Background/Question/Methods Less than 25% of Artemisia tridentata wyomingensis (Wyoming big sagebrush) communities retain an intact herbaceous understory. The invasive annual, Bromus tectorum, is a substantial component of the understory in more than 60% of the remaining communities.  The same ecological processes drive species’ abundances and community composition and invasibility.  Previous studies have shown that soil and landscape properties are important determinants of species’ abundances in these communities. However, these studies did not consider the potential role of cattle grazing disturbance or facilitation by sagebrush as a drivers in these communities.
The primary objective of this study was to gain a better understanding of the importance of abiotic stress, disturbance, and facilitation as drivers of species’ abundances and community composition in the face of B. tectorum invasion.  Twenty environmental-disturbance factors and species abundance data were measured on 30 plots located along two cattle grazing disturbance gradients in southeast Oregon.  Non-metric multidimensional scaling (NMS) ordination was used to examine the relationships between the community and environmental data. Hierarchical agglomerative cluster analysis was used to identify groups differing in species’ abundances or composition.  Multi-response permutation procedures (MRPP) and Indicator Species Analysis were used to evaluate group differences in environmental and species attributes respectively.
Results/Conclusions Preliminary findings indicate that native species were strongly associated with soil properties (texture and depth), landscape properties (heat loads), and cattle grazing disturbance.  The abundance and dominance of most native bunchgrasses and forbs were strongly negatively associated with cattle grazing disturbance.  The abundance and dominance of individual bunchgrass species were strongly related to soil depth and texture, and heat loads.   B. tectorum abundance and dominance was positively associated with cattle grazing and abiotic stress driven primarily by higher heat loads. B. tectorum abundance (14% cover) and dominance (20-29% of the understory) was highest in plots characterized by a combination of the highest disturbance levels and heat loads.
Ordination of differences in abundance data between sagebrush canopies and interspaces revealed a strong stress gradient associated with increasing cattle disturbance levels and abiotic stress levels driven by increasing heat loads.  Differences were  related positively to this axis for all bunchgrass species and B. tectorum (r = .58-.67).  This finding suggests increasing net facilitative effects with increasing abiotic stress and disturbance consistent with the stress-gradient hypothesis. These findings suggest that complex interactions between landscape and soil properties and cattle grazing disturbance and facilitation are important drivers in A. t. wyomingensis communities.