fashion, desert environments suffer and can even disappear. More insidious is the long-term threat to water supplies and to the desert rivers that provide them. The delta of the great Colorado River was once one of the richest estuarine habitats anywhere, but no longer. Today, the river barely trickles into the Gulf, which it had once flushed with nutrients carried from the land. Thus, what goes on in deserts deep within the interior of the continent can have an environmental impact on the very sea itself.
Edward Ricciuti has covered conservation issues around the globe. His specialties include natural history, environmental and conservation issues, science and law enforcement. He was a curator for the New York Zoological Society, now the Wildlife Conservation Society. He has written more than 80 books, with his most recent “The Snake Almanac” (The Lyons Press).