Intermountain Salt Desert Scrub

This extensive ecological system includes open-canopied shrublands of typically saline basins, alluvial slopes and plains across the Intermountain western U.S. This type also extends in limited distribution into the southern Great Plains. Substrates are often saline and calcareous, medium- to fine-textured, alkaline soils, but include some coarser-textured soils. The vegetation is characterized by a typically open to moderately dense shrubland composed of one or more Atriplex species, such as Atriplex confertifolia, Atriplex canescens, Atriplex obovata, Atriplex polycarpa, or Atriplex spinifera. Other shrubs present to codominant may include Artemisia tridentata ssp. wyomingensis, Chrysothamnus viscidiflorus, Ericameria nauseosa, Ephedra nevadensis, Grayia spinosa, Krascheninnikovia lanata, Lycium spp., Picrothamnus desertorum, or Tetradymia spp. Northern occurrences may lack Atriplex species and are typically dominated by Grayia spinosa, Krascheninnikovia lanata, and/or Picrothamnus desertorum. In Wyoming, occurrences are typically a mix of Atriplex confertifolia, Grayia spinosa, Artemisia tridentata ssp. wyomingensis, Sarcobatus vermiculatus, Krascheninnikovia lanata, and various Ericameria or Chrysothamnus species. Some places are a mix of Atriplex confertifolia and Artemisia tridentata ssp. wyomingensis. In the Great Basin, Sarcobatus vermiculatus is generally absent but, if present, does not codominate. The herbaceous layer varies from sparse to moderately dense and is dominated by perennial graminoids such as Achnatherum hymenoides, Bouteloua gracilis, Elymus lanceolatus ssp. lanceolatus, Pascopyrum smithii, Pleuraphis jamesii, Pleuraphis rigida, Poa secunda, or Sporobolus airoides. Various forbs are also present.

The vegetation is characterized by a typically open to moderately dense shrubland composed of one or more Atriplex species, such as Atriplex confertifolia, Atriplex canescens, Atriplex polycarpa, or Atriplex spinifera. Grayia spinosa tends to occur on coppice dunes that may have a silty component to them. Northern occurrences lack Atriplex species and are typically dominated by Grayia spinosa and Krascheninnikovia lanata. Other shrubs present to codominant may include Artemisia tridentata ssp. wyomingensis, Chrysothamnus viscidiflorus, Ericameria nauseosa, Ephedra nevadensis, Grayia spinosa, Lycium spp., Picrothamnus desertorum, or Tetradymia spp. In Wyoming, occurrences are typically a mix of Atriplex confertifolia, Grayia spinosa, Artemisia tridentata ssp. wyomingensis, Sarcobatus vermiculatus, Krascheninnikovia lanata, and various Ericameria or Chrysothamnus species. In the Great Basin, Sarcobatus vermiculatus is generally absent but, if present, does not codominate. The herbaceous layer varies from sparse to moderately dense and is dominated by perennial graminoids such as Achnatherum hymenoides, Bouteloua gracilis, Elymus lanceolatus ssp. lanceolatus, Pascopyrum smithii, Pleuraphis jamesii, Pleuraphis rigida, Poa secunda, or Sporobolus airoides. The vegetation description is based on several references, including Beatley (1976), Campbell (1977), Brown (1982), West (1979, 1983b), Knight et al. (1987), Knight (1994), Shiflet (1994), Holland and Keil (1995), Reid et al. (1999), Ostler et al. (2000), Barbour et al. (2007), and Sawyer et al. (2009).

Climate: This is a semi-arid system of extreme climatic conditions, with warm to hot summers and cold winters. Annual precipitation ranges from approximately 13-33 cm. In much of this shrubland's distribution the season of greatest moisture is mid to late summer, although in the more northern areas a moist period is to be expected in the winter and spring. Precipitation is extremely irregular in the southern part of its distribution, such that long-term seasonal or monthly averages do not convey the full story (Blaisdell and Holmgren 1984).

Physiography/landform: This salt desert shrubland system is a matrix system in the Intermountain West. This system occurs on lowland and upland sites usually at elevations between 1520 and 2200 m (4987-7218 feet). Sites can be found on all aspects and include valley bottoms, alluvial and alkaline flats, mesas and plateaus, playas, drainage terraces, washes and interdune 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." Soil surface is often very barren in occurrences of this system. The interspaces between the characteristic plant clusters are commonly covered by a biological soil crust (West 1982).

Soils/substrates/hydrology: Soils are shallow to moderately deep, poorly-developed, and often alkaline or saline. The soils of much of the area are poorly-developed Entisols, a product of an arid climate. Vegetation within this system is tolerant of these soil conditions but not restricted to it. Other sites include level pediment remnants where coarse-textured and well-developed soil profiles have been derived from sandstone gravel and are alkaline, or on Mancos shale badlands, where soil profiles are typically fine-textured and non-alkaline throughout (West and Ibrahim 1968). They can also occur in alluvial basins where parent materials from the other habitats have been deposited over Mancos shale and the soils are heavy-textured and saline-alkaline throughout the profile (West and Ibrahim 1968). The environmental description is based on several other references, including Branson et al. (1967, 1976), Beatley (1976), Campbell (1977), Brown (1982), West (1983b), Knight et al. (1987), Knight (1994), Shiflet (1994), Holland and Keil (1995), Reid et al. (1999), Ostler et al. (2000), Barbour et al. (2007), and Sawyer et al. (2009).

West (1982) stated that "salt desert shrub vegetation occurs mostly in two kinds of situations that promote soil salinity, alkalinity, or both. These are either at the bottom of drainages in enclosed basins or where marine shales outcrop." However, salt-desert shrub vegetation may also occur in climatically extremely dry, non-saline sites, as well as physiologically dry (saline) soils (Blaisdell and Holmgren 1984). Not all salt desert shrub soils are saline, and their hydrologic characteristics may often be responsible for the associated vegetation (Naphan 1966). That is, they are flooded or wetted enough to mobilize but not flush soil salt content, and therefore the ephemeral hydrology precipitates and concentrates salts. Species of the salt desert shrub complex have different degrees of tolerance to salinity and aridity, and they tend to sort themselves out along a moisture/salinity gradient (West 1982). Thus these saltbush shrublands are dependent on a certain amount of ephemeral flooding and warm temperatures causing evaporation. The effects of these physical, chemical, moisture, and topographic gradients on species and communities occur through complex relations that are not well understood and are in need of further study (Blaisdell and Holmgren 1984). In northern, cool desert locations of this system, soil moisture accumulation and storage within this system typically occur in the winter months. There is generally at least one good snowstorm per season that will provide sufficient moisture to the vegetation. The winter moisture accumulation amounts will affect spring plant growth. Plants may grow as little as a few inches to 1 m. Unless more rains come in the spring, the soil moisture will be depleted in a few weeks, growth will slow and ultimately cease, and the perennial plants will assume their various forms of dormancy (Blaisdell and Holmgren 1984). 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 (Blaisdell and Holmgren 1984). Atriplex confertifolia shrubs often develop large leaves in the spring, which increase the rate of photosynthesis. As soil moisture decreases, the leaves are lost, and the plant takes on a dead appearance. During late fall, very small overwintering leaves appear which provide some photosynthetic capability through the remainder of the year (Reid et al. 1999).

The variation of plant communities found within this ecological system is maintained by intra- or inter-annual cycles of flooding followed by extended drought, which favor accumulation of transported salts. The moisture supporting these intermittently flooded communities is usually derived off-site, and they are dependent upon natural watershed function for persistence (Reid et al. 1999). As a result, these desert communities of perennial plants are dynamic and changing. The composition within this system may change dramatically and may be both cyclic and unidirectional. Superimposed on the compositional change is great variation from year to year in growth of all the vegetation, the sum of varying growth responses of individual species to specific conditions of different years (Blaisdell and Holmgren 1984). Desert plants grow when temperature is satisfactory, but only if soil moisture is available at the same time. Because the amount of moisture is variable from year to year and because different species flourish under different seasons of soil moisture, seldom do all components of the vegetation thrive in the same year (Blaisdell and Holmgren 1984).

Insects are an important component of many shrub steppe and grassland systems. Mormon crickets and grasshoppers are natural components of many rangeland systems (USDA-APHIS 2003, 2010). There are almost 400 species of grasshoppers that inhabit the western United States with 15-45 species occurring in a given rangeland system (USDA-APHIS 2003). Mormon crickets are also present in many western rangelands and, although flightless, are highly mobile and can migrate large distances consuming much of the forage while travelling in wide bands (USDA-APHIS 2010). Following a high population year for grasshoppers or Mormon crickets and under relatively warm dry spring environmental conditions that favor egg hatching and grasshopper and Mormon cricket survival, there may be large population outbreaks that can utilize 80% or more of the forage in areas as large as 2000 square mile. Conversely, relatively cool and wet spring weather can limit the potential for outbreaks. These outbreaks are naturally occurring cycles and, especially during drought, can denude an area of vegetation leaving it exposed to increased erosion rates from wind and water (USDA-APHIS 2003).

Disturbance scale was variable during presettlement. Droughts and extended wet periods could be region-wide, or more local. A series of high water years or drought could affect whole basins. Mormon cricket disturbances could affect hundreds to perhaps thousands of acres for a few years to 1-2 decades (LANDFIRE 2007a).

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 2310810):
A) Early Development 1 All Structures (25% of type in this stage): Shrub cover is 0-5%. Dominated by continuous grass with widely scattered shrubs and relatively younger shrubs than in classes B and C. Over 10 years, vegetation moves to class B as the primary succession pathway. Replacement fire occurs every 300 years on average, and will set back succession to year zero. Extended wet periods (every 35 years) will also have a stand-replacing effect. During a drought (mean return interval of 35 years), vegetation will follow an alternative succession pathway to class C.

B) Mid Development 1 Open (45% of type in this stage): Characterized by mature shrubs (5-20% cover). Discontinuous grass patches and higher shrub canopy cover than in class A. Extended wet periods (every 35 years on average) will cause a stand-replacing transition to class A. During extended drought periods (every 35 years), vegetation will shift to class C. Replacement fire is rare (mean FRI of 500 years). Class B will be maintained in the absence of disturbance.

C) Mid Development 2 Open (30% of type in this stage): Characterized by mature shrubs (21-30% cover). Grass is lacking and shrub canopy cover is even higher than class B. During extended wet periods (35 years), vegetation will transition to class A. After 20 years, vegetation moves back to class B through succession. Drought (mean return interval of 35 years) will maintain vegetation in class C. Fire would not carry in this class and is not modeled.

Under reference conditions disturbances were unpredictable, but flooding, drought, insects and fire may all occur in this system. Extended wet periods were modeled as occurring every 35 years, and drought periods every 35 years. Extended wet periods tended to favor perennial grass development, while extended drought tended to favor shrub development. Fire was rare and limited to more mesic sites (and moist periods) with high grass productivity. Mixed-severity fire was modeled as occurring with a mean FRI of 500-1000 years (LANDFIRE 2007a).

In summary, desert communities of perennial plants are dynamic and changing. The composition within this system may change dramatically over time and may be both cyclic and unidirectional. Superimposed on the compositional change is great variation from year to year in growth of all the vegetation - the sum of varying growth responses of individual species to specific conditions of different years (Blaisdell and Holmgren 1984). Desert plants grow when temperature is satisfactory, but only if soil moisture is available at the same time. Because amount of moisture is variable from year to year and because different species flourish under different seasons of soil moisture, seldom do all components of the vegetation thrive in the same year (Blaisdell and Holmgren 1984).

Conversion of this type has commonly come from invasive annual plant species, which displace natural composition and provide fine fuels that significantly increase spread of catastrophic fire. The primary land uses that alter the natural processes of this system are associated with livestock grazing and introduction of exotic annual grasses. Some of the salt desert shrub species are more palatable; Atriplex canescens, Kochia americana, Krascheninnikovia lanata, and Picrothamnus desertorum are at greater risk of overuse by livestock (West 1983b). There is evidence that palatable grasses such as Achnatherum hymenoides, Elymus elymoides, Pleuraphis jamesii, and Sporobolus cryptandrus may have been more abundant before grazing (West 1983). Excessive grazing stresses the system through soil disturbance, diminishing or eliminating the biological soil crust, altering the composition of perennial species, and increasing the establishment of native disturbance-increasers and annual species, particularly Bromus madritensis, Bromus tectorum, Schismus spp., and other exotic annual grasses. The introduction of exotic annual grasses has altered many stands by increasing the amount of fine fuels present that can substantially increase fire frequency and intensity which reduces the cover of shrubs (Sawyer et al. 2009).

When grasshopper and Mormon cricket populations reach outbreak levels, they cause significant economic losses for ranchers and livestock producers, especially when accompanied by a drought (USDA-APHIS 2003, 2010). Both rangeland forage and cultivated crops can be consumed by grasshoppers. The U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) is the Federal agency responsible for controlling economic infestations of grasshoppers on western rangelands with a cooperative suppression program. They work with federal land managing agencies to conduct grasshopper suppression. The goal of APHIS's grasshopper program is not to eradicate them but to reduce outbreak populations to less economically damaging levels (USDA-APHIS 2003). This APHIS effort dampens the natural ecological outbreak cycles of grasshoppers and Mormon crickets, but does not eradicate the species.

Human development has impacted many locations throughout the range of this type. High- and low-density urban and industrial developments have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system occurs in the intermountain western U.S., extending in limited distribution into the southern Great Plains. In the Great Basin, this ecological system occupies sites west of the Wasatch Mountains, east of the Sierra Nevada, south of the Idaho batholith and north of the Mojave Desert. In Wyoming, this system occurs in the Great Divide and Bighorn basins.