Columbia Plateau Steppe and Grassland

This system occurs throughout much of the Columbia Plateau. It is a bunchgrass-dominated grassland or steppe that is similar floristically to big sagebrush-dominated steppe, but is defined by a more frequent fire regime and the absence or low cover of shrubs over large areas. These are large, extensive grasslands, not grass-dominated patches within the sagebrush shrub-steppe ecological system. Soils are variable, ranging from relatively deep, fine-textured often with coarse fragments, and non-saline often with a microphytic crust, to stony volcanic-derived clays to alluvial sands. This grassland is dominated by perennial bunchgrasses and forbs (>25% cover), sometimes with a sparse (<10% cover) shrub layer. Associated graminoids include Achnatherum hymenoides, Elymus elymoides, Elymus lanceolatus ssp. lanceolatus, Hesperostipa comata, Festuca idahoensis, Koeleria macrantha, Poa secunda, and Pseudoroegneria spicata. Common forbs are Phlox hoodii, Arenaria spp., and Astragalus spp. Shrubs such as Chrysothamnus viscidiflorus, Ericameria nauseosa, Tetradymia spp., or Artemisia spp. are often present in disturbed stands. Areas with deeper soils are rare because of conversion to other land uses. The rapid fire-return regime of this ecological system maintains a grassland structure by retarding shrub invasion, and landscape isolation and fragmentation limit seed dispersal of native shrub species. Fire frequency is presumed to be less than 20 years. Through isolation from a seed source, combined with repeated burning, these are "permanently" (more than 50 years) converted to grassland.

This grassland is dominated by perennial bunchgrasses and forbs (>25% cover), sometimes with a sparse (<10% cover) shrub layer; Chrysothamnus viscidiflorus, Ericameria nauseosa, Tetradymia spp., or Artemisia spp. may be present in disturbed stands. Associated graminoids include Achnatherum hymenoides, Elymus elymoides, Elymus lanceolatus ssp. lanceolatus, Hesperostipa comata, Festuca idahoensis, Koeleria macrantha, Poa secunda, and Pseudoroegneria spicata. Common forbs are Phlox hoodii, Arenaria spp., and Astragalus spp.

These are large extensive grassland ecosystems, not grass-dominated patches within sagebrush shrub-steppe ecological system. This system occurs throughout much of the Columbia Plateau and is found at slightly higher elevations farther south. Soil depth and soil texture within precipitation zones largely drive the distribution of shrub-steppe and grassland (WNHP 2011). Geographically (climatically), this steppe system is associated with Inter-Mountain Basins Big Sagebrush Steppe (CES304.778), rings the driest portion of the basin that supports the big sagebrush shrubland and the semi-desert shrub-steppe systems, and is bounded by montane woodlands and the Palouse prairie. It is found in landscapes that favor frequent ignition sources and fuels that spread fire, and few natural firebreaks. Biological soil crust is very important in this ecological system (WNHP 2011).

Climate: Climate is semi-arid, cool temperate with annual precipitation ranging from 18-40 cm and high inter-annual variation. Much of the precipitation falls as snow or spring rain; however, growing-season drought is characteristic. Temperatures are continental with large annual and diurnal variation. Winter precipitation dominates and promotes cool-season grasses.

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

Soil/substrate/hydrology: Soils are variable, ranging from relatively deep, fine-textured often with coarse fragments, and non-saline often with a biological soil crust, to stony volcanic-derived clays to alluvial sands. Burrowing animals and their predators likely played important roles in creating small-scale patch patterns (WNHP 2011).

In the Columbia Plateau this grassland ecosystem occurs in a mosaic with sagebrush steppe vegetation and includes sagebrush steppe habitats where fire has removed the sagebrush; thus, due to change in fire regime, this type has expanded at the expense of sagebrush steppe (LANDFIRE 2007a).

Columbia Plateau ecosystems are more sensitive to grazing than grasslands in the Great Plains as they did not evolve with the same duration, seasonality, and severity of large native ungulate grazing (Mack and Thompson 1982, Burkhart 1996). In general, native ungulate grazing was dispersed and occurred during the winter and spring when forage was available.

These grasslands are defined by a more frequent fire regime and the absence or low cover of shrubs over large areas, occasionally entire landforms. The historic frequency was 30-100 years (LANDFIRE 2007a). 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. Post-fire shrub recruitment is limited and rate is estimated to be 25 acres in 50 years under ideal conditions for Artemisia tridentata (WNHP 2011). These shrubs produce large quantities of small seeds beginning at age 3-4 years of which 90% of the seed is dispersed within 9 m (30 feet) of the parent and few seeds are carried more than 30 m (100 feet) (Tirmenstein 1999c). Biological soil crust is very important in this ecological system (LANDFIRE 2007a).

LANDFIRE developed a somewhat different VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 0911230):
A) Early Development 1 All Structures (herbaceous-dominated - 5% of type in this stage): Herbaceous cover is variable (10-50%). Grassland having just burned. Young, green vegetation. Lasts one year before natural succession to class B.

B) Mid Development 1 Open (herbaceous-dominated - 80% of type in this stage): Herbaceous cover 51-90%. Perennial bunchgrass with solid cryptogam cover, large bluebunch wheatgrasses, lower Poa secunda and forb cover, greater forb diversity. Patches are anywhere from 2-50 years old. Replacement fire is the primary disturbance (MFR=50 years).

C) Late Development 1 Closed (herbaceous-dominated - 15% of type in this stage): Herbaceous cover 51-90%. Shrub cover is 0-30%. Native grassland with shrubs beginning to get a foothold, or small pockets of remnants from the original fire expanding into the grassland. It equals the early-seral states in Wyoming big sagebrush steppe ecological system. Patches within this matrix die back due to competition/maintenance, but this does not have a profound effect on class condition. Replacement fire occurs every 16-17 years on average.

Shrubs may increase following heavy grazing and/or with fire suppression, particularly in moist portions in the northern Columbia Plateau where it forms a landscape mosaic pattern with shallow-soil scabland shrublands.

Conversion of this type has commonly come from conversion to invasive non-native species such as Bromus tectorum, Centaurea solstitialis, Hypericum perforatum, and Poa pratensis. These invasive species increase post disturbance including long-term excessive grazing by livestock, or direct soil disturbance from severe trampling by livestock and roads. Altered fire regimes such as repeated, high-frequency fire has 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 shrublands (LANDFIRE 2007a, WNHP 2011). The primary land uses that alter the natural processes of this system are associated with livestock practices, annual exotic species, fire regime alteration, direct soil surface disturbance, and fragmentation from roads and agriculture (WNHP 2011).

Ecosystems in the Columbia Basin are more sensitive to livestock grazing than grasslands in the Great Plains as they did not evolve with the same duration, seasonality, and severity of large native ungulate grazing (Mack and Thompson 1982, Burkhart 1996). In the early 1900s, heavy sheep and cattle grazing led to an increase of shrubs into much of the area. Excessive grazing stresses the system through soil disturbance, trampling and displacing the biological soil crust, altering the composition of perennial species, and increasing the establishment of native disturbance-increasers and exotic annual grasses, particularly Bromus tectorum (Pellant 1990, 1996). Persistent grazing will further diminish perennial cover, expose bare ground, and increase exotic annuals. Currently, fire further stresses livestock-altered vegetation by increasing exposure of bare ground and consequent increases in exotic annuals and decrease in perennial bunchgrass. In more mesic steppe, fire is not as important in maintenance of perennial grasses and forbs. Fescue dominates more heavily on north aspects and moist sites, which have a lower fire frequency (LANDFIRE 2007a). Shrubs may increase with fire suppression, particularly in moist portions in the northern Columbia Plateau where it forms a landscape mosaic pattern with shallow-soil scabland shrublands.

Any disturbances to soil and bunchgrass layers, such as vehicle tracks and chaining shrubs, will increase the probability of alteration of vegetation structure and composition and response to fire as discussed above. Johnson and Swanson (2005) note that Festuca idahoensis decreases following fire, but following a flush of annuals, these sites regain pre-fire cover after a few years. Repeated, high-frequency fire has eliminated the sagebrush and the seed sources of sagebrush, creating extensive grasslands (LANDFIRE 2007a). Currently, cheatgrass and other introduced grasses often invade these habitats after fire. Too much fire has turned steppe into annual grasslands in many areas and has turned large areas of shrubland into grasslands (LANDFIRE 2007a).

Fragmentation of shrub-steppe by agriculture increases cover of annual grass, total 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. 2000, 2001). These fragmentation responses are similarly expected in steppe vegetation (WNHP 2011).

This system occurs throughout the Columbia Plateau region, from north-central Idaho, south and west into Washington, Oregon, southern Idaho, and northern Nevada. Whether it also occurs in northeastern California, in the western ranges of Wyoming, or the central Wyoming Basins is unclear.