Physiognomy and Structure: The vegetation is characterized by a sparse cover of microphyllous evergreen shrubs, usually 1-2 m in height. Shrub cover can be variable, but on average is less than 25% for steppe sites and 10-70% cover for shrubland sites. Perennial cespitose graminoids form a matrix surrounding the shrubs in steppe, occasionally matching them in height, or form a sparse to moderately dense graminoid layer in shrublands. Forb species may be frequent, but are usually of low canopy cover. The ground surface is covered with mosses and lichens in good condition stands, except for fluvial sites.

Floristics: The vegetation included in this alliance is characterized by a somewhat sparse to moderately dense (10-70% cover) shrub layer of Artemisia tridentata ssp. tridentata or Artemisia tridentata ssp. xericensis. Shrub associates include Ericameria nauseosa (= Chrysothamnus nauseosus) or Chrysothamnus viscidiflorus which increase with disturbance. Other shrubs occasionally present include Atriplex spp., Gutierrezia sarothrae, and Symphoricarpos longiflorus. Occasionally individual trees are present in some stands. The open to dense herbaceous layer is dominated by bunchgrasses that occupy patches in the shrub matrix. The most widespread species is Pseudoroegneria spicata, which occurs from the Columbia Basin to the Northern Rockies. Other locally dominant or important species include Achnatherum hymenoides (= Oryzopsis hymenoides), Elymus elymoides, Elymus lanceolatus, Festuca idahoensis, Hesperostipa comata (= Stipa comata), Koeleria macrantha, Leymus cinereus, Muhlenbergia richardsonis, Pascopyrum smithii, Pleuraphis jamesii (= Hilaria jamesii), and Poa secunda. Forbs are generally of low importance and are highly variable across the range. Mosses and lichens are important ground cover in some stands. Diagnostic of this alliance is the Artemisia tridentata ssp. tridentata- or Artemisia tridentata ssp. xericensis-dominated shrub layer. The graminoid layer typically has >20% cover perennial graminoids or has over 40% total cover of shrubs.

Dynamics:  Complex ecological interactions between fire regimes, grazing history, and climate patterns result in equally complex patterns of species structure and composition in Artemisia tridentata. These present corresponding difficulties in the classification of these shrublands, which have been compounded by the influence of human settlement and agricultural patterns. What follows is a summary of some of the influences of altered fire regimes and grazing history on Artemisia tridentata shrublands and shrub-herbaceous vegetation that can result in the conversion of stands of this alliance into stands of exotic annual grassland. Exotic plants have invaded many stands, especially where disturbed (Daubenmire 1970). 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. The winter precipitation recharges soil moisture, and the typically dry summers favor shrubs and deep-rooted grasses (West 1983c, 1988). The exotic annual grass Bromus tectorum competes favorably with these cool-season, perennial bunchgrasses in these stands by geminating in the fall, establishing a root system during the winter, then utilizing and depleting soil moisture early in the spring while the bunchgrasses are still mostly dormant (West 1983c).

Artemisia tridentata (ssp. tridentata, ssp. xericensis) shrub-herbaceous communities may represent either moister or less disturbed examples of the Artemisia tridentata (ssp. tridentata, ssp. xericensis) shrubland complex. Shrub densities typically increase with overgrazing of the bunchgrass component or with increasing summer drought (West 1983c). Artemisia tridentata is inhibited by fire, and excessive grazing may decrease fire frequency due to consumption of herbaceous forage, resulting in increased shrub density. Conversely, invasion by non-native annual grasses (e.g., Bromus tectorum) may increase fire frequency sufficiently to eliminate the shrubs from the stands (Daubenmire 1970). With a change in fire frequency, species composition will be altered as well (West 1983c). With a high fire frequency (every 2-5 years), perennial grasses and shrubs are eliminated and non-native annual grasses dominate. At fire-return intervals of 10-30 years, short-lived resprouting shrubs, such as Chrysothamnus or Tetradymia spp. dominate. At fire intervals of 30-70 years, a mixture of perennial bunchgrasses and shrubs is maintained. Finally, in the complete absence of fire, deep-rooted shrubs such as Artemisia tridentata become the dominant shrubs.