Physiognomy and Structure: Vegetation included in this alliance has a sparse to moderately dense layer of medium-tall graminoids, primarily perennial bunch grasses. Forbs are present but usually not abundant. Shrubs are rarely present except in livestock-grazed sites.

Floristics: Vegetation in this alliance is sparse to moderately dense grassland communities that are dominated or codominated by the perennial medium-tall, warm-season bunchgrasses Sporobolus cryptandrus and Aristida purpurea var. longiseta and cool-season bunchgrass Poa secunda. Cool-season bunchgrass Hesperostipa comata may be present in lesser amounts, especially on the more upland sites. Pseudoroegneria spicata is generally absent or has low cover. Associated perennial forbs include Achillea millefolium, Astragalus inflexus, Calochortus macrocarpus, Erigeron pumilus, Heterotheca villosa, and Opuntia polyacantha. Common annuals are the forbs Epilobium brachycarpum (= Epilobium paniculatum) and Plantago patagonica, and the grasses Vulpia octoflora (= Festuca octoflora) and Vulpia microstachys var. pauciflora (= Festuca pacifica). The shrub Ericameria nauseosa (= Chrysothamnus nauseosus) is present in some stands, and will increase with overgrazing (Daubenmire 1970). The introduced annual grasses Bromus tectorum, Bromus arvensis (= Bromus japonicus), and Bromus briziformis and diverse non-native forbs such as Hypericum perforatum and Erodium cicutarium are abundant in many stands. Diagnostic of this grassland alliance is the dominance or codominance of Sporobolus cryptandrus.

Dynamics:  Sporobolus cryptandrus occurs throughout the western U.S. as a minor species, occasionally becoming locally dominant in disturbed or sandy sites in the midgrass prairie (Weaver and Albertson 1956). In the headwaters of the Columbia River basin these stands occur as climax grasslands in canyons on river bars and terraces, and on upland benches and terraces as grazing disclimax and edaphic climax grasslands (Daubenmire 1970, Johnson and Simon 1987, Tisdale 1986). Many of these grasslands have been severely disturbed by grazing livestock and exotic weeds (Daubenmire 1970, Johnson and Simon 1987, Tisdale 1986). Early-season grazing reduces the abundance of the very palatable cool-season bunchgrass Pseudoroegneria spicata and favors the warm-season Sporobolus cryptandrus that greens up later in the season. Also, Sporobolus cryptandrus is a C4 plant that grows on sites that stay moist when it is hot giving the C4 the advantage over the cool-season (C3) bunch grasses. Poa secunda, another cool-season grass, is also affected by early grazing, but is better able to withstand it because of its short stature. Aristida purpurea var. longiseta is an unpalatable warm-season grass that increases as the palatable grasses decline. As native perennial grasses decline, exotic species fill the gaps (Tisdale 1986). Exotic annual grasses (Bromus tectorum, Bromus arvensis, and Bromus briziformis) and others may make up 20-50% of the vegetation cover. These annual grasses are especially abundant during wet years (Ganskopp 1979, Tisdale 1986). Exotic forbs, both perennials, such as Hypericum perforatum, and annuals, such as Arenaria serpyllifolia, Epilobium brachycarpum, Erodium cicutarium, Draba verna, Holosteum umbellatum, Lactuca serriola, Lepidium perfoliatum, Myosotis stricta, Tragopogon dubius, and Veronica arvensis, make up significant cover and diversity in many stands. The newest exotic threats are four species of Centaurea that are invading large areas in this region. Fire appears to have little effect on the species composition of these grasslands (Tisdale 1986).