Environmental and vegetation relationships of the Artemisia tridentata spp. wyomingensis alliance

The Artemisia tridentata spp. wyomingensis (Beetle & A. Young) S.L. Welsh alliance is the most extensive of the big sagebrush complex in the Intermountain West and is characterized by a wide range of environments and vegetation heterogeneity. The purpose of this study was to identify environmental factors driving variation in plant species composition and determine the potential for using environmental factors to explain vegetation characteristics of the A. tridentata spp. wyomingensis alliance. Seventeen environmental factors and seven vegetation response variables were measured on 107 relatively undisturbed, late seral A. tridentata spp. wyomingensis sites across southeastern Oregon and northern Nevada. Non-metric multidimensional scaling (NMS) was used to identify environmental factors correlated with plant species composition as indexed by canopy cover. Stepwise multiple linear regressions were used to develop models correlating plant cover and structural characteristics with environmental factors. Vegetation composition variation appears to be driven by soil characteristics. Canopy cover of perennial grasses and forbs was moderately correlated with direct incident radiation and soil characteristics, particularly soil texture in the upper 15 cm of the profile. Total herbaceous cover variation was better explained by environmental factors (soil water-holding capacity, incident radiation, depth to Bt horizon, and percent sand in the upper 15 cm of the soil profile) (P<0.0001, R2=0.52) than any other vegetation characteristic. Vegetation structural characteristics (e.g. sagebrush height, canopy volume and canopy cover, density, and plant visual obstruction) exhibited weak or no relationships with measured environmental variables. Limited correlation among environmental factors and some vegetation characteristics was likely due to the large ecological amplitudes and ecotypic variations expressed by many of the plant species in the sagebrush steppe. This study expands our understanding of the A. tridentata spp. wyomingensis alliance and elucidates the complexity of environmental–vegetation relationships.