Physiognomy and Structure: Vegetation in this alliance is characterized by a sparse to dense cover of graminoids that is dominated by perennial bunch grasses less than 1 m tall. There is also sparse to moderate cover of perennial forbs. Occasional scattered shrubs and dwarf-shrubs may be present. Annual forbs and grasses are seasonally present. Nonvascular cover is important in some stands.

Floristics: The vegetation is characterized by a sparse to moderately dense graminoid layer of Pseudoroegneria spicata codominated by Aristida purpurea var. longiseta and/or Poa secunda. Pseudoroegneria spicata is sometimes absent or has low cover. Sporobolus cryptandrus and Opuntia polyacantha are especially abundant on disturbed sites or harsh windswept ridgetops and steep upper slopes. Other associated grasses include Danthonia unispicata, Elymus elymoides, and Hesperostipa comata. Important forbs include Achillea millefolium, Allium acuminatum, Antennaria spp., Arnica sororia, Balsamorhiza sagittata, Castilleja spp., Calochortus spp., Crepis acuminata, Erigeron pumilus, Eriophyllum lanatum, Lomatium cous, Lupinus spp., Plantago patagonica, Scutellaria angustifolia, and Sedum stenopetalum. Deciduous shrubs such as Holodiscus discolor, Phlox longifolia, Physocarpus malvaceus, Ribes spp., Rhus glabra, and Symphoricarpos spp. are occasionally present and may increase in abundance with fire exclusion. The shrub Ericameria nauseosa (= Chrysothamnus nauseosus) is present in some stands, and will increase with overgrazing and other disturbance (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.

Dynamics:  Fire has variable effects on Pseudoroegneria spicata. Plants usually survive burning, and growth is often stimulated, except when fire occurs in the driest month when the crowns will burn because of low moisture in the vegetation, and the meristems are damaged (Johnson and Simon 1987). Grazing impacts are concentrated on the gentler slopes accessible to livestock. Pseudoroegneria spicata shows an inconsistent reaction to grazing, increasing on some grazed sites while decreasing on others. It seems to recover more quickly from overgrazing than Festuca campestris (Mueggler and Stewart 1980). It tolerates dormant-period grazing well, but is sensitive to defoliation during the growing season. Light spring use or fall grazing can help retain plant vigor. It is particularly sensitive to defoliation in late spring (Comer et al. 1999).

Poa secunda-dominated grasslands usually have relatively shallow, rocky soils with limited water-holding capacity. The winter precipitation wets these soils, but they typically dry out completely to bedrock by midsummer (Ganskopp 1979). Poa secunda is well-adapted to these conditions because it starts growing early in the spring and completes its reproductive cycle early while there is still moisture in the soil (Daubenmire 1970). If there is late summer or fall precipitation, dominant Poa secunda can green up quickly again. Shrubs are uncommon. Daubenmire (1970) and Johnson and Simon (1987) suggest that bedrock present under these grasslands is not fractured enough to support deeper-rooted shrubs. In addition to drought tolerance, Poa secunda is also tolerant of grazing and trampling by livestock (Daubenmire 1970, Ganskopp 1979).

Stands in this alliance are generally considered to be late-seral with species composition controlled by the harsh edaphic conditions of the site (Daubenmire 1970, Johnson and Simon 1987). However, some stands, especially stands of ~Aristida purpurea var. longiseta - Poa secunda Grassland (CEGL001781)$$ that occur on deeper soils, may be early-seral forms of a Pseudoroegneria spicata grassland, because Poa secunda increases under moderate grazing (Daubenmire 1970).

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) bunchgrasses. 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).

Other disturbances such as frost heaving also favor exotic species invasion. Frost heaving causes local soil disturbance in the winter when these thin, saturated soils freeze and push soil and plants up out of the ground. 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).