Intermountain Mountain Sagebrush Steppe

This ecological system includes sagebrush communities occurring at foothills (in Wyoming) to montane and subalpine elevations across the western U.S. from 1000 m in eastern Oregon and Washington to over 3000 m in the Southern Rockies. In Montana, it occurs on isolated mountains in the north-central portion of the state and possibly along the Boulder River south of Absarokee and at higher elevations. In British Columbia, it occurs between 450 and 1650 m in the southern Fraser Plateau and the Thompson and Okanagan basins. Climate is cool, semi-arid to subhumid. This system primarily occurs on deep-soiled to stony flats, ridges, nearly flat ridgetops, and mountain slopes. In general, this system is found on fine-textured soils, some source of subsurface moisture or more mesic sites, zones of higher precipitation, and areas of snow accumulation. Across its range, this is a compositionally diverse system. It is composed primarily of Artemisia tridentata ssp. vaseyana, Artemisia cana ssp. viscidula, and related taxa such as Artemisia tridentata ssp. spiciformis. Purshia tridentata may codominate or even dominate some stands. Artemisia arbuscula ssp. arbuscula-dominated shrublands commonly occur within this system on rocky or windblown sites. Other common shrubs include Symphoricarpos spp., Amelanchier spp., Ericameria nauseosa, Peraphyllum ramosissimum, Ribes cereum, and Chrysothamnus viscidiflorus. Artemisia tridentata ssp. wyomingensis may be present to codominant if the stand is clearly montane as indicated by montane indicator species such as Festuca idahoensis, Leucopoa kingii, or Danthonia intermedia. Most stands have an abundant perennial herbaceous layer (over 25% cover, in many cases over 50% cover), but this system also includes Artemisia tridentata ssp. vaseyana shrublands. Common graminoids include Danthonia intermedia, Festuca arizonica, Festuca idahoensis, Hesperostipa comata, Poa fendleriana, Elymus trachycaulus, Bromus carinatus, Poa secunda, Deschampsia cespitosa, Calamagrostis rubescens, and Pseudoroegneria spicata. Species of Achnatherum are common, including Achnatherum nelsonii ssp. dorei, Achnatherum nelsonii ssp. nelsonii, Achnatherum hymenoides, and others. In many areas, wildfires can maintain an open herbaceous-rich steppe condition, although at most sites, shrub cover can be unusually high for a steppe system (>40%), with the moisture providing equally high grass and forb cover. This ecological system is closely related to the cool-moist sagebrush in the resistance-resilience framework.

Across its range of distribution, this is a compositionally diverse system. It is composed primarily of Artemisia tridentata ssp. vaseyana, Artemisia cana ssp. viscidula, and related taxa such as Artemisia tridentata ssp. spiciformis (= Artemisia spiciformis). Purshia tridentata may codominate or even dominate some stands. Artemisia arbuscula ssp. arbuscula-dominated shrublands commonly occur within this system on rocky or windblown sites. Other common shrubs include Symphoricarpos spp., Amelanchier spp., Ericameria nauseosa, Peraphyllum ramosissimum, Ribes cereum, and Chrysothamnus viscidiflorus. Artemisia tridentata ssp. wyomingensis may be present to codominant if the stand is clearly montane as indicated by montane indicator species such as Festuca idahoensis, Leucopoa kingii, or Danthonia intermedia. Most stands have an abundant perennial herbaceous layer (over 25% cover, in many cases over 50% cover), but this system also includes Artemisia tridentata ssp. vaseyana shrublands that generally have lower herbaceous cover and may have higher shrub cover. Common graminoids include Achnatherum nelsonii ssp. dorei, Achnatherum nelsonii ssp. nelsonii, Achnatherum hymenoides, Bromus carinatus, Calamagrostis rubescens, Carex spp., Danthonia intermedia, Danthonia parryi, Deschampsia cespitosa, Elymus trachycaulus, Festuca arizonica, Festuca idahoensis, Festuca thurberi, Hesperostipa comata, Koeleria macrantha, Leucopoa kingii, Pascopyrum smithii, Poa fendleriana, Poa secunda, or Pseudoroegneria spicata. Forbs are often numerous and an important indicator of health. Forb species may include species of Castilleja, Potentilla, Erigeron, Phlox, Astragalus, Geum, Lupinus, and Eriogonum, as well as Balsamorhiza sagittata, Achillea millefolium, Antennaria rosea, Eriogonum umbellatum, Fragaria virginiana, Artemisia ludoviciana, Hymenoxys hoopesii (= Helenium hoopesii), etc. In many areas, wildfires can maintain an open herbaceous-rich steppe condition, although at most sites, shrub cover can be unusually high for a steppe system (>40%), with the moisture providing equally high grass and forb cover. The vegetation description is based on several other references, including Daubenmire (1970), Young et al. (1977), Mueggler and Stewart (1980), Brown (1982a), Hironaka et al. (1983), West (1983c), Barbour and Billings (1988), Padgett et al. (1989), Knight (1994), Hansen et al. (1995), Holland and Keil (1995), Howard (1999), Johnson (2000b), West and Young (2000), Barbour et al. (2007a), and Sawyer et al. (2009).

This ecological system includes sagebrush communities occurring at foothills (in Wyoming) to montane and subalpine elevations across the western U.S. from 1000 m elevation in eastern Oregon and Washington to over 3000 m in the Southern Rockies. In Montana, it occurs in isolated mountains in the north-central portion of the state and possibly along the Boulder River south of Absarokee and at higher elevations. In British Columbia, it occurs between 450 and 1650 m in the southern Fraser Plateau and the Thompson and Okanagan basins.

Climate: Climate is cool, semi-arid to subhumid with yearly precipitation ranging from 25 to 90 cm/year. Much of this precipitation falls as snow. In general, this system occurs on fine-textured soils, some source of subsurface moisture or more mesic sites, zones of higher precipitation, and areas of snow accumulation.

Physiography/landform: This system primarily occurs on deep-soiled to stony flats, ridges, nearly flat ridgetops, and mountain slopes. Stands occur on all aspects, but the higher-elevation occurrences may be restricted to south- or west-facing slopes.

Soil/substrates/hydrology: Soils generally are moderately deep to deep, well-drained, and of loam, sandy loam, clay loam, or gravelly loam textural classes; soils often have a substantial volume of coarse fragments and are derived from a variety of parent materials.

The environmental description is based on several other references, including Daubenmire (1970), Young et al. (1977), Mueggler and Stewart (1980), Brown (1982a), Hironaka et al. (1983), West (1983c), Barbour and Billings (1988), Padgett et al. (1989), Knight (1994), Hansen et al. (1995), Holland and Keil (1995), Howard (1999), Johnson (2000b), 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 (Johnson 2000b). Healthy stands often have a very productive herbaceous understory that is high quality forage for livestock.

Like other big sagebrush subspecies, Artemisia tridentata ssp. vaseyana stands are inhibited by fire as Artemisia tridentata does not sprout after being top-killed by fire and may take over 10 years to form stands with 20% or more cover (Johnson 2000b, Sawyer et al. 2009). Winward (1991) suggests Artemisia tridentata ssp. vaseyana shrublands have a natural fire regime of 10-30 years. Presettlement fires tended to be patchy, forming a mosaic of different age and density of shrubs because of different fire intensity across the landscape (Winward 1991, Tart 1996). Regeneration of mountain big sagebrush is from on-site or off-site seed and, depending on circumstances of the environment and seed source, Artemisia tridentata ssp. vaseyana seeds may sprout abundantly or very sparsely the following spring after burning (Johnson 2000b). Establishment after severe fires may proceed more slowly (Bunting et al. 1987, Johnson 2000b). Increasing fire frequency significantly will eliminate the shrubs from the stands (Daubenmire 1970, Johnson 2000b). Stand species composition will be altered with changes in fire frequency (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 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), trees such as Pinus monophylla, Juniperus occidentalis, and Juniperus osteosperma may invade and eventually dominant sites (Young et al. 1977, Bunting 1990, Johnson 2000b).

In addition, 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 affected areas are relatively small (10-100 acres). Of the three big sagebrush subspecies and black sagebrush, Artemisia tridentata ssp. vaseyana was found to be the most palatable browse for elk (Wambolt 1995, 1996). These big game preference differences may make it more sensitive to effects of browsing. Heavy grazing in these mountain shrub-steppes may decrease fire frequency due to consumption of herbaceous forage (fine fuels), resulting in increased shrub density (woody fuel buildup) that leads to severe, stand-replacement fires (LANDFIRE 2007a, BpS 1211260).

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 1211260). These are summarized as:

A) Early Development 1 Open (herbaceous-dominated - 20% of type in this stage): Herbaceous cover is variable but typically >50% (50-80%). Shrub cover is 0-5%. Replacement fire (mean fire return interval (FRI) of 80 years) setbacks succession by 12 years. Succession to class B after 12 years.

B) Mid Development 1 Open (shrub-dominated - 50% of type in this stage): Shrub cover 6-25%. Mountain big sagebrush cover up to 20%. Herbaceous cover is typically >50%. Initiation of conifer seedling establishment. Replacement fire mean FRI is 40 years. Succession to class C after 38 years.

C) Late Development 1 Closed (shrub-dominated - 15% of type in this stage): Shrubs are the dominant lifeform with canopy cover of 26-45+%. Herbaceous cover is typically <50%. Conifer (juniper, pinyon-juniper, ponderosa pine or white fir) cover <10%. Insects and disease every 75 years on average will thin the stand and cause a transition to class B. Replacement fire occurs every 50 years on average. In the absence of fire for 80 years, vegetation will transition to class D. Otherwise, succession keeps vegetation in class C.

D) Late Development 1 Open (conifer-dominated - 10% of type in this stage): Conifers are the upper lifeform (juniper, pinyon-juniper, ponderosa pine, limber pine or white fir). Conifer cover is 11-25%. Shrub cover generally less than mid-development classes but remains between 26-40%. Herbaceous cover <30%. The mean FRI of replacement fire is 50 years. Insects/diseases thin the sagebrush, but not the conifers, every 75 years on average, without causing a transition to other classes. Succession is from class C to class D after 50 years.

E) Late Development 2 Closed (conifer-dominated - 5% of type in this stage): Conifers are the dominant lifeform (juniper, pinyon-juniper, ponderosa pine, limber pine or white fir). Conifer cover ranges from 26-80% (pinyon-juniper 36-80% (Miller and Tausch 2001), juniper 26-40% (Miller and Rose 1999) and white fir 26-80%). Shrub cover 0-20%. Herbaceous cover <20%. The mean FRI for replacement fire is longer than in previous states (75 years). Conifers are susceptible to insects/diseases that cause diebacks (transition to class D) every 75 years on average (LANDFIRE 2007a). The woodland systems that this montane sagebrush system would succeed into vary by location.

Conversion of Artemisia tridentata ecosystems to invasive, non-native grasses and forbs causes habitat degradation, fragmentation and loss for several species, including sage-grouse (Centrocercus spp.) which is now being considered for federal listing (Knick et al. 2003). Another potential means to ecological conversion of mountain sagebrush shrubland system is succession to conifer woodlands. With severe fire regime alteration and extended fire suppression, trees, especially junipers and pinyons, colonized these shrublands and grow to the eventual exclusion of the shade-intolerant sagebrush (LANDFIRE 2007a).

The primary land uses that alter the natural processes of this system are associated with domestic livestock grazing and introduction of exotic grasses. Excessive grazing stresses the system through soil disturbance, altering the composition of perennial species, and increasing the establishment of native disturbance-increasers and grasses, particularly Poa pratensis, a grazing-tolerant, exotic perennial grass. Excessive cattle grazing will decrease the abundance of native bunch grasses such as Festuca idahoensis, Koeleria macrantha, and Pseudoroegneria spicata, and increase the cover of grazing-tolerant grass species and forbs (Tart 1996). Overgrazing by sheep will also decrease some forbs, such as species of Geranium, Ligusticum, Packera, and Potentilla, and increase others, such as species of Achillea, Antennaria, Arenaria, and Lupinus (Tart 1996). In general, heavy grazing also favors shrubs that increase in density and cover and reduces the herbaceous layer (fine fuels) that allows fire to spread, which reduces fire frequency (Tart 1996).

Domestic livestock grazing is a widespread disturbance factor in sagebrush systems and can affect ecosystem condition (Veblen et al. 2011). 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). An assessment was conducted by Veblen et al. (2011) to evaluate rangewide impacts of livestock grazing across the sagebrush biome. 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 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 biome, a relatively small percentage of allotments are being significantly impacted by livestock grazing.

Human development has impacted many locations throughout the type's 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 is found at montane and subalpine elevations across the western U.S. from 1000 m in eastern Oregon and Washington to over 3000 m in the Southern Rockies. In British Columbia, it occurs in the southern Fraser Plateau and the Thompson and Okanagan basins. This system occurs in mapzone 20 on the Rocky Mountain island ranges and on the western edge with mapzone 19.