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Inter-Mountain Basins 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 Montana and South Dakota. It is found at slightly higher elevations farther south. In central Montana, this system differs slightly, with more summer rain than winter precipitation, more precipitation annually, and it occurs on glaciated landscapes. 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 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 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).

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