Physiognomy and Structure: Vegetation in this alliance has a sparse to moderately dense herbaceous layer that is dominated by perennial bunch grasses less than 0.5 m tall. There is also a sparse, deciduous dwarf-shrub layer that is diagnostic of this type, and a sparse layer of perennial forbs. Mosses and lichens may be common (<0.1 m tall). Annual forbs and grasses are seasonally present.

Floristics: Vegetation included in this alliance is typically open with total vascular cover much less than 60%. A sparse (10-25% cover) woody layer of scattered Artemisia rigida, a cold-deciduous dwarf-shrub, is diagnostic of these stands. Occasionally this dwarf-shrub layer exceeds 25% cover and may include woody Eriogonum spp. Stands have a sparse to moderately dense herbaceous layer (<0.5 m tall) that is dominated by either of the cool-season, perennial bunchgrasses Poa secunda or Pseudoroegneria spicata. Other characteristic species may include occasional graminoids such as Danthonia unispicata, Elymus elymoides, or Festuca idahoensis; perennial forbs such as Achillea millefolium, Allium spp., Antennaria spp., Balsamorhiza incana, Erigeron chrysopsidis, Eriogonum spp., Lithophragma glabrum, Lomatium spp., Phlox hoodii, Sedum lanceolatum, Sedum stenopetalum, or Trifolium macrocephalum; and the annuals Collinsia parviflora, Epilobium brachycarpum, Idahoa scapigera, Microsteris gracilis (= Phlox gracilis), Plantago patagonica, and Vulpia microstachys var. pauciflora. Moss and lichen cover in undisturbed stands is often high and ranges from 1-60% cover.

Dynamics:  This xeric shrubland vegetation occurs under relatively extreme soil-moisture conditions. Stands are found on shallow rocky soils with limited water-holding capacity. These soils are often saturated from fall to spring by winter precipitation, but they typically dry out completely to bedrock by mid-summer (Daubenmire 1970, 1992, Johnson and Simon 1987). 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, 1992, Johnson and Simon 1987). Also, if there is late-summer or early-fall precipitation, dormant Poa secunda can respond quickly and green up. Daubenmire (1970) and Johnson and Simon (1987) suggest that the basalt bedrock present under these dwarf-shrub/grassland stands is fractured enough to support deeper-rooted dwarf-shrubs. Moss does well in this habitat. In addition to drought tolerance, Poa secunda is also tolerant of grazing and trampling by livestock (Daubenmire 1970, Ganskopp 1979). Artemisia rigida is favored winter browse for elk and deer, and moderately palatable to livestock (Johnson and Clausnitzer 1992). The saturated spring soils are vulnerable to trampling, but the rocky soils discourage livestock (Daubenmire 1992). However, in areas excluded from grazing entirely, Pseudoroegneria spicata and Festuca idahoensis may dominate with Artemisia rigida in portions of the alliance''s range, also growing in rock fractures.

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). With disturbance, such as livestock impacts, comes an increase in erosion pavement and bare ground, and a decrease in moss and lichen cover (Daubenmire 1970, Johnson and Simon 1987). Frost heaving may be severe, causing local soil disturbance in the winter when these thin, saturated soils freeze and push soil and plants up out of the ground. Pedestalled Artemisia rigida plants and bunch grasses are common (Daubenmire 1970, Hironaka et al. 1983). Exotic plants have invaded many stands especially where disturbed (Tisdale 1986, Daubenmire 1992). Common exotics may include annual grasses such as Bromus tectorum, Bromus arvensis, and Bromus briziformis, which may be abundant during wet years, and annual forbs such as Epilobium brachycarpum, Erodium cicutarium, Lactuca serriola, Tragopogon dubius, and the perennial forb Hypericum perforatum.

Fire is thought to be unimportant because it is unlikely that the sparse vegetation in these stands could carry a fire. However, if it did occur, the Artemisia rigida plants are not tolerant and would be killed (Johnson and Simon 1987, Daubenmire 1992, Johnson and Clausnitzer 1992).