Native Grasslands & Shrub-Steppe
Throughout the Columbia Basin Ecoregion, a number of different plant association zones occur as climatic climaxes (i.e., the plant association or community expected to occur in typical sites in the absence of disturbance). The largest and driest of these zones is the big sagebrush (Artemisia tridentata) / bluebunch wheatgrass (Pseudoroegneria spicata [=Agropyron spicatum]) association. This association occupies the center of the Columbia Basin Ecoregion, which includes the Hanford Reach National Monument. In general, the big sagebrush / bluebunch wheatgrass association is characterized by four layers of vegetation—an overstory layer composed mostly of big sagebrush up to two meters tall, a tall understory layer of bluebunch wheatgrass, a short understory dominated by Sandberg’s bluegrass (Poa sandbergii [included within Poa secunda]), and a layer of algae, lichens and mosses on the soil surface (i.e., the microbiotic crust). The microbiotic crust is a critical component of native grasslands and shrub-steppe communities. This diminutive community of mosses, lichens, liverworts, algae and bacteria stabilizes the soils and fills the interstitial space between bunchgrass clumps. Perennial forbs are a minor constituent of the tall understory layer, whereas most annual forbs occur in the short understory layer. Other shrubs that may be present include rabbitbrush (Chrysothamnus spp.), bitterbrush (Purshia tridentata), spiny hopsage (Grayia spinosa), and three-tip sagebrush (Artemisia tripartita). Additional locally abundant bunchgrasses include needle-and-thread (Stipa comata), Indian ricegrass (Oryzopsis hymenoides), Cusick’s bluegrass (Poa cusickii [included within Poa secunda]) and Idaho fescue (Festuca idahoensis).
Other associations, such as big sagebrush / Idaho fescue, bluebunch wheatgrass / Sandberg’s bluegrass, and bluebunch wheatgrass / Idaho fescue can occur as topographic climaxes on moister sites within the big sagebrush / bluebunch wheatgrass association. Certain edaphic (soil-related) plant associations also are of ecological importance within the ecoregion. On deep soils dominated by gravel, sand, or strongly weathered volcanic ash, needle-and-thread and/or Indian ricegrass replaces bluebunch wheatgrass as the dominant grass in several associations. The dominant shrub in these associations can be either big or three-tip sagebrush or bitterbrush. On stony soils or extremely shallow soils over bedrock (lithosols), various species of buckwheat (Eriogonum) and/or stiff sage (Artemisia rigida) dominate the shrub layer and Sandberg’s bluegrass dominates the understory. As the hottest, driest, and lowest elevation part of the ecoregion, the Hanford Site also possesses a series of three plant associations found on reasonably deep, loamy (but dry) soils. These are the big sagebrush / Sandberg’s bluegrass, spiny hopsage / Sandberg’s bluegrass, and winterfat (Atoides [=Eurotia] lanata) / Sandberg’s bluegrass associations. Each of these associations is characterized by the lack of large, perennial bunchgrasses (Sandberg’s bluegrass is relatively small) and low overall plant diversity.