No. 17. Dwarf Shrub-steppe

Rex C. Crawford and Jimmy Kagan

Geographic Distribution. Dwarf-shrub and related scabland habitats are located throughout the Columbia Plateau and in adjacent woodland and forest habitats. They are more common in southern Oregon than in Washington. 

Low sagebrush steppe is common in the Basin and Range and the Owyhee Uplands in eastern Lake, Harney, and Malheur counties and is a minor type in eastern Washington and northeastern Oregon. It usually occurs on low, scabby plateaus above lake basins. Stiff sagebrush/Sandberg bluegrass is a major type widely distributed in the Columbia Basin, particularly associated with the channeled scablands, High Lava Plains, and in isolated spots throughout the Blue Mountains and the Palouse. Black sagebrush steppe is not found in Washington and is rare in Oregon, occurring along the Nevada border in southern Lake, Harney, and Malheur counties, in the southern Basin and Range and Owyhee Uplands Physiographic Province.

Physical Setting. This habitat appears on sites with little soil development that often have extensive areas of exposed rock, gravel, or compacted soil. The habitat is characteristically associated with flats, plateaus, or gentle slopes although steep slopes with rock outcrops are common. Scabland types within the shrub-steppe area occur on barren, usually fairly young basalts or shallow loam over basalt <12 inches (30 cm) deep. In woodland or forest mosaics, scabland soils are deeper (still <26 inches [65 cm]) but too droughty or extreme soils for tree growth. Topoedaphic drought is the major process influencing these communities on ridge tops and gentle slopes around ridgetops. Spring flooding is characteristic of scablands in concave topographic positions. This habitat is found across a wide range of elevations from 500 to 7,000 ft (152 to 2,134 m).

Landscape Setting. These scabland habitats form a mosaic with Shrub-steppe habitat, Eastside Grassland habitat, and with Western Juniper and Mountain Mahogany Woodland or Ponderosa Pine Forest and Woodlands habitats. Low sagebrush stands are often extensive and occasionally occur in a mosaic with big sagebrush, stiff sagebrush, or black sagebrush steppe or within lower treeline woodlands. Stiff sagebrush stands may also be extensive, but usually occur in a mosaic with grassland, big sagebrush or occasionally in juniper (Juniperus occidentalis) or Ponderosa pine (Pinus ponderosa) woodlands. Black sagebrush stands are extensive and may occur in a mosaic with low sagebrush or mountain or Wyoming big sagebrush.

Structure. These low shrub (<1.6 ft [0.5 m] high) communities have an undergrowth of short grasses and forbs with extensive exposed rock and cryptogamic crusts. More productive sites have an open, native medium-tall bunchgrass layer with scattered low shrubs. Some scablands in the Columbia Basin have few to no dwarf shrubs and the habitat is entirely dominated by grasses and forbs. Total vegetation cover is open to sparse. Individual trees can appear among the low shrubs when this habitat appears in the forest matrix.

Composition. Several dwarf-shrub species characterize this habitat: low sagebrush (Artemisia arbuscula), black sagebrush (A. nova), stiff sagebrush (A. rigida), or several shrubby buckwheat species (Eriogonum douglasii, E. sphaerocephalum, E. strictum, E. thymoides, E. niveum, E. compositum). These dwarf-shrub species can be found as the sole shrub species or in combination with these or other low shrubs. Purple sage (Saliva dorrii) can dominate scablands on steep sites with rock outcrops.

Sandberg bluegrass (Poa sandbergii) is the characteristic and sometimes the dominant grass making up most of this habitat’s sparse vegetative cover. Taller bluebunch wheatgrass (Pseudoroegneria spicata) or Idaho fescue (Festuca idahoensis) grasses may occur on the most productive sites with Sandberg bluegrass. Bottlebrush squirreltail (Elymus elymoides) and Thurber needlegrass (Stipa thurberiana) are typically found in low cover areas, although they can dominate some sites. One-spike oatgrass (Danthonia unispicata), prairie junegrass (Koeleria macrantha), and Henderson ricegrass (Achnatherum hendersonii) are occasionally important. Exotic annual grasses, commonly cheatgrass (Bromus tectorum), increase with heavy disturbance and can be locally abundant. Common forbs include serrate balsamroot (Balsamorhiza serrata), Oregon twinpod (Physaria oregana), Oregon bitterroot (Lewisia rediviva), big-head clover (Trifolium macrocephalum), and Rainier violet (Viola trinervata). Several other forbs (Arenaria, Collomia, Erigeron, Lomatium, and Phlox spp.) are characteristic, early blooming species. A diverse lichen and moss layer is a prominent component of these communities.

Medium-tall shrubs, such as big sagebrush (Artemisia tridentata), Silver sagebrush (A. cana), antelope bitterbrush (Purshia tridentata), and rabbitbrush (Chrysothamnus spp.) occasionally appear in these scablands.

Other Classifications and Key References. This habitat is called scabland, biscuit-swale topography, lithosolic steppe, or low shrub-steppe. Quigley and Arbelbide ¹⁸¹ called this habitat low sagebrush cover type and "Low Sagebrush-Xeric" and "Low Sagebrush-mesic" potential vegetation groups. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Type ¹²⁷ that would represent this type is low-dwarf sagebrush. Kuchler ¹³⁶ did not distinguish this habitat but included it within Sagebrush Steppe. Franklin and Dyrness ⁸⁸ discussed this habitat as lithosolic sites in steppe and shrub-steppe zones of Washington and as plant associations in steppe and shrub-steppe zones of central and southern Oregon. Other references describe this habitat ^{60, 64, 122, 123, 207}.

Natural Disturbance Regime. Scabland habitats often do not have enough vegetation cover to support wildfires. Bunchgrass sites with black or low sagebrush may burn enough to damage shrubs and decrease shrub cover with repetitive burns. Many scabland sites have poorly drained soil and because of shallow soil are prone to winter flooding. Freezing of saturated soil results in "frost-heaving" that churns the soil and is a major disturbance factor in vegetation patterns. Stiff sagebrush is a preferred browse for elk as well as livestock. Native ungulates use scablands in early spring and contribute to churning of the soil surface.

Succession and Stand Dynamics. Grazing reduces the cover of bunchgrasses and increases the abundance of common yarrow (Achillea millefolium), phlox species, bighead clover, serrate balsamroot, bottlebrush squirreltail and annual bromes on dwarf shrublands. Increased ground disturbing activities increases exotic plant abundance, particularly on deeper soil sites. All dwarf-shrub species are intolerant of fire and do not sprout. Consequently, redevelopment of dwarf shrub-steppe habitat is slow following fire or any disturbance that removes shrubs.

Effects of Management and Anthropogenic Impacts. Scabland habitats provide little forage and consequently are used only as a final resort by livestock. Heavy use by livestock or vehicles disrupts the moss/lichen layer and increases exposed rock and bare ground that create habitat for exotic plant invasion. Exotic annual bromes have become part of these habitats with natural soil churning disturbance.

Status and Trends. Quigley and Arbelbide ¹⁸¹ concluded that the low sagebrush cover type is as abundant as it was before 1900. They concluded that "Low Sagebrush-Xeric" successional pathways have experienced a high level of change from exotic invasions and that some pathways of "Low Sagebrush-Mesic" are unaltered. Twenty percent of Pacific Northwest dwarf shrub-steppe community types listed in the National Vegetation Classification are considered imperiled or critically imperiled ¹⁰.