Great Basin Big Sagebrush Steppe

This widespread matrix-forming ecological system occurs throughout much of the Columbia Plateau and northern Great Basin, east into the Wyoming Basins, central Montana, and north and east onto the western fringe of the Great Plains in South Dakota. It is found at slightly higher elevations farther south. Relative to other portions of the distribution, in central Montana this system occurs in areas with more summer rain than winter snow precipitation, more overall annual precipitation, and it occurs on glaciated landscapes. Across the entire distribution of this type, soils are typically deep and non-saline, often with a microphytic crust. This shrub-steppe is dominated by perennial grasses and forbs (>25% cover) with Artemisia tridentata ssp. tridentata (this is not at all important in Wyoming occurrences), Artemisia tridentata ssp. xericensis, Artemisia tridentata ssp. wyomingensis, Artemisia tripartita ssp. tripartita (Snake River valley in Wyoming), Artemisia cana ssp. cana, and/or Purshia tridentata dominating or codominating the open to moderately dense (10-40% cover) shrub layer. Atriplex confertifolia, Chrysothamnus viscidiflorus, Ericameria nauseosa, Sarcobatus vermiculatus, Tetradymia spp., or Artemisia frigida may be common especially in disturbed stands. In Montana and Wyoming, stands are more mesic, with more biomass contributed by grasses, have less shrub diversity than stands farther west, and 50 to 90% of the occurrences are dominated by Artemisia tridentata ssp. wyomingensis with Pascopyrum smithii. Associated graminoids can include Achnatherum hymenoides, Calamagrostis montanensis, Elymus lanceolatus ssp. lanceolatus, Koeleria macrantha, Poa secunda, Pascopyrum smithii, Hesperostipa comata, Nassella viridula, Bouteloua gracilis, and Pseudoroegneria spicata. Important rhizomatous species include Carex filifolia and Carex duriuscula, which are very common and important in the eastern distribution of this system in both Wyoming and Montana. Festuca idahoensis is uncommon in this system, although it does occur in areas of higher elevations/precipitation; Festuca campestris is also uncommon. In Wyoming, both Nassella viridula and Pseudoroegneria spicata rarely occur, with the latter typically found in eastern Wyoming on ridgetops and rocky slopes outside of this system. In Montana, there is an absence of Festuca spp., except Vulpia octoflora. Common forbs are Phlox hoodii, Arenaria spp., Opuntia spp., Sphaeralcea coccinea, Dalea purpurea, Liatris punctata, and Astragalus spp. Areas with deeper soils more commonly support Artemisia tridentata ssp. tridentata but have largely been converted for other land uses. The natural fire regime of this ecological system likely maintains a patchy distribution of shrubs, so the general aspect of the vegetation is a grassland. Shrubs may increase following heavy grazing and/or with fire suppression, particularly in moist portions of the northern Columbia Plateau where it forms a landscape mosaic pattern with shallow-soil scabland shrublands. Where fire frequency has allowed for shifts to a native grassland condition, maintained without significant shrub invasion over a 50- to 70-year interval, the area would be considered Columbia Basin Foothill and Canyon Dry Grassland (CES304.993). This ecological system is closely related to the warm-dry sagebrush in the resistance-resilience framework.

This shrub-steppe is dominated by perennial grasses and forbs (>25% cover) with Artemisia tridentata ssp. tridentata (not at all important in Wyoming occurrences), Artemisia tridentata ssp. xericensis, Artemisia tridentata ssp. wyomingensis, Artemisia tripartita ssp. tripartita (Snake River valley in Wyoming), Artemisia cana ssp. cana, and/or Purshia tridentata dominating or codominating the open to moderately dense (10-40% cover) shrub layer. Atriplex confertifolia, Chrysothamnus viscidiflorus, Ericameria nauseosa, Sarcobatus vermiculatus, Tetradymia spp., or Artemisia frigida may be common, especially in disturbed stands. In Montana and Wyoming, stands are more mesic, with more biomass of grass, have less shrub diversity than stands farther west, and 50 to 90% of the occurrences are dominated by Artemisia tridentata ssp. wyomingensis with Pascopyrum smithii. In addition, Bromus arvensis (= Bromus japonicus) and Bromus tectorum are indicators of disturbance, and Bromus tectorum is typically not as abundant as in the Intermountain West, possibly due to a colder climate. Associated graminoids can include Achnatherum hymenoides, Calamagrostis montanensis, Elymus lanceolatus ssp. lanceolatus, Koeleria macrantha, Poa secunda, Pascopyrum smithii, Hesperostipa comata, Nassella viridula, Bouteloua gracilis, and Pseudoroegneria spicata. Important rhizomatous species include Carex filifolia and Carex duriuscula, which are very common and important in the eastern distribution of this system in both Wyoming and Montana. Festuca idahoensis is uncommon in this system, although it does occur in areas of higher elevations/precipitation; Festuca campestris is also uncommon. In Wyoming, both Nassella viridula and Pseudoroegneria spicata rarely occur, with the latter typically found in eastern Wyoming on ridgetops and rocky slopes outside of this system. In Montana, there is an absence of Festuca spp., except Vulpia octoflora. Common forbs are Phlox hoodii, Arenaria spp., Opuntia spp., Sphaeralcea coccinea, Dalea purpurea, Liatris punctata, and Astragalus spp. Areas with deeper soils more commonly support Artemisia tridentata ssp. tridentata but have largely been converted for other land uses. The natural fire regime of this ecological system likely maintains a patchy distribution of shrubs, so the general aspect of the vegetation is a grassland. Shrubs may increase following heavy grazing and/or with fire suppression, particularly in moist portions of the northern Columbia Plateau where it forms a landscape mosaic pattern with shallow-soil scabland shrublands. Where fire frequency has allowed for shifts to a native grassland condition, maintained without significant shrub invasion over a 50- to 70-year interval, the area would be considered Columbia Basin Foothill and Canyon Dry Grassland (CES304.993). The floristic description is based on several references, including Daubenmire (1970), Mueggler and Stewart (1980), Brown (1982a), Hironaka et al. (1983), West (1983c), Barbour and Billings (1988), Knight (1994), Holland and Keil (1995), Howard (1999), Tirmenstein (1999c), West and Young (2000), Barbour et al. (2007a), and Sawyer et al. (2009).

This widespread matrix-forming ecological system occurs throughout much of the Columbia Plateau and northern Great Basin, east into the Wyoming Basins, central Montana, and north and east onto the western fringe of the Great Plains in Montana and South Dakota. It is found at slightly higher elevations farther south.

Climate: Climate is semi-arid and continental with annual precipitation ranging from 18-40 cm and with high inter-annual variation. Precipitation amount and time vary depending on location, with stands in the western portion of its range receiving winter/spring precipitation and very little summer precipitation, whereas stands in the eastern portion of its range receive both winter and summer precipitation. Much of the precipitation falls as snow, and growing-season drought is characteristic. Temperatures are continental with large annual and diurnal variation. In central Montana, this system differs slightly, with more summer rain than winter precipitation, more precipitation annually, and it occurs on glaciated landscapes.

Physiography/landform: Stands occur on stream terraces, point bars, valley floors, alluvial fans, floodplains, washes, gullies, stabilized dunes, mesic uplands, swales, and rocky slopes. Slopes are variable from gentle to very steep.

Soil/substrates/hydrology: Soils are typically deep and non-saline, often with a microphytic crust.

The environmental description is based on several references, including Daubenmire (1970), Mueggler and Stewart (1980), Brown (1982a), Hironaka et al. (1983), West (1983c), Barbour and Billings (1988), Knight (1994), Holland and Keil (1995), Howard (1999), Tirmenstein (1999c), West and Young (2000), Barbour et al. (2007a), and Sawyer et al. (2009).

The natural fire regime of this ecological system likely maintains a patchy distribution of shrubs, so the general aspect of the vegetation is a grassland. Shrubs may increase following heavy grazing and/or with fire suppression, particularly in moist portions of the northern Columbia Plateau where it forms a landscape mosaic pattern with shallow-soil scabland shrublands. Response to grazing can be variable depending on the type of grazer and the season in which grazing occurs. Hesperostipa comata can increase in abundance in response to either grazing or fire. In central and eastern Montana (and possibly elsewhere), complexes of prairie dog towns are common in this ecological system. Microphytic crust is very important in this ecological system.

Complex ecological interactions of fire regimes and climate patterns result in equally complex patterns of species structure and composition in Artemisia tridentata stands. Prolonged drought on the more xeric sites may reduce shrub cover. Flooding may also cause mortality if the soil remains saturated for an extended period of time. The Aroga moth is capable of defoliating large acreages (i.e., >1000 acres, but usually 10-100 acres). Heavy grazing by wildlife can remove the fine fuels that support mixed-severity fires and result in woody fuel buildup that leads to severe, stand-replacement fires (LANDFIRE 2007a, BpS 1210800).

Big sagebrush stands are inhibited by fire as Artemisia tridentata does not sprout after burning (Tirmenstein 1999c). Conversely, increasing fire frequency significantly will eliminate the shrubs from the stands (Daubenmire 1970, Tirmenstein 1999c). With a change in fire frequency, species composition will be altered as well (West 1983c). With a high fire frequency (every 2-5 years), perennial grasses and shrubs are eliminated and non-native annual grasses dominate. At fire-return intervals of 10-30 years, short-lived resprouting shrubs such as Chrysothamnus or Tetradymia spp. dominate. At fire-return intervals of 30-70 years, a mixture of perennial bunchgrasses and non-sprouting shrubs is maintained (Johnson 2000b). Finally, in the complete absence of fire, deep-rooted shrubs such as Artemisia tridentata become dominant. At higher-elevation sites with absence of fire (>100 years), Pinus monophylla and Juniperus osteosperma trees may invade and eventually dominate sites (Tirmenstein 1999c).

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 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).

Climatic variability may have been as important a disturbance agent as fire in these areas. Prolonged drought may have helped to reduce the density and cover of sagebrush. The size of the area affected by the drought would vary from 100s-1000s of acres and may be related to soil type (LANDFIRE 2007a).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has four classes in total (LANDFIRE 2007a, BpS 0911250). These are summarized as:

A) Early Development 1 All Structures (15% of type in this stage): Herbaceous canopy cover is variable (0-50%). This class is dominated by forbs with varying presence of grasses. Post-fire cover and recovery rates vary greatly depending on fire severity and post-fire precipitation amounts and timing as well as pre-fire species composition. This stage lasts 9-15 years, depending on how quickly sagebrush is able to begin reoccupying the area. Replacement fire (MFRI= 100 years) resets.

B) Mid Development 1 Open (30% of type in this stage): Dominant lifeform is herbaceous (20-40% cover), shrub cover 0-10%. Scattered and usually small sagebrush are present, but perennial grasses and forbs continue to dominate. The general formation is that of a shrub savanna. Sagebrush cover is usually 1-5% in this stage. Stands are 15-35 years old. Succession to class C. Replacement fire (MFRI= 100 years) reset to class A. Surface fires (MFRI=1000 years) maintain in class B.

C) Late Development 1 Open (35% of type in this stage): Shrubs have canopy cover of 11-20%. Sagebrush is codominant with the perennial grasses and forbs. The general formation is that of a shrub-steppe. Stands are 35-70 years old; succession to class D. Replacement fire (MFRI=100 years) reset to class A. Mixed fire (MFRI= 50 years) opens the stand to class B. Surface fire (MFRI=1000 years) keeps in class C.

D) Late Development 1 Closed (20% of type in this stage): Shrubs have canopy cover of 21-30%. Sagebrush is dominant with relatively low cover of perennial grasses and forbs. Sagebrush cover can be variable, with the lowest productivity sites reaching only about 15% canopy cover with large areas of bare ground and rock in the interspaces. The general formation is that of a shrubland. Stands are greater than about 70 years old. Replacement fire (MFRI=85 years) reset to class A. Mixed fire (MFRI=85 years) opens the stand to class B.

Conversion of this type has commonly come from agriculture (wheat farming and non-native hay production) where soils are deeper. Rangeland management such as sagebrush reduction treatments (frequent burning, herbicide spraying, and mechanical techniques such as plowing or mowing, and planting Agropyron cristatum) also convert large areas (Wambolt and Payne 1986, Beck et al. 2012). Another major conversion type is due to invasive non-native species such as Bromus tectorum, Centaurea solstitialis, Hypericum perforatum, Poa pratensis, Taeniatherum caput-medusae and Ventenata dubia (Young and Evans 1971, 1973, Mack 1981b, Pellant 1990, 1996, D'Antonio and Vitousek 1992, Chambers et al. 2007a, 2013, D'Antonio et al. 2009). These invasive species increase post-disturbance, including long-term excessive grazing by livestock, or direct soil disturbance from severe trampling by livestock, ORVs and roads. Altered fire regimes, such as repeated, high-frequency fires have eliminated shrubs and created extensive grasslands dominated by non-native invasive annual grass Bromus tectorum and other non-native annual species. Additionally, in some places fire suppression has allowed succession and conversion to woodlands (Tirmenstein 1999c, LANDFIRE 2007a, WNHP 2011).

The primary land uses that alter the natural processes of this system are associated with livestock management practices, invasive annual plant species, fire regime alteration, direct soil surface disturbance, and fragmentation. 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 grasses, particularly Bromus tectorum and other exotic annual bromes. If soil moisture is sufficient and sagebrush seeds are available, grazing can result in increased shrub density. There are strong links between foliose lichens and ecosystem health. Severe trampling breaks lichens into fragments too small to re-establish and eventually leads to foliose lichen elimination (Rosentreter and Eldridge 2002). Domestic livestock grazing is a widespread disturbance factor in sagebrush systems and can affect ecosystem condition. Inappropriate livestock grazing, in terms of stocking rate or season of use, can alter species composition, ecosystem function and structure (Dyksterhuis 1949, as cited by Veblen et al. 2011).

Fire further stresses livestock-altered vegetation by increasing exposure of bare ground and consequently increases exotic annuals and decreases perennial bunchgrass and sagebrush abundance. Fire suppression, even in the absence of livestock grazing impacts, can increase shrub density that in turn reduces bunchgrass cover or results in increased grass litter and fire fuel. Both conditions increase the probability of fire and vegetation responses that increase annual grass abundance following fire (Davies et al. 2009). Any soil and bunchgrass layer disturbances, such as vehicle tracks or chaining shrubs, will increase the probability of alteration of vegetation structure and composition, and response to fire as discussed above. Loss of shrub density and degradation of the bunchgrass layer's native diversity decreases obligate shrub-steppe birds (Vander Haegen et al. 2000).

Fragmentation of shrub-steppe by agriculture increases cover of annual grasses, total annual/biennial forbs, and bare ground, and decreases cover of perennial forbs and biological soil crusts and reduces obligate insects (Quinn 2004) and obligate birds and small mammals (Vander Haegen et al. 2001). Fine fuel adjacency from alien annual grasses, such as Bromus tectorum, currently represents the most important fuelbed component in the system and can substantially increase the fire frequency. With a high fire frequency (every 2-5 years), perennial grasses and shrubs are eliminated and non-native annual grasses dominate. At fire-return intervals of 10-30 years, short-lived resprouting shrubs such as Chrysothamnus or Tetradymia spp. dominate. This expansion of juniper trees into Artemisia tridentata-dominated ecosystems has many effects on the ecology of the site, including reduction of understory cover and species, increased fuel load as trees grow and expand, resulting eventually in large, high-severity fires with high tree and shrub mortality (Miller et al. 2011, 2014). These severely burned areas are highly susceptible to invasion by annual grasses, often resulting in conversion to Bromus tectorum-dominated stands (Chambers et al. 2007a, Condon et al. 2011). Conversion of Artemisia tridentata ecosystems to invasive non-native annual grasses causes habitat degradation, fragmentation and loss for several species, including sage-grouse (Centrocercus spp.) which is now at risk for federal listing (Knick et al. 2003).

An assessment was conducted by Veblen et al. (2011) to evaluate rangewide impacts of livestock grazing across the sagebrush distribution. Most information on range condition is at the local scale and not consistently collected for regional or rangewide assessment; however, the study was able to compile and utilize available data. Using Land Health Standards (LHS) data and sagebrush vegetation characteristics, the study compared LHS across a subset of allotments within the sagebrush biome. Results showed 798 allotments (70%) that met and 333 allotments that did not meet LHS. Livestock grazing was identified as the reason for unmet standards for 132 (approximately 15%) of the 333 allotments that did not meet standards. Therefore, across the sagebrush distribution, a relatively small percentage of allotments are being significantly impacted by livestock grazing.

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 type distribution. 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 throughout much of the Columbia Plateau, the northern Great Basin, central and southeastern Montana, and Wyoming, and is found at slightly higher elevations farther south.