Desert shrub plant communities commonly dominated by mesquite (Prosopis glandulosa) or creosote bush (Larrea tridentata) are prevalent in the Mojave, Sonoran and Chihuahuan Deserts. These desert shrub communities are often called desert scrub or creosote bush scrub. Desert scrub is not the only plant community in these deserts. The Mojave Desert includes salt desert shrub, pinyon juniper and sagebrush communities (described elsewhere on this website). The Sonoran Desert plant communities include tundra on the San Francisco Peaks near Flagstaff, AZ, coniferous forests in the higher mountain ranges, temperate deciduous forest, grasslands, chaparral, desert shrub and cacti. The Chihuahuan Desert is occupied by desert scrub, grasslands, pinyon-juniper, coniferous forests and riparian vegetation. The Chihuahuan Desert region supports more cacti than any other region in the world. Many botanists believe that these plants originated here or to the south and radiated out through the New World. Common cacti include the prickly pears, hedgehogs, living rocks, nipple cacti, and cory cacti.
Desert shrub plant communities occur in the Mojave, Sonoran and Chihuahuan Deserts (Fig. 1). The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona in the United States. The Mojave Desert displays typical basin and range topography. Higher elevations above 610 m (2,000’) (610 m) in the Mojave are commonly referred to as the High Desert; however, Death Valley is the lowest elevation in North America at 86 m (282’) below sea level and is one of the Mojave Desert’s more notorious places.
The Mojave Desert’s (Fig. 2) boundaries are generally defined by the presence of Yucca brevifolia (Joshua trees), considered an indicator species for this desert. The topographical boundaries include the Tehachapi, San Gabriel and San Bernardino mountain ranges on the west. This mountain boundary is quite distinct since it is outlined by the two largest faults in California: the San Andreas and the Garlock. The Great Basin shrub steppe lies to the north, and the warmer Sonoran Desert to the south and east.
The Sonoran Desert (Fig. 3) as currently defined covers approximately 260,000 km2(100,000 mi2) and includes 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 latitude desert; the rest is rain shadow desert. It is lush in comparison to most other deserts. The visually dominant elements of the landscape are two life forms that distinguish the Sonoran Desert from the other North American deserts: legume trees and large columnar cacti. This desert also supports many other life forms, encompassing a rich spectrum of some 2000 species of plants, 550 species of vertebrates, and unknown thousands of invertebrate species.
The Chihuahuan Desert (Fig. 4) straddles the U.S.-Mexico border in the central and northern portions of the Mexican Plateau. It is bordered on the west by the extensive Sierra Madre Occidental range, along with overlaying northern portions of the Sierra Madre Oriental. On the U.S. side it occupies central and southern New Mexico, the portion of Texas west of the Pecos River, and southeastern Arizona. On the Mexican side, it covers the northern half of the state of Chihuahua, along with the majority of Coahuila, north-eastern Durango, the extreme northern part of Zacatecas, and small western portions of Nuevo León. With an area of about 362,000 km2 (139,769 mi2), it is the third largest desert of the Western Hemisphere and the second largest in North America, after the Great Basin Desert.
The Mojave Desert is found at elevations of 609 to 1524 m (2,000 to 5,000’), and is considered a “high desert.” It is a transition desert between the hot Sonoran Desert to the south, and the cold Great Basin Desert to the north. The climate of the Mojave Desert has extreme fluctuations of daily temperatures, strong seasonal winds, and clear skies.
The Mojave Desert lies in the rain shadow of the Coast Ranges and receives an average annual precipitation of 5 inches. Most of the rain falls between November and April. There is, however, a summer thunderstorm season from July to September, with violent and heavy rainstorms possible. In 1986 only 3.81 cm (1.5 in) of rain fell on the eastern Mojave Desert, while in 1983 precipitation was 16.51 cm (6.5 in). May and June are usually the driest months. During El Niño years more rain falls on the Mojave Desert than usual. The runoff has resulted in shallow ponds in the normally dry washes and playas.
Recorded temperatures have been as low as -13°C (8°F) in January and as high as 48° C (119°F) in August. In May the temperature will begin to climb in excess of 38° C (100°F) and continue into October. The night temperatures in July and August can at times be in the low to mid 30s C (90s F).
In late winter and early spring the wind is a prominent feature, with dry winds blowing in the afternoon and evening. Winds in excess of 40 kph (25 mph), with gusts of 120 kph (75 mph) or more are not uncommon. Although it is windy during all months, November, December and January are the calmest. The humidity is below 40% most of the year. During most winter nights, and during and after summer rains the humidity can get above 50%.The amount and seasonality of rainfall are defining characteristics of the Sonoran Desert. Much of the area has a bi-seasonal rainfall pattern, though even during the rainy seasons most days are sunny. 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.
The Sonoran Desert differs prominently from the other three North American deserts 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 thorn scrub to the south; their life cycles are 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. The Chihuahuan Desert is considered a “high-elevation” desert because so much of the desert lies above 4000’ in elevation. The Rio Grande valley forms the lowest portion of the desert at 305 m (1000’), while the mountain ranges of Mexico approach 10,000’ (3050 m) in elevation.
Winters in the Chihuahuan Desert are typically cool, especially in the northern reaches of the desert where nighttime temperatures drop below freezing over one hundred times a year on average. In the summer, daytime high temperatures in the Bolson de Mapimi in Mexico have reached a reported 50° C (122º F). The dry early-summer months of May and June are typically the hottest part of the year in the Chihuahuan Desert.
The Chihuahuan Desert is dry because it is bounded by Mexico’s two great mountain ranges: the Sierra Madre Oriental and the Sierra Madre Occidental. As warm, moist air off the oceans rises to move across these mountains, the air is cooled rapidly, causing rainfall on the ocean-side of the mountains. The now-hot, dry air flows across the mountains and descends into the basins of the Chihuahuan Desert region.
Unlike the other North American deserts, the Chihuahuan Desert does not have a winter rainy season. Instead, over 90% of the annual rainfall occurs between the months of July and October. This summer monsoon or rainy season is the result of thunderstorms that build in the afternoon and significantly cool the summer days.
Creosote bush-burro bush communities dominate the Mojave Desert and extend into adjacent northwestern portions of the Sonoran Desert. Stretching from Antelope Valley in the west to Eureka Valley in the north, creosote bush-burro bush communities extend south into the northwestern portion of the Sonoran Desert. This community is found on sandy alluvial fans and bajadas, as well as rocky uplands and slopes.
Creosote bush-brittle bush scrub communities are found on south facing slopes in the southern portions of the Mojave Desert up into the lower elevations of Death Valley and Panamint Valley. Desert holly scrub thrives in the hottest and most arid portions of the Mojave and is the most drought tolerant atriplex in North America.
Blackbrush (Coleogyne ramosissima) ranges from the Colorado Plateau to the Mojave Desert and the southern Great Basin and the western borders of the Sonoran Desert. It dominates on rocky or shallow soils on upper bajadas, pediments and rocky slopes in the transition zone between the warm deserts and the Great Basin. There is a debate as to whether blackbrush is part of the Great Basin vegetation or part of the Mojave Desert.
In the Chihuahuan Desert creosote shrublands occur on bajada slopes and alluvial fans and mesquite shrublands occurs on sandy soils often with a calcium carbonate layer at variable depths. Short, multi-stemmed mesquite plants accumulate blowing sand until a mound forms around each plant. Interspaces between plants are scoured of loose soil resulting in sparse herbaceous cover.
In the Sonoran Desert creosote bush and bursage communities occur on coarser soils and may stretch for miles. Where the water table is high, honey or velvet mesquite may form dense bosques or woodlands. Bursage and desert saltbush often occupy the bajadas and alluvial lowlands.
Current Plant Communities
Creosote bush (Larrea tridentata) is a prominent species in the Mojave, Sonoran, and Chihuahuan Deserts of western North America. Creosote bush scrub communities of the Mojave Desert are dominated by creosote bush, but also heavily populated by burro-weed or white bursage (Ambrosia dumosa), ocotillo (Fouquieria splendens), Mojave and Schotts indigo bush (Psorothamnus arborescens and schottii ), Baja desert thorn and Anderson’s desert thorn (Lycium brevipes and andersonii ), cheesebush, (Hymenoclea salsola), brittlebush, and rayless encelia (Encelia farinose and frutescens), apricot or desert mallow (Sphaeralcea ambigua), and beavertail, teddybear and silver or golden cholla (Opuntia basilaris, bigelovii and echinocarpa).
Large areas of the Sonoran Desert are dominated with creosote bush and white bursage. Ocotillo, brittlebrush, foothill palo verde (Parkinsonia microphylla), fourwing saltbush (Atriplex canescens), and ironwood (Olneya tesota) may also occur with creosote bush. Saguaro (Carnegiea gigantea) is a characteristic cactus. Western honey mesquite, ironwood, catclaw acacia (Acacia greggii), blue palo verde (Parkinsonia florida), desert willow (Chilopsis spp), and smoketree are usually associated with washes. Big galleta grass (Pleuraphis rigida formerly Hilaria rigida)is an important grass species. Invasive weedy species include exotic species such as buffelgrass(Cenchrus ciliaris), red brome (Bromus madritensis), filaree (Erodium spp), prickly lettuce, Russian thistle (Salsola kali), and London rocket (Sisymbrium irio).
Creosote bush (Larrea tridentata) is a dominant element of the Chihuahuan Desert Scrub and can cover vast expanses. Stem succulents such as lechuguilla (Agave lechuguilla), sotol(Dasylirion wheeleri), and various types of yuccas (yucca spp.) are also prominent in the desert scrub. Other common shrubs include mimosa, (Mimosa spp.), acacia (Acacia spp.), mesquite (Prosopis spp.), mariola (Parthenium incanum), fourwing saltbush, tarbush, javelin bush, skeleton-leaf goldeneye, allthorn, and ocotillo. Bush muhly (Muhlenbergia porteri), fluff grass, black grama (Boutaloua eriopida) and a variety of forbs may occur in association with creosote bush. Honey mesquite (Prosopis glandulosa) is the dominant plant in the Chihuahuan Desert’s mesquite shrublands that occur on sandy soils. Associated plants include salt bush (Atriplex canescens), broom snakeweed (Gutierrezia sarothrae), dropseeds (Sporobolus spp), threeawns (Aristida spp.), and a variety of forbs.
The current desert shrub vegetation of the Mojave Desert arrived about 9000 years BP following radical climatic changes in the Pleistocene and Holocene. The broad extent of creosote bush, bursage, and many associated species, suggests that these are effective colonizers and ecological generalists. The recent colonization of many sites by desert scrub species suggests that the equilibrium and stability of many of these communities has not yet been reached and continuing shifts in species composition should be expected. Observations have shown species composition shifts in fewer than 50 years.
Throughout the desert southwest, the densest and most abundant breeding and migrating birds are found in riparian areas supporting the Sonoran cottonwood-willow riparian forest plant community. Examples of breeding birds dependent on riparian areas include summer tanager, yellow-breasted chat, Abert’s towhee, Bullock’s oriole, common yellowthroat, and several others. Often riparian areas are too small to support a large number of breeding riparian obligates. However, these oases serve as resting habitat for a great variety of migrating neotropical migrant landbirds.
Mule deer (Odocoileus hemionus), desert bighorn sheep (Ovis Canadensis), javelina (Tayassu tajacu), mountain lion (Felis concolor), ringtail cat (Bassariscus astutes), bobcat (Felis rufus), and coyote (Canis latrans) are large mammals occupying this vegetation community, while smaller, less wide-ranging mammals abound, including Merriam’s kangaroo rat (D. merriami), little pocket mouse (Perognathus longimembris), white-tailed antelope squirrel (Ammospermophilus leucurus), desert woodrat (Neotoma lepida), southern grasshopper mouse (Onychomys torridus), long-tailed pocket mouse (Perognathus formosus), cactus mouse (Peromyscus eremicus), Harris antelope squirrel (Ammospermophilus harrisii), and canyon mouse (Peromyscus crinitus). Many of the bird and reptile species typical of this vegetation community are subspecies or subpopulations of species found in other desert vegetation communities in Arizona (Brown 1994). Bird species include black-tailed gnatcatcher (Polioptila melanura), great horned owl (Bubo virginianus), Phainipepla (Phainopepla nitens), black-throated sparrow (Amphisipiza bilineata), cactus wren (Campylorhynchus brunneicapilus), common raven (Corvus corax), rock wren (Salpinctes obsoletus), ash-throated flycatcher (Myiarchus cinerascens), loggerhead shrike (Lanius ludovicianus), mourning dove (Zenaida macroura), red-tailed hawk (Buteo jamaicensis), house finch (Carpodaucus mexicanus), and Gambels quail (Lophortyx gambelii). Reptiles include desert spiny lizard (Sceloporus magister), Mojave desert tortoise (Gopherus agassizii), zebra-tailed lizard (Callisaurus draconoides), side-blotched lizard (Uta stansburiana stejnegeri), long-nosed leopard lizard (Gambelia wislizenii), Mojave rattlesnake (Crotalus scutulatus scutulatus), coachwhip (Masticophis flagellum), and Mojave fringe-toed lizard (Uma scoparia).
Mammals typical to this arid region include mule deer (Odocoileus hemionus), desert bighorn sheep (Ovis Canadensis), javelina (Tayassu tajacu), mountain lion (Felis concolor), ringtail cat (Bassariscus astutes), bobcat (Felis rufu), grey fox (Urocyon cinereoargenteus), kit fox (Vulpes velox), white-tailed antelope squirrel (Ammospermophilus leucurus), black-tailed jack rabbit (Lepus californicus), desert pocket mouse (Chaetodipus penicillatus), and desert and Merriam’s kangaroo rats (Dipodomys deserti and D. merriami), as well as the ubiquitous coyote (Canis latrans). Because of its sparse vegetation this vegetation community is the poorest of the Sonoran Desert for birds, because of its sparsely vegetated and structurally shorter habitats. Typical bird species include lesser numbers of arid adapted species, such as the LeContes thrasher (Toxostoma lecontei), white-winged dove (Zenaida asiatica), elf owl (Micrathene whitneyi), black-throated sparrow (Amphisipiza bilineata), loggerhead shrike (Lanius ludovicianus), cactus wren (Campylorhynchus brunneicapilus), red-tailed hawk (Buteo jamaicensis), ash-throated flycatcher (Myiarchus cinerascens), gilded flicker (Colaptes chysoides), mourning dove (Zenaida macroura), Gambels quail (Lophortyx gambelii), and verdin (Auriparus flaviceps). Amphibians include Couchs spadefoot toad (Scaphiopus cochii), western green toad (Bufo debilis insidior), and Woodhouse’s toad (Bufo woodhousii). This vegetation community supports a diverse and productive community of reptiles. The sandy plains and dunes of the Lower Colorado River Sonoran Desert Scrub support a number of unique sand-adapted lizards and snakes, such as fringe-toed lizards (Uma inornata), banded sand snake (Chilomeniscus cinctus), and sidewinder (Crotalus cerastes).
The Chihuahuan Desert is dominated by several mammals including the Southern grasshopper mouse (Onychomys torridus), white-throated woodrat (Neotoma albigula), silky pocket mouse (Perognathus flavus), chisel-tooth kangaroo rat (Dipodomys microps), antelope jack rabbit (Lepus alleni), yellow-nosed cotton rat (Sigmodon ochrognathus), and Ords and Merriam kangaroo rats (Dipodomys ordii and D. merriami). Birds include quail (Callipepla squamata), Chihuahuan raven (Corvus cryptoleucus), pyrrhuloxia (Cardinallis sinuatus), Swainsons hawk (Buteo swainsoni), scaled quail (Callipepla squamata), black-throated sparrow (Amphisipiza bilineata), common poorwill (Phalaenoptilus nuttallii), red-tailed hawk (Buteo jamaicensis), Verdin (Auripasus flaviceps), cactus wren (Campylorchynchus brunneicapilus), lesser nighthawk (Chordeiles acutipenisi), loggerhead shrike (Lanius ludovicianus), ash-throated flycatcher (Myiarchus cinerascens), rock wren (Salpinctes obsoletus), and Gambel’s quail (Lophortyx gambelii). Amphibians and reptiles include Woodhouse’s toad (Bufo woodhousii) and plains spadefoot toad (Spea bombifrons), Texas banded gecko (Coleonyx brevis), greater earless lizard (Cophosaurus texanus), and little striped and marbled whiptails (Cnemidophorus inornatus, C. tigris marmoratus).
There is evidence that the Chihuahuan and Sonoran Deserts were once a grassland or a grassland with scattered shrubs (Buffington and Herbal 1965). Naturally occurring wildfires helped shape the desert grasslands of the southwest. Lightning from summer thunderstorms and fires started by Native Americans were common prior to the 1900s. Natural fire frequencies for these grasslands are estimated to between 10 and 20 years and favored perennial grasses over shrubs and cacti, helping to maintain the grassland. Human impacts since settlement have reduced the frequency and spread of wildfires, allowing present-day communities to be dominated by mesquite (Prosopis spp.), catclaw (Acacia spp.) or creosote bush with understories of burro weed and cacti (Opuntia) to dominate former desert grasslands, causing these communities to more closely resemble desert shrub communities (Robinett 1993).
Creosote bush density and cover are generally decreased by disturbance. In a comparison between vegetation on disturbed and undisturbed Mojave Desert sites, creosote bush was dominant on all control sites and subdominant to white bursage on disturbed sites. Desert succession can be described using life-history strategies: species with high recruitment and mortality rates, such as white bursage, are dominant in the colonizing stage and species with low recruitment and mortality, such as creosote bush, eventually dominate the landscape, although colonizing species usually remain present.
Recruitment of creosote bush is infrequent in the Mojave Desert. Despite the abundance of potentially suitable areas beneath white bursage, young creosote bush have been found beneath only 1% of all white bursage. Total densities of young creosote bush were between 12 and 15 plants per hectare. The density of white bursage plants was ten times that of creosote bush. Although large-scale creosote bush seedling establishment does not occur after disturbance, relict creosote bush usually increases in size by cloning. Creosote bush canopies may grow to exceed the coverage of white bursage by more than six times.
Creosote bush exhibits root-mediated allelopathy. In a laboratory study, creosote bush test roots grew freely through soil occupied by white bursage roots, but white bursage test roots grew at reduced rates into soil occupied by creosote bush. Mature creosote bush may be allelopathic to their own seedlings, encouraging an open community structure.
Yearly and Seasonal Variation
Creosote bush leafs out in response to spring, summer, or fall rains. Creosote bush usually flowers in May in the Mojave Desert, but it can flower anytime during the summer if it receives enough rain. In the Sonoran Desert, most creosote bush seeds are shed in the summer, but creosote bush in the Chihuahuan Desert does not shed its seeds until fall. Creosote bush seeds germinate after rains from mid-June to mid-September in the Mojave Desert.
Spring bud break in honey mesquite can vary by as much as six weeks from year to year. Bud break is dependent upon both photo- and thermal periods and rarely occurs until after the last spring frost has passed or the photoperiod exceeds 11.5 hours. Honey mesquite apparently has a cold requirement that must be met before bud burst occurs.
Following bud burst, twig elongation and leaf growth are rapid and generally completed in about 6 weeks. New foliage is generally very dense following a wet spring and fall, but less foliage is produced if the preceding spring and fall were dry. Inflorescences emerge in the spring with the leaves. By the time the leaves are fully expanded, miniature fruit pods have begun to develop. It takes two to three months for the fruits to mature, and by late summer they fall from the plant. More than one fruit crop per year is possible but uncommon. Sometimes a wet period late in the flowering season causes a flush of new growth, producing new leaves and flowers and, consequently, a second fruit crop. Flowering may occur up to four times in one growing season. Flower production varies with amount of available soil moisture. Heavy flowering and fruiting often occur when soil moisture is low; high soil moisture at the time of flowering appears to suppress fruit production.
Leaf drop generally occurs in November or December and is often initiated by a killing frost or leaf removal by insects. Plants from northern populations show early dormancy and are more resistant to freezing damage than plants from southern populations.
Fire and Invasion
Creosote bush is poorly adapted to fire because of its limited sprouting ability. Creosote bush survives some fires that burn patchily or are of low severity. Historically, infrequent fires may have limited the invasion of desert grasslands by creosote bush.
Most fires in the desert are infrequent and of low severity because production of annual and perennial herbs seldom provides a fuel load capable of sustaining fire. Creosote bush-white bursage communities have been described as “essentially nonflammable” because the shrubs are too sparse to carry fire. The resinous foliage of creosote bush, however, is very flammable.
Season of burning, fuel quantity, fire temperature, and age of existing creosote bush may affect the ability of creosote bush to sprout. Researchers have noted that burning creosote bush during different seasons at the Santa Rita Experimental Range near Tucson, AZ, resulted in significant differences in sprout production. The most sprouts were produced following February and August fires. The least sprouts were produced following June and July fires. The seasonal pattern of sprout production closely followed trends in growth of terminal shoots. Sprouting in creosote bush decreased with increasing fuel quantity and decreased as soil temperature and duration of heating increased. Young plants produced fewer sprouts after burning than mature plants.
Alterations to fire regimes can be especially damaging to ecosystem sustainability since native components have specific adaptations and traits that allow them to be successful under a certain regime. When regimes are altered in either direction, it can be catastrophic.
Historically, fires have been relatively absent or extremely rare in creosote bush scrub but the frequency and extent have increased significantly since the 1950s as alien annual grasses and human ignition sources have become increasingly more common.
As fire becomes more common in creosote bush scrub, especially in localized areas this type of disturbance dramatically alters the landscape. Since most native shrub components of creosote bush scrub lack traits that enable resiliency to fire disturbance, fires can result in long-lasting alterations to vegetation. In addition, invasive species common to the desert, such as Bromus madritensis, Schismus spp., and Erodium cicutarium, are aggressive fire followers in the immediate post-fire growing season or at least regain-exceed their pre-fire abundance within several years after fire. Since these invasive annual grasses are also competitive with mature shrubs, the combination of fire and the particular suite of invasive plants are threats to desert ecosystems.
The impact of multiple fires is especially of interest since an invasive grass – fire cycle has been reported from desert vegetation of the southwest. Besides anecdotal evidence of type conversion to invasive annual grassland following repeated fires, examples and information are lacking in the literature about the consequences of multiple fires on creosote bush shrub perennials.
Fires are not considered common historically in southwestern deserts but increasing in extent in recent decades partly because of fuel provided by non-native annual grasses (e.g., red brome and buffelgrass) are having significant economic and environmental impacts. Fire has killed the charismatic Joshua tree (Yucca brevifolia) in areas of Joshua Tree National Park in California and the renowned giant saguaro cactus (Carnegiea gigantea) in Saguaro National Park in Arizona. Joshua tree and saguaro are not considered well-adapted to fire (which was not part of their evolutionary environment); as fire readily kills them and they rarely resprout. These species often require protection (e.g., shading) by existing vegetation (“nurse plants”) from the harsh desert environment for reproduction, so regeneration of new individuals is slow because the nurse plants must first become established after fire.
Invasive annual grasses and forbs have invaded desert shrub communities leading to an increase in the frequency of fires. Many of the desert shrub species are not tolerant of fire. Thus the potential for species composition changes due to invasion of annual plants is great. Research has shown that invasive grasses and forbs including filaree (Erodium cicutarium), and Asian mustard (Brassica tournefortii) can all individually reduce native annual abundance in desert environments. When one of these invaders is removed, remaining invasive species usually increase. Studies in California have successfully controlled invasive annual plants following fire with a resulting increase in native annual plant cover and species richness.
Uncontrolled grazing occurred on these desert ranges until 1934 when the Taylor Grazing Act was passed. As a result of heavy domestic livestock grazing, vegetative composition changed. In many areas more palatable species such as black sage were eliminated. A distinct increase in shadscale came about and introduction of Russian thistle and halogeton took place.
Creosote bush is susceptible to severe drought during short-term climate changes like El Nino. During dry years, creosote bush undergoes severe moisture stress and subsequent defoliation. Older branches do not produce new foliage, but sprouting may occur. The cumulative result of El Nino can be a 60-80 percent stem dieback. Dead stemwood remains standing within the shrub biomass for several years.
Vehicle traffic, construction and mining activities are a source of disturbance especially for cryptogrammic crusts. Because of their thin, fibrous nature, cryptogrammic crusts (Fig. 5) are extremely fragile systems; a single footprint or tire track is sufficient to disrupt the soil crust and damage the organisms. While some species within the soil crust system may regrow within a few years of a disturbance, the damage to slow-growing species may require more than a century before the delicate crust returns to its former productivity.
Conversion of natural habitat to urban, suburban, industrial, and agricultural uses has and likely will continue to result in extensive habitat loss. Increasing recreational use of the desert is resulting in habitat damage and declines in some species. Since the end of World War II, the “sunbelt area” of the Southwest has experienced the largest immigration in human history. In 1990, the Sonoran Desert ecoregion contained 6.9 million residents, nearly double the population in 1970. The population is expected to reach 12 million by 2020.
Areas containing these species are national and international tourist destinations. From an economic standpoint, the tourism industry (e.g., resorts, golf courses, businesses dependent on visitation to area parks) is concerned about having scenery and tourist attractions disrupted . Furthermore, fires have threatened human habitations and cost millions of dollars to suppress, such as the 100,000 ha Cave Creek Fire in 2005 near Phoenix, AZ, in the Sonoran Desert.
It has long been understood that some alien annuals greatly shorten the fire return cycle on semiarid and arid lands throughout western America. Shortened fire return intervals are known to eliminate fire intolerant shrubs and herbs from natural vegetation and disrupt many natural processes in associated systems.
Domestic grazing by sheep and cattle has occurred on public lands in the Mojave Desert since the late 1500s, with stocking densities peaking at the turn of the 20th century. The Mojave Desert is commonly used in the winter and spring prior to moving to mountain pastures.
Wild burros (Equus asinus) have been one of the most contentious environmental management issues in arid lands of the southwestern United States (Fig. 6). Considered native to arid northeastern Africa and domesticated more than 5000 years ago, burros are believed to have been brought to the American Southwest in the sixteenth century by Spanish explorers. Owing to their desert hardiness, burros were deemed excellent pack animals and were used extensively to assist with mining operations in the 1800s in the deserts of Arizona, California, Nevada, and neighboring states. Following a decline in mining and the development of other transportation methods in the late 1800s, many burros were released or escaped and became feral. With few predators, burro density increased in the Southwest in the early and mid-1900s, alarming ecologists and resource managers who believed that these nonnative animals negatively affected desert soils, native bighorn sheep (Ovis canadensis nelsoni), and plant communities.
Resource managers of some public lands initiated shooting, translocation, and other control measures in attempts to reduce burro density and perceived impacts on desert ecosystems. However, public outcry about population control activities resulted in the U.S. government passing the 1971 Wild Free-Roaming Horse and Burro Act (Public Law 92-195). This act declared that wild burros were part of national heritage and should be considered where found as integral components of federal public land, with the exception that national parks and wildlife refuges were exempt. However, burros continued to reside on these exempt lands, partly by movement from other adjacent public land, lack of public support for population control measures, and costs and constraints on control.
Because of low natural productivity, little open water, and limited potential for intensive agriculture, most of the desert shrub region has been traditionally regarded as wasteland. Very little of this type was homesteaded or otherwise claimed for transfer into private ownership. The only major uses have been for range livestock grazing, mining of the accumulated minerals, and military testing and maneuvers. The major managers are thus the Bureau of Land Management, Department of Defense and Department of Energy.
Grazing Management Practices
Low forage production and resistance of many desert plants to grazing makes livestock production impractical in much of the Mojave and Sonoran deserts. Grazing continues to be an important practice in the Chihuahuan Desert but control of mesquite and creosote bush has proven economically impractical (Holochek et al. 2004). Creosote bush is unpalatable to livestock and most browsing wildlife. Consumption of creosote bush may be fatal to sheep.
Creosote bush may be used to rehabilitate disturbed environments in southwestern deserts. Once established, creosote bush may improve sites for annuals that grow under its canopy by trapping fine soil, organic matter, and symbiotic propagules. It may also increase water infiltration and storage.
Creosote bush should be transplanted rather than spot-seeded. Researchers have increased germination success by leaching seeds in running water for 12 hours. At Organ Pipe National Monument, the survival rate for creosote bush was 78 percent when seeds were germinated in grow tubes filled with nursery soil mix and allowed to harden-off before being transplanted outside. Creosote bush should be planted in the spring or fall. It is recommend that seedlings be heavily pruned one month before transplanting. Rodent protectors are necessary.
In addition to the recreational values of wildlife, the vast and largely uninhabited salt-desert shrub ranges provide unique areas for camping, hiking, biking, caving, rock-hounding, and enjoyment of the scenery or the solitude. Unfortunately, the wheels of off-road vehicles used to reach these recreational areas can destroy vegetation and damage the soil, often causing accelerated wind and water erosion. Regulations of recreational activities and repair of existing damage are serious management problems. Several areas have been designated as State or National Parks, most of the desert shrub rangelands are not esthetically pleasing to most people and, therefore, attract little attention for this purpose.
Native American used creosote bush to treat disease, with a balm made from the plant. The balm was used to treat chickenpox, tuberculosis, STDs, dysmenorrhea and snakebites. Creosote is currently being tested as an anti-cancer treatment. Historically, many desert peoples have used the bitter leaves to make a tea, and it is still consumed to treat ailments such as fever, influenza, colds, upset stomach, gas, gout, arthritis, sinusitis, anemia, PMS, diarrhea, allergies, and fungus infections. The plant also has antiseptic properties. Although U.S. officials discourage the medicinal use of the plant, it is still commonly used for medicinal purposes in Mexico. Desert peoples also used the substance secreted by the plant as a sealant or glue. Prolonged use of the medicinal plant has been linked to some kidney or liver failure.