. Vegetation of Oregon and Washington. Botany Oregon Ecology; Botany Washington (State) Ecology. reduced in stature. There is no evidence these shrub thickets are spreading; grassland and shrub thickets form a stable mosaic. Fire has little effect on the Festuca/Sym- phoricarpos association, since all important species can regenerate from underground or- gans. Heavy grazing results in a major and ir- reversible change, however; perennial grasses, shrubs, and most forbs are eliminated, and an invader, Poa pratensis, becomes dominant. The Poa pratensis community is not replaced with the eliminat
. Vegetation of Oregon and Washington. Botany Oregon Ecology; Botany Washington (State) Ecology. reduced in stature. There is no evidence these shrub thickets are spreading; grassland and shrub thickets form a stable mosaic. Fire has little effect on the Festuca/Sym- phoricarpos association, since all important species can regenerate from underground or- gans. Heavy grazing results in a major and ir- reversible change, however; perennial grasses, shrubs, and most forbs are eliminated, and an invader, Poa pratensis, becomes dominant. The Poa pratensis community is not replaced with the elimination of grazing and can be- come an essentially pure sward. This replace- ment of native grasses and herbs by Poa pra- tensis is characteristic of all habitats (except lithosolic sites) in the Festuca/Symphoricar- pos, Festuca/Rosa, and lower forested zones. ASSOCIATIONS ON SPECIALIZED HABITATS Within the steppe region are a wide variety of habitats which have soils sufficiently un- usual in physical or chemical properties to de- velop climax communities not assignable to any of the zonal associations. Some of these are associated with particular zones as defined by the nine zonal associations; many are not. Consequently, some of the associations typi- cal of such sites are handled separately in this section. Sandy or Gravelly Soils Deep soils dominated by gravel or sand have low moisture-holding capacity. Associa- tions found on such soils within the drier in- terior steppe zones share a dominance of Stipa comata. In this series of Stipa comata associations, a shrubby species is usually pres- ent: Artemisia tridentata, A. tripartita, oxPur- shia tridentata. These shrub dominants appear insensitive to soil differences so conspicuously reflected in the compositional shift of the herbaceous cover. Lithosols Lithosolic sites are those where soils are stony and extremely shallow to bedrock. These sites provide an extremely rigorous plant environment, with heat and drought in the sum
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