Washington Native Plant Society: Plant Assemblages of the Shrub-Steppe

Plant Assemblages of the Shrub-Steppe

Shrub-steppe refers to the dominant components of this ecosystem: shrubs (such as sagebrush, hopsage, greasewood, and bitterbrush); and steppe, or perennial bunchgrasses (such as bluebunch, needle-and-thread, Idaho fescue and Sandberg’s bluegrass). Numerous annual and perennial wildflowers (such as phlox, mariposa lily, fleabanes and locoweeds) thrive in the spaces between shrubs and bunchgrasses. Sagebrush and other shrubs are important in capturing litter and providing shade, which influences other components of the plant community. Cryptobiotic soil crusts in this system play a crucial role in fixing nutrients and retaining soil stability, which in turn influences the ability of different plant species to germinate and become established. Cryptobiotic crusts serve as a protective layer and are composed of lichens, mosses and algae. Without this protective layer, bare ground is susceptible to rapid erosion by wind and water, and provides an ideal site for invasive plants to establish.

From a fast-moving highway, shrub-steppe landscapes may appear uniform. However, they are composed of diverse plant assemblages that change with soil type, slope, aspect and geomorphic characteristics that influence soil-water relations and insolation.

The most stunning wildflower displays of the shrub-steppe are often seen on lithosols, thin-soiled basalt formations that have poor water-holding capacity. Lithosols support distinct and beautiful species such as bitterroot, thyme-leaved buckwheat and Gairdner’s penstemon.

Deeper soils support plant assemblages of bluebunch wheatgrass and sagebrush where there is moderate precipitation, while sandier, drier sites support needle and thread grass, Indian ricegrass or Sandberg’s bluegrass in combination with sagebrush, hopsage or winterfat shrubs. Higher elevations support more productive grasslands that are dominated by closely-spaced species such as Idaho fescue, phlox, lupine and balsamroot. Along the eastern Cascades, high elevation shrub-steppe intermingles with the ponderosa pine and garry oak ecosystem. Riparian habitats are critical to most animal species using the shrub-steppe, and support trees such as cottonwood and alder, a wide variety of shrubs such as mock-orange, willow, and greasewood, and bunchgrasses such as basin wildrye. Extremely rare moist meadows support sedges, rushes and wildflowers such as shooting star, saxifrage and bistort.