Intermountain Semi-Desert Grassland

This widespread ecological system includes the driest grasslands throughout the intermountain western U.S. It occurs on xeric sites over an elevation range of approximately 1450 to 2320 m (4750-7610 feet) on a variety of landforms, including swales, playas, mesas, alluvial flats, and plains. This system may constitute the matrix over large areas of intermountain basins, and also may occur as large patches in mosaics with shrubland systems dominated by Artemisia tridentata ssp. tridentata, Artemisia tridentata ssp. wyomingensis, Atriplex spp., Coleogyne spp., Ephedra spp., Gutierrezia sarothrae, or Krascheninnikovia lanata. Grasslands in areas of higher precipitation, at higher elevation, typically belong to other systems. Substrates are often well-drained sandy or loam soils derived from sedimentary parent materials but are quite variable and may include fine-textured soils derived from igneous and metamorphic rocks. The dominant perennial bunchgrasses and shrubs within this system are all drought-resistant plants. Dominant or codominant species are Achnatherum hymenoides, Aristida spp., Bouteloua gracilis, Hesperostipa comata, Muhlenbergia spp., Pleuraphis jamesii, or Sporobolus spp. Scattered shrubs and dwarf-shrubs often are present, especially Artemisia tridentata ssp. tridentata, Artemisia tridentata ssp. wyomingensis, Atriplex spp., Coleogyne spp., Ephedra spp., Ericameria spp., Gutierrezia sarothrae, and Krascheninnikovia lanata. This system is typically composed of cool-season grasses in the western portion of its range where winter precipitation dominates, and a mix of cool- and warm-season grasses where precipitation occurs during both winter and summer seasons (Colorado Plateau). Grasslands in the basins of south-central and southwestern Wyoming, dominated by Pseudoroegneria spicata and Poa secunda and containing cushion-form forbs and other species typical of dry basins, are included in this system.

This grassland system may constitute the matrix over large areas of intermountain basins and may also occur as large patches in mosaics with shrubland systems dominated by Artemisia tridentata ssp. tridentata, Artemisia tridentata ssp. wyomingensis, Atriplex spp., Coleogyne spp., Ephedra spp., Gutierrezia sarothrae, or Krascheninnikovia lanata. The dominant perennial bunchgrasses and shrubs within this system are all drought-resistant plants. Dominant or codominant species are Achnatherum hymenoides, Aristida spp., Bouteloua gracilis, Hesperostipa comata, Muhlenbergia spp., or Pleuraphis jamesii. Additional perennial warm-season grasses found in this system include Aristida purpurea, Bouteloua curtipendula, Bouteloua eriopoda, Muhlenbergia asperifolia, Muhlenbergia pungens, Muhlenbergia richardsonis, Muhlenbergia torreyi, Pleuraphis rigida, Sporobolus airoides, Sporobolus contractus, and Sporobolus cryptandrus. Scattered shrubs and dwarf-shrubs often are present, especially Artemisia tridentata ssp. tridentata, Artemisia tridentata ssp. wyomingensis, Atriplex spp., Coleogyne spp., Ephedra spp., Gutierrezia sarothrae, and Krascheninnikovia lanata. Grasslands in the basins of south-central and southwestern Wyoming, dominated by Pseudoroegneria spicata and Poa secunda and containing cushion-forming forbs and other species typical of dry basins, are included in this system. In the Columbia Plateau, this semi-desert ecological system does not include Pseudoroegneria spicata-dominated or -codominated associations such as Pseudoroegneria spicata - Achnatherum hymenoides Grassland (CEGL001674) or Pseudoroegneria spicata - Poa secunda Grassland (CEGL001677). Additionally, Poa cusickii Marsh (CEGL001655) is restricted to relatively mesic sites there and does not occur in this semi-desert system as it occurs in the Columbia Plateau, but may be found in this system in Wyoming. The vegetation description is based on several other references, including Barbour and Major (1977), Brown (1982), West (1983e), Knight (1994), Reid et al. (1999), West and Young (2000), Tuhy et al. (2002), Barbour et al. (2007), and Sawyer et al. (2009).

This widespread semi-arid ecological system consists of lower-elevation dry grasslands found on plains, mesas and foothills throughout the intermountain western U.S. Elevation ranges from approximately 1450 to 2320 m (4750-7610 feet).

Climate: Climate usually includes hot summers and cold winters with freezing temperatures and snow common. Annual precipitation is usually from 20-40 cm (7.9-15.7 inches). A significant portion of the precipitation falls in July through October during the summer monsoon storms, with the rest falling as snow during the winter and early spring months (bimodal precipitation). However, precipitation in the western portion of this system's range occurs primarily in the winter.

Physiography/landform: These grasslands occur on a variety of aspects, slopes and landforms, including swales, playas, mesas, alluvial flats, plains and hillslopes. Stands are found in lowland and upland areas usually on xeric sites. Grasslands in areas of higher precipitation, at higher elevation, typically belong to other systems.

Soil/substrate/hydrology: Substrates range from deep to shallow, frequently well-drained sandy or loam soils derived from sandstone or shale parent materials but are quite variable and may include fine-textured soils derived from igneous and metamorphic rocks. Some occurrences on sandy soils have a high cover of cryptogams on the soil surface. These cryptogams tend to increase the stability of the highly erodible sandy soils of these grasslands during torrential summer rains and heavy wind storms (Kleiner and Harper 1977). Muhlenbergia-dominated grasslands which flood temporarily, combined with high evaporation rates in this dry system, can have accumulations of soluble salts in the soil. Soil salinity depends on the nature of the parent material and on the amount and timing of precipitation and flooding. Growth-inhibiting salt concentrations are diluted when the soil is saturated, allowing the growth of less salt-tolerant species. As the saturated soils dry, the salt concentrates until it precipitates out on the soil surface (Dodd and Coupland 1966, Ungar 1968). The environmental description is based on several other references, including Barbour and Major (1977), Brown (1982), West (1983e), Knight (1994), Reid et al. (1999), West and Young (2000), Tuhy et al. (2002), Barbour et al. (2007), and Sawyer et al. (2009).

Disturbance dynamics in this semi-arid grassland system are variable because of variation in the composition; however, most are dominated by perennial bunchgrasses that are adapted to low- to medium-frequency (<30 to <100 years) and low- to medium-intensity fires (Howard 1997a, b, Tirmenstein 1999e, Zlatnik 1999a, b, Johnson 2000c, Simonin 2000a, b, c, Anderson 2003a, Sawyer et al. 2009). Most of the species are classified as resistant or tolerant of fire, with the exception of Bouteloua eriopoda, which is classified as sensitive, but will recover quickly if there is adequate summer moisture (Simonin 2000a). Season of burn is also important for predicting post-burn recovery.

The majority of characteristic grass species, such as Achnatherum hymenoides, Aristida spp., Bouteloua eriopoda, Bouteloua gracilis, Hesperostipa comata, Pleuraphis jamesii, Poa secunda, Pseudoroegneria spicata, Sporobolus airoides, and Sporobolus cryptandrus, will be top-killed after burning, then resprout from rootcrowns unless the fire was very severe (Howard 1997a, b, Tirmenstein 1999e, Zlatnik 1999a, b, Johnson 2000c, Simonin 2000a, b, c, Anderson 2003a, Sawyer et al. 2009). This grassland system is maintained by fires that kill or reduce cover of the more fire-sensitive shrub species.

The dominant perennial grass species are well-adapted to the semi-arid conditions. Achnatherum hymenoides is one of the most drought-tolerant, cool-season grasses in the western U.S. (USFS 1937, Tirmenstein 1999e). It is also a valuable forage grass in arid and semi-arid regions. Hesperostipa comata is a deep-rooted, cool-season grass that uses soil moisture below 0.5 m depth during the dry summers. It is prone to litter accumulations at plant bases, which can increase intensity of fire, making it more susceptible to mortality (Zlatnik 1999a). Bouteloua gracilis is a drought- and very grazing-tolerant warm-season grass that generally forms a short sod. Pleuraphis jamesii, also a warm-season grass, is only moderately palatable to grazers, but decreases when heavily utilized during drought and in the more arid portions of its range where it is the dominant grass (West et al. 1972). This grass reproduces extensively from scaly rhizomes, which make the plant resistant to trampling by large wildlife or livestock and have good soil-binding properties (Weaver and Albertson 1956, West 1972).

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 2,000 square miles. 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).

LANDFIRE developed this VDDT model for this system for the Great Basin using two classes (LANDFIRE 2007a, BpS 1211350).

A) Early Development 1 Open (grass-dominated - 20% of type in this stage): Dominated by grasses (Achnatherum hymenoides, Hesperostipa comata) and post-fire-associated forbs, and remnant Artemisia tridentata. Perennial grasses and forbs dominate (generally 25-40% cover) where woody shrub canopy has been top-killed/removed by wildfire. Shrub cover is less than 5%. Replacement fire occurs every 120 years on average. Succession to class B after 20 years.

B) Mid Development 1 Open (grass with shrubs - 80% of type in this stage): Dominated by grasses (Achnatherum hymenoides, Hesperostipa comata) and Artemisia tridentata. Shrubs compose the upper layer lifeform (5-25% cover) with diverse perennial grass and forb understory dominant. Mean fire-return interval (FRI) is 75 years with 80% replacement fire (mean FRI of 94 years) and 20% mixed-severity fire (mean FRI of 375 years). Mixed-severity fire, insect/disease (return interval of 75 years), and weather-related stress (return interval of 100 years) maintain vegetation in class B.

Conversion of this type has commonly come from the combination of heavy livestock use and drought, which can push these grassland communities over thresholds that are often irreversible because of soil loss and arroyo formation. Relatively intact sites will have both native perennial grasses and intact biological soil crusts. Conversions occur as biological soil crusts decrease, shrubs increase, and non-native species begin to invade, such as Bromus rubens, Bromus tectorum, Centaurea solstitialis, Hypericum perforatum, and Poa pratensis. The final endpoint on severely altered sites is non-native grasses and severe soil loss. This has been well established on both the Colorado Plateau and Great Basin.

The primary land uses that alter the natural processes of this system are associated with livestock practices, invasive annual plant invasion, fire regime alteration, direct soil surface disturbance, and fragmentation (WNHP 2011). 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 including Aristida spp., Ericameria spp., and Gutierrezia sarothrae, and annual grasses, particularly Bromus tectorum and other exotic annual bromes. Persistent grazing will further diminish perennial grass cover, expose bare ground, increase exotic annuals, and may lead to higher density of Ericameria spp. or Gutierrezia sarothrae. Fire further stresses livestock-altered vegetation by increasing exposure of bare ground to erosion and consequent increases in exotic annuals and decrease in perennial bunchgrasses. The introduction of Bromus tectorum into these communities has altered fuel loads and fuel distribution. More frequent fire favors cool-season annuals that complete their life cycles in early spring, leaving abundant fine fuels that burn hot and damage and kill perennial grasses. Fragmentation of grasslands by agriculture also increases cover of annual grass, annual/biennial forbs, bare ground, decreases cover of perennial forbs and biological soil crusts, and reduces obligate insects (Quinn 2004), obligate birds and small mammals (Vander Haegen et al. 2001).

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 system. High- and low-density urban and industrial developments can 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.

Common stressors and threats include fragmentation from roads, ORV use (LANDFIRE 2007a, WNHP 2011), altered fire regime from too frequent fires caused by build ups of fine fuels from invasion of non-native annual grasses (Pellant 1990, 1996), altered fire regime from active fire suppression and indirect fire suppression from livestock grazing and fragmentation, and introduction of invasive non-native species (WNHP 2011). The most serious current threat is from the interaction between livestock grazing and long-term drought, which together exceed the resilience of system and leads to degradation and conversion.

This system occurs throughout the intermountain western U.S. on dry plains and mesas, at approximately 1450 to 2320 m (4750-7610 feet) elevation. In the Bighorn Basin of north-central Wyoming, there may be some desert grasslands, but this is uncertain.