Geomorphic controls on Great Basin riparian vegetation at the watershed and process zone scales

Riparian ecosystems supply valuable resources in all landscapes, but especially in semiarid regions such as the Great Basin of the western United States. Over half of Great Basin streams are thought to be in poor ecological condition and further deterioration is of significant concern to stakeholders. A thorough understanding of how physical processes acting at multiple scales work independently and interactively to shape riparian communities is necessary for successful management and restoration. I investigated: 1) geomorphic influences on riparian vegetation pattern at the watershed scale; 2) the ecological significance of process zones, a hydrogeomorphic classification scheme; and 3) relationships between geomorphic characteristics, riparian community distribution, and plant species abundances.The study was conducted in small, upland watersheds in several mountain ranges of central Nevada, USA. The watershed-scale component used a GIS-based approach to quantify riparian extent, vegetation type proportions, and geomorphic characteristics of eighteen watersheds. The process zone component involved field-mapping process zones and vegetation types, followed by sampling of geomorphic and vegetation data at 184 sites along main stream channels in four watersheds. Regression and multivariate analyses related geomorphic variables to patterns of riparian vegetation. An ANOVA tested for differences in physical characteristics of process zones.Riparian extent was positively related to percent intrusive bedrock, and was negatively related to percent carbonate bedrock and relative stream power. Riparian forest and shrub types were more abundant in small, rugged watersheds of high relief; meadow vegetation types were more abundant in large, low gradient, and less rugged watersheds. Process zones exhibited some significant differences in valley and channel geomorphic attributes, but were not strongly associated with specific assemblages of vegetation types. Riparian plant communities were primarily distinguished by elevation and contributing area, and secondarily by valley and channel attributes. Variables describing the larger-scale geomorphic context (e.g. watershed, bedrock, contributing area, elevation) also were strong predictors of plant species abundances. Watershed geomorphology significantly constrained riparian corridor extent and relative abundance of woody or herbaceous vegetation within the corridor. In addition, riparian community composition and species abundances were strongly influenced by the larger-scale geomorphic context, such as differences in watershed characteristics and site bedrock, elevation and contributing area. Although I did not identify distinct process zone-vegetation type assemblages with the classification applied in this study, my results suggest that incorporation of such variables into the classification scheme should make it more meaningful for explaining and predicting patterns of riparian vegetation in these small upland watersheds