Great Basin Big Sagebrush Shrubland

This ecological system occurs throughout much of the interior western U.S., typically in broad basins between mountain ranges, plains and foothills between 800 and 2500 m elevation. Soils are typically deep, well-drained and non-saline. These shrublands are dominated by Artemisia tridentata ssp. tridentata (not as common in Wyoming or Montana but possibly on stabilized part of Killpecker Dunes in Wyoming) and/or Artemisia tridentata ssp. wyomingensis (predominant in Wyoming and Montana). Scattered Juniperus spp., Sarcobatus vermiculatus, and Atriplex spp. may be present in some stands. Ericameria nauseosa, Chrysothamnus viscidiflorus, Purshia tridentata (not commonly in Montana or Wyoming), or Symphoricarpos oreophilus may codominate disturbed stands (e.g., in burned stands, these may become more predominant). Perennial herbaceous components typically contribute less than 25% vegetative cover. Common graminoid species can include Achnatherum hymenoides, Bouteloua gracilis, Elymus lanceolatus, Festuca idahoensis (not in Montana or Wyoming), Hesperostipa comata, Leymus cinereus, Pleuraphis jamesii (not present in northeastern portions of the range), Pascopyrum smithii, Poa secunda, or Pseudoroegneria spicata (not in Wyoming). Dunes in the Red Desert have areas of large basin big sage with very dense canopies. In Wyoming, this system is likely to only contain Artemisia tridentata ssp. tridentata.

These shrublands are dominated by Artemisia tridentata ssp. tridentata (not as common in Wyoming or Montana but possibly on the stabilized part of Killpecker Dunes in Wyoming) and/or Artemisia tridentata ssp. wyomingensis (predominant in Wyoming and Montana). Scattered Juniperus spp., Sarcobatus vermiculatus, and Atriplex spp. may be present in some stands. Ericameria nauseosa, Chrysothamnus viscidiflorus, Purshia tridentata (not commonly in Montana or Wyoming), or Symphoricarpos oreophilus may codominate disturbed stands (e.g., in burned stands, these may become more predominant). Perennial herbaceous components typically contribute less than 25% vegetative cover. Common graminoid species can include Achnatherum hymenoides, Bouteloua gracilis, Elymus lanceolatus, Festuca idahoensis (not in Montana or Wyoming), Hesperostipa comata, Leymus cinereus, Pleuraphis jamesii (not present in northeastern portions of the range), Pascopyrum smithii, Poa secunda, or Pseudoroegneria spicata (not in Wyoming). Some semi-natural communities are included that often originate on abandoned agricultural land or on other disturbed sites. In these locations, Bromus tectorum or other annual bromes and invasive weeds can be abundant. Most Artemisia tridentata ssp. wyomingensis communities in Wyoming are placed in Inter-Mountain Basins Big Sagebrush Steppe (CES304.778); the shrubland system is more restricted in environmental setting than the steppe. Dunes in the Red Desert have areas of large basin big sagebrush with very dense canopies. In Wyoming, this system is likely to only contain Artemisia tridentata ssp. tridentata. The vegetation description is based on several references, including Brown (1982a), West (1983a), Barbour and Billings (1988), Knight (1994), Shiflet (1994), Holland and Keil (1995), Reid et al. (1999), West and Young (2000), Barbour et al. (2007a), and Sawyer et al. (2009).

This ecological system occurs throughout much of the interior western U.S., typically in broad basins between mountain ranges, plains and foothills between 1500 and 2500 m elevation.

Climate: The climate where this system occurs is semi-arid with annual precipitation ranging from 18-40 cm and high inter-annual variation. Much of the precipitation falls as snow, and growing-season drought is characteristic. Temperatures are continental with large annual and diurnal variation. In drier regions, these shrublands are usually associated with perennial or ephemeral stream drainages with water tables less than 3 m from the soil surface.

Physiography/landform: Sites supporting this system include flat to steeply sloping uplands on alluvial fans and terraces, toeslopes, lower and middle slopes, draws, badlands, and deep, well-drained alluvial bottomlands foothills and basins and plains (Barker and McKell 1983).

Soil/substrates/hydrology: In drier regions, these shrublands are usually associated with perennial or ephemeral stream drainages with water tables less than 3 m from the soil surface. Substrates are typically deep, well-drained and non-saline, fine- to medium-textured alluvial soils with some source of subirrigation during the summer season, but moderately deep upland soils with ample moisture storage also support these shrublands. Some stands occur on deep, sandy soils, or soils that are highly calcareous (Hironaka et al. 1983). Although this system may grade into sites with alkaline soils at the edge of internally drained basins, Artemisia tridentata is a non-halophyte and requires low salinity for optimum growth. The importance of perennial bunch grasses, the most typical herbaceous associates, is favored with greater spring and summer rain, which increases northward and eastward.

The environmental description is based on several references, including Brown (1982a), Hironaka et al. (1983), West (1983a), Barbour and Billings (1988), Knight (1994), Shiflet (1994), Holland and Keil (1995), Reid et al. (1999), West and Young (2000), Barbour et al. (2007a), and Sawyer et al. (2009).

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 result in lower shrub cover. Flooding may also cause plant 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 reproduces from seed only, so stands are inhibited by fire as Artemisia tridentata does not sprout after burning (Howard 1999, Tirmenstein 1999c). Increasing fire frequency can eliminate the shrubs from the stands (Daubenmire 1970, Tirmenstein 1999c). With a change in fire frequency, species composition will be altered as well (West 1983a). With a high fire frequency (every 2-5 years), perennial grasses and shrubs are eliminated and non-native annual grasses dominate (Whisenant 1990, D'Antonio and Vitousek 1992). 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 bunch grasses 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 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).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total and two classes (classes D & E) that model conversion to forest systems (LANDFIRE 2007a, BpS 1210800). These are summarized as:

A) Early Development 1 - All Structures (15% of type in this stage): Early development is dominated by grasses and forbs with scattered shrubs representing <10% upper canopy cover. Post-replacement disturbance; grass-dominated with scattered shrubs. Fuel loading discontinuous. Surface fire occurs every 200 years on average but has no effect on succession. Succession to class B after 20 years.

B) Mid Development 1 Open (shrub-dominated - 50% of type in this stage): Shrub cover 11-50%. Shrubs and herbaceous vegetation can be codominant, fine fuels bridge the woody fuels, but fuel discontinuities are possible. Replacement fire accounts for 80% of fire activity (mean FRI of 125 years), whereas mixed-severity fire occurs every 500 years on average (20% of fire activity) and maintains vegetation in class B. Succession to class C after 40 years.

C) Mid Development 1 Closed (shrub-dominated - 25% of type in this stage): Shrubs dominate the landscape; fuel loading is primarily woody vegetation. Shrub density sufficient in old stands to carry the fire without fine fuels. Establishment of pinyon and juniper seedlings and saplings widely scattered. Replacement fire (mean FRI of 100 years) and rare flood events (return interval of 333 years) cause a transition to class A. Prolonged drought (mean return interval of 100 years) and insect/disease (every 75 years on average) cause a transition to class B. Succession to class D after 40 years.

D) Late Development 1 Open (5% of type in this stage): Shrubs may still represent the dominant lifeform with pinyon and juniper saplings common (1-15% upper canopy cover). Pinyon-juniper encroachment where disturbance has not occurred for at least 100 years (tree species cover <15%). Saplings and young trees are the dominant lifeform. Sagebrush cover (<25%) and herbaceous cover decreasing compared to class C. Replacement fire occurs every 125 years on average. Insect/disease (every 75 years) and prolonged drought (every 100 years) thin both trees and shrubs, causing a transition to class C. Succession to class E after 50 years.

E) Late Development 1 Closed (5% of type in this stage): Shrubland encroached with mature pinyon and/or juniper (cover 16-90%) where disturbance does not occur for at least 50 years in class D. Shrub cover <10% and graminoids scattered. Replacement fire occurs every 125 years on average. Prolonged drought thins trees, causing a transition to class B.

Conversion of this type has come from agriculture (wheat farming and non-native hay production) where soils are deeper and water sources are available. 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). Substantial area has been lost 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, D'Antonio and Vitousek 1992, Chambers et al. 2007a, D'Antonio et al. 2009, Chambers et al. 2013, Miller et al. 2014). 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 (Pellant 1990, 1996). 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, annual exotic plant species, fire regime alteration, direct soil surface disturbance, and landscape 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 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 depending on the amount of cheatgrass in the understory. 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).

Fire further stresses livestock-altered vegetation by increasing exposure of bare ground and consequently increases exotic annuals and decreases perennial bunchgrass and sagebrush abundance. Presettlement stand-replacing fire frequency was 40-60 years, with smaller fires every 20-25 years (Wright et al. 1979). Repeated burning every few years or burning in summer will reduce the cover of perennial grasses and allow invasive forbs and cheatgrass to increase. Following a fire, sagebrush must re-establish itself from seed, and recovery is slow (Bunting et al. 1987). Fire favors shrubs such as Ericameria nauseosa that can resprout after fire (Tirmenstein 1999b). Fine fuel adjacency from exotic annual grasses, such as Bromus tectorum, Taeniatherum caput-medusae, Bromus madritensis, and Schismus spp., currently represents the most important fuelbed component in the system and can substantially increase the fire frequency. Locally in areas with a high fire frequency (every 2-5 years) or high-severity fire, perennial grasses and shrubs may be eliminated and non-native annual grasses will dominate (Pellant 1990, 1996).

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). Fire suppression can lead to pinyon-juniper encroachment with subsequent loss of shrub and herbaceous understory where adjacent to pinyon-juniper woodlands (LANDFIRE 2007a).

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 has been found to decrease the presence of 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 populations of obligate insects (Quinn 2004), obligate birds and small mammals (Vander Haegen et al. 2000, 2001).

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 and energy development facilities 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 interior western U.S., typically in broad basins between mountain ranges, plains and foothills. Its core distribution is in the Great Basin, but it extends north into the Columbia Basin and west into the foothills of the Sierra Nevada and Cascades, and east into the Colorado Plateau, Wyoming Basins and central and eastern Montana, although much of the sagebrush in this region is more steppe in physiognomy.