This extensive ecological system includes open-canopied shrublands of typically saline desert basins, alluvial slopes and plains across the intermountain western United States. The vegetation is characterized by a typically open to moderately dense shrubland composed of one or more Atriplex species. Substrates are often saline and calcareous, medium-to fine-textured, alkaline soils but include some coarser-textured soils.
A major portion of the temperate desert region of North America is occupied by shrubs and half-shrubs in the Chenopodiaceae Family. Because they are generally associated with halomorphic soils, they are called “salt desert shrub.” Some chenopod dominated areas are not salty but lie in the dry rain shadow of the Sierra Nevada in California and Nevada.
These habitats occur at low elevations and receive low annual precipitation. Although these areas may seem harsh and inhospitable to life, a variety of shrubs and grasses that are adapted to salt, drought and toxic minerals thrive in these desert zones. Portions of this vegetation type occur in eight western states, including parts of the Mojave, Sonoran, Chihuahuan and Great Basin deserts, Central Valley of California and valleys within the Rocky Mountains. According to Kuchler (1970) the salt desert shrub covers about 16.9 million ha (41.76 million acres).
Salt desert shrub communities occur in a region of extreme climatic conditions with hot summers and freezing winters. These plant communities occupy desert or semi-desert regions in the orographic rain shadow of the Sierra Nevada, Cascades and the Rocky Mountain ranges. The Sierra Nevada/Cascades are an orographic barrier to storms coming from the Pacific Ocean and the Rocky Mountains are a barrier to the warm and moist summer fronts coming from the Gulf of Mexico. Even the low elevation Coast Range in California creates a rain shadow effect in California’s San Joaquin Valley where some salt desert shrub communities occur.
Average annual precipitation ranges from 13 to 33 cm (5 – 13 in) but is commonly less than 20 cm (8 in). Seasonality of precipitation varies from winter concentrations in the western Great Basin to late summer peaks in the Colorado Plateau and Rio Grande River drainages where monsoonal precipitation is more influential. Diurnal and seasonal variations in temperature are among the greatest on the continent. This is the result of extremely clear air, very low average relative humidity and low elevational position of the salt desert basins.
The latitude and elevation of the salt desert shrub area control its temperature. This “cold desert” has warm rather than hot summers and cold winters with several weeks of temperatures below (usually much below) freezing. Over the wide geographic range of the desert, again because of latitudinal and elevational variation and also because of local relief and aspect, there is variation in length of the summer frost-free period, usually between 100 to 150 days. The term “frost-free period” has less significance for desert vegetation than for many agricultural crops because many desert plant species grow when daytime temperatures are above the freezing point and the soil is unfrozen, but the night or early morning temperatures fall below the freezing level. Frost-free time in arid country is by no means synonymous with “growing season.” The presence or absence of moisture determines whether plants will grow when the temperature is optimum. The desert is sunny, more so in summer than in winter, and the relative humidity of the air is low. Wide daily ranges in temperature are therefore common on the desert, particularly in the valleys of interior drainage. Differences between day and night temperatures of 45°F (25°C) are not uncommon when the sky is clear.
Topography and Soils
Most salt desert communities occur on lowland and upland sites at elevation of 1500 to 2300 m (5000 to 7200 ft.). These communities can be found on all aspects and include valley bottoms, alluvial and alkaline flats, mesas and plateaus, playas, drainage terraces, washes and inter-dune basins, bluffs, and gentle to moderately steep sandy or rocky slopes. Slopes are typically gentle to moderately steep, but are sometimes unstable and prone to surface movement. Many areas within this system are degraded due to erosion and may resemble “badlands.”
Soils of the salt desert shrub are often shallow to moderately deep, poorly developed, and a product of an arid climate and little precipitation. Soils are often alkaline or saline. Salt desert shrub occurs mostly in two kinds of situations which promote extreme soil salinity and/or alkalinity. These are either on flat deep lacustrine deposits at the bottom of drainages in enclosed basins (Figure 2) or badlands (Figure 3) where marine shales outcrop. Development on lacustrine deposits depends greatly on their salt content, texture and water table.
Badland soils are excessively drained with surface soil often eroding almost as fast as underlying rock weathers. Soil moisture input is so low that there is not enough to leach salts from the soil or encourage much vegetation. Erosion rates are great because of the steep slopes and low vegetative cover. High sodium content in the soils cause fine soil particles to disperse, reducing infiltration further increasing surface runoff. These soils are in the Entisol order.
Current Plant Communities
Most of the dominant plants in the salt desert shrub are members of the Chenopodiaceae Family. Members of the Asteraceae, Barassicaceae, Fabaceae and Poaceae may also occur but they are not dominants. Few plants can stand the high salinity and/or extreme aridity and both high and low temperatures.
In the uplands where the water table is well below 1 m, bud sage (Picrothamnus desertorum, formerly Atriplex spinescens), shadscale (A. confertifolia), fourwing saltbush (Atriplex canescens) and spiny hopsage (A. (Grayia) spinosa) are dominant shrubs. Half-shrubs include mat saltbush (A. corrugata), valley saltbush (A. cuneata), sickle saltbush (A. falcata), Gardner’s saltbush (A. gardneri), basin saltbush (A. tridentata), winterfat (Krascheninnikovia lanata, formerly Cerantoides lanata) and green molly (Neokochi americana). Herbs may include cheatgrass (Bromus tectorum), basin wildrye (Leymus cinereus), halogeton (Halogeton glomeratus), clasping pepperweed (Lepidium perfoliatium), Indian ricegrass (Oryzopsis hymenoides), Russian thistle (Salsola kali), squirreltail (Elymus elymoides, formerly Sitanion hystrix) and alkali sacaton (Sporobolus airoides).
In the lowland sites where the water table is within 1 m of the surface and occasionally present at the surface the dominant shrub is greasewood (Sarcobatus vermiculatus, Figure 5). Half-shrubs include pickleweed or iodine bush (Allenrolfea occidentalis), Utah pickleweed (Salicornia utahensis) and iodine weed (Suaeda torreyana). Salt grass (Distichlis spicata) is about the only herb in these lowland sites.
Cryptogramic crusts are well-developed in the salt desert shrub and represent a high proportion of the living ground cover. A cryptogramic crust is composed of living cyanobacteria, green algae, brown algae, fungi, lichens, and/or mosses and are commonly found in arid regions around the world. These soil crusts are important components of desert ecosystems that occupy the areas between shrubs and other plants and contribute to ecosystem processes and services by 1) stabilizing the soil surface and preventing erosion, 2) increasing water infiltration, 3) improving moisture retention, and 4) fixing atmospheric nitrogen. However, because of their thin, fibrous nature, cryptogramic crusts are extremely fragile.
Historic Plant Communities
Few species can survive in the high salinity and arid conditions of the salt desert shrub. Consequently historic changes to this vegetation type are less apparent than that of other desert plant communities. However, halogeton, Russian thistle and cheatgrass have invaded these communities resulting in species composition changes from historic conditions. Additionally where black sage (Artemisia nova) was present, it may have decreased under heavy grazing. Shadscale may have increased with the decrease in black sage.
Species of the salt-desert shrub complex have different degrees of tolerance to salinity and aridity, and tend to sort themselves out along a moisture/salinity gradient. The harsh environmental conditions of these systems slow down community dynamics. The same species or species similar in stature or appearance often succeed each other after disturbances. Fire frequency was historically very low in this system. Heavy sheep-grazing practices can significantly impact vigor and cover of the principal shrub species, leading to an increase of annual bromes (Bromus species) and other exotic annual forbs. Sites invaded with exotic annuals are changing the dynamics of this system by increasing fire potential, severity, and spread.
Principle big game use of the salt desert shrub is by pronghorn antelope (Antilocapra americana) but mourning dove (Zenaida macroura), mule deer (Odocoileus hemionus), and chukar partridge (Alectoris chukar) may also be present.
Pronghorn antelope is the most important game animal on most salt-desert shrub ranges, but numbers are often sparse. For example, in western Utah there is only about one animal per 13 km2 (5 mi2) of potentially usable range, whereas pronghorn food requirements and composition of the vegetation indicate that such range should be able to support 5 animals per 2.6 km2 (1 mi2), a 25-fold increase. This low antelope production has been a perplexing problem in Utah for many years. However, studies have shown the importance of adequate water during the summer, fawn losses due to predation, and competition with domestic livestock for preferred winter forage species.
Although many factors affect the amount of water necessary to maintain healthy and productive antelope herds, they cannot live without drinking water for extended periods during the summer on most salt-desert shrub ranges, even when forage is above average in succulence. Because free water is usually in short supply, providing drinking water through spring development, construction of guzzlers, hauling, or other means will expand the area of usable range and increase the density of antelope.
Upland game birds
Salt-desert shrub ranges provide especially good habitat for mourning doves, which are widely distributed wherever free water is available. Although doves nest at a considerable distance from water, it is vital to their survival and welfare in this desert habitat. Dove hunting is a fairly popular sport. Birds will accommodate more hunters than will larger game. Chukars and sage grouse may also occupy salt-desert shrub ranges, especially peripheral areas where preferred habitat is available. Chukars are found near rocky escape cover and areas dominated by winter annuals, whereas sagebrush communities must be present to attract sage grouse. Water is needed for all of these birds, and habitat improvement is often restricted to water development. Installations are commonly of the “guzzler” type with a collecting apron, storage tank, and a small drinking fountain with a float valve for minimal waste and evaporation.
Other species of some prominence in the salt-desert shrub rangelands may be roughly classified as songbirds, rodents and rabbits, reptiles, and predators (coyotes, bobcats, kit foxes, badgers, owls, hawks and eagles). Although some observations have been made of their abundance and activities, little information is available on ecological interrelations and impacts. Until these are better known, it appears prudent to aim at maintenance of somewhat natural populations of these native species to avoid serious disruption of ecosystems that have existed for long periods.
Most salt desert shrub vegetation occurs in two situations that promote soil salinity, alkalinity, or both. These are either at the bottom of drainages in enclosed basins or on marine shale outcrops. However, salt-desert shrub vegetation may be an indication of climatically dry as well as physiologically dry soils. Species of the salt-desert shrub complex have different degrees of tolerance to salinity and aridity. Species and communities are apparently sorted out along physical, chemical, moisture, and topographic gradients. Generally sagebrush and winterfat occur on soils with low soluble salts compared to shadscale and Gardner saltbush that occur on soils with intermediate levels of soluble salts and greasewood occupies soils with higher soluble salts.
The chenopods and other species tolerate salty and arid conditions and are thus able to colonize this harsh landscape. Grazing and weather act as disturbances that can result in retrogression but, if not heavily impacted, the community moves back to a stable state. Heavy grazing of palatable species may result in their replacement by other natives or by invasives such as cheatgrass, halogeton and Russian thistle.
Yearly and Seasonal Variation
Desert plants grow when temperature is satisfactory, but only if soil moisture is available at the same time. The winter months in the salt desert shrub are a good time for soil moisture accumulation and storage. There is generally at least one good snow storm per season that will provide sufficient moisture to the vegetation. The winter moisture accumulation amounts will affect spring plant growth. Production of shadscale and other plants is strongly influenced by spring (April – June) precipitation. When spring moisture is depleted, perennial plants will assume their various forms of dormancy. If effective rain comes later in the warm season, some of the species will renew their growth from the stage at which it had stopped. Others, having died back, will start over as if emerging from winter dormancy.
Since the 1980s cheatgrass has invaded many salt desert shrub communities, providing fuel that can carry fires. Fire was a very rare occurrence with fire return intervals greater than 500 years and may have been limited to more mesic sites during the pre-settlement period. Native American manipulation of salt desert shrub plant communities was probably less than nearby higher-elevation communities. Grass seed may have been one of the more important salt desert shrub crops. It is unlikely that Native Americans manipulated the vegetation to encourage grass seed.
Both spiny hop sage and winterfat are capable of sprouting following fire. Researchers have found that recruitment of bud sage is severely restricted following fire, occurring in only two of 24 burned sites in one study. The co-dominant shrub, shadscale, occurred in most burned sites, although on average its percent cover was one-third lower than adjacent unburned sites. Biotic soil crust cover was four times lower, and non-native species cover five times higher in burned sites compared to unburned.
Halogeton was accidentally introduced into the western U. S. during the 20th century. Because it is highly poisonous to sheep (Ovis aries), this rather diminutive herb became the center of attention for biological research on Intermountain rangelands during the 1950s. Grazing management for halogeton involves procedures to prevent accidental poisoning of the grazing animals, and management to encourage the density and vigor of competing perennial vegetation to biologically suppress halogeton. Halogeton became most abundant in salt desert rangelands and the lower elevation portions of the sagebrush (Artemisia)/bunchgrass zone. In the sagebrush zone the introduced perennial, crested wheatgrass (Agropyron desertorum) was widely planted to both suppress halogeton and to provide alternative forage for livestock. In the salt deserts, the management of native chenopod shrubs was the key to suppressing halogeton. The key species in salt deserts was the highly preferred semi-woody species winterfat. In many parts of the Intermountain region, halogeton has declined in importance because of the reduced importance of the range sheep industry and improved range condition. In the south central Great Basin, halogeton is still considered a serious problem.
Like other intermountain ecosystems, cheatgrass has become a major problem in salt desert shrub communities. It increased dramatically during the high precipitation years of the early 1980s. Its flashy, continuous fuel has set the stage for larger, more frequent fires in these communities.
Drought years occur periodically. Shadscale populations are expected to decline with each drought and eventually be replaced by winterfat and bud sage, if lightly grazed or ungrazed (Holmgren and Hutchings). Severe drought (every 70 years on average), flooding and insect outbreaks (60 years return interval) were the most common disturbances. The length and severity of drought in the Great Basin has increased since the beginning of the 20th century.
Several studies have documented shrub die-offs in the western United States following successive wet years. Increased soil salinity, anaerobic conditions in the root zone, and soil borne disease have been implicated in the die-off. Additionally, plants in desert ecosystems are already under the usual stresses associated with desert climates. Added to these stresses are several more associated with excess water such as the possible increased soil bicarbonate that induces iron deficiency, the parasite dodder, and loss of drought tolerance because of the wet years.
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.
Vehicle traffic, construction and mining activities are a source of disturbance, especially for cryptogrammic crusts. Because of their thin, fibrous nature, cryptogrammic crusts 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.
Existing and emerging threats to the salt desert shrub vegetation type include 1) livestock mismanagement, 2) invasive introduced weeds, 3) climate change, 4) altered fire regimes, and 5) surface disturbance from mining, energy development and recreational activities.
In the early decades of the 20th century there were many more sheep than cattle on the desert, but since the 1930s sheep numbers have declined about 80%. Cattle have taken their place on many allotments. Most studies of sheep grazing response of plants have shown that the palatable species were most damaged by grazing, especially during the season of early and rapid growth. Response to cattle grazing was similar to sheep grazing except that different species were impacted as a result of different dietary preferences.
The early (1930-40s) devastation of much of the salt-desert now appears to have been curtailed and some areas are trending upward. True desertification was once described for areas that are now stable. The most severely depleted ranges are still in very poor condition, but others are grazed with fewer animals at seasons more compatible with plant survival, and range condition has improved.
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.
The salt desert shrub received little use by Native Americans. Because of low natural productivity, little open water, and limited potential for intensive agriculture, most of this area 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. Salt desert shrub is frequently used for hazardous waste disposal because of environmental conditions and the lack of human populations in these areas. Much of Tooele County in Utah has low-level radioactive waste and chemical weapons disposal sites in salt desert shrub.
It wasn’t until the arrival of the first sheep flocks in the mid-1800s that this vegetation type received significant use. Sheep numbers peaked during WWI, WWII and the Korean War due to the military’s need for wool during those periods. The spread of halogeton during 1950s led to reduction in sheep grazing.
Grazing Management Practices
Season of grazing has traditionally been during winter and well into the spring, and for most areas that is still the case. On many ranges the spring season has been shortened by removing the animals at an earlier date, a practice that benefits the forage species. On the other hand, as watering places for livestock have been developed, some desert ranges have been made usable for warm season grazing by cattle, the season when top removal is most damaging to plants.
Studies have shown that season of grazing has more of an effect on successional direction than grazing intensity. Spring grazing has very different impacts on vegetation than late fall or no grazing. Because vegetation is actively growing in the spring, foliage removal is more damaging than during the late fall when vegetation is dormant.
Stocking rates based on past records of use can serve as an index to grazing capacity. If the range has shown no improvement or if the desirable forage species are being injured or destroyed, fewer animals should be grazed or the grazing season shortened. If the range is in good condition and more herbage is left ungrazed than is needed to keep the desirable forage species thrifty and productive, more livestock can be grazed.
Management measures to reduce halogeton poisoning include: 1) avoid overgrazing that creates habitat for halogeton; 2) develop grazing management programs that result in improving range condition; 3) reduce grazing pressure during droughts; 4) avoid late spring grazing that injures native perennials; 5) supply adequate water; 6) observe the sheep and know what they are grazing; 7) allow time for rumen microorganisms to adapt to oxalates; 8) introduce animals to halogeton-infested areas gradually; 9) do not unload animals from trucks into halogeton patches unless there is supplemental feed and water; 10) never allow hungry animals to graze in large, dense patches of halogeton; and, 11) do not trail thirsty animals into watering places surrounded by halogeton without food supplement. The continued use by the Great Basin range sheep industry of non-resident herders, often from countries with vastly differing environments, has contributed to the occurrence of halogeton poisoning. Adequate training of such herders is often hampered by language barriers.
Ecosystem processes in the salt desert shrub have been altered by the persistent effects of past land-use and invasion of exotic annuals. Restoring ecosystems to pre-disturbance conditions is not always feasible because abiotic and biotic thresholds may have been crossed. Thus restoration of ecosystem structure and function is the remaining alternative. Principles and tools of recovery include: 1) assess the underlying above- and below-ground processes responsible for invasive dominance, 2) develop and apply effective management strategies that affect the causes of invasion and reduce invasive plant dominance, and 3) re-establish native and introduced plant species with appropriate traits to perform well in a restoration setting. The primary challenge to restoring rangelands today is to integrate these three elements.
Post fire intervention in salt desert communities is important. Studies of post-fire invasion of cheatgrass into salt desert communities found that cheatgrass invaded quickly into greasewood and black sagebrush/shadscale communities following fires. After a few years, cheatgrass density and cover was higher on burned plots whether seeded or not. While seeding success was variable there was a trend toward lower densities of cheatgrass in burned drill seeded plots vs. burned and not seeded plots in both shrub communities.
In addition to the recreational values of wildlife, the vast and largely uninhabited salt-desert shrub ranges provide unique areas for camping, 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. Although a few badland and other scenic areas have been designated as State or National Parks, most of the salt-desert shrub rangelands are not esthetically pleasing to most people and, therefore, attract little attention for this purpose.