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'Multi-scale drivers of riparian vegetation across the interior Pacific Northwest: a case from the interior Columbia River basin.'
Nate Hough-Snee (nate@natehough-snee.org), Utah State University; Brett Roper (broper@fs.fed.us), US Forest Service; Joe Wheaton (joe.wheaton@usu.edu), Utah State University

Riparian vegetation both shapes and responds to physical processes and instream properties that are also driven by larger-scale climatic, hydrologic and geomorphic processes. These processes may be thought of as environmental filters that restrict local riparian vegetation to those species that can colonize and persist through multiple and sometimes hierarchical filters. Accordingly, riparian vegetation is frequently monitored as an indicator of watershed integrity as the environmental filters that shape riparian vegetation may shift rapidly following land-use changes and/or disturbance. When assessing riparian vegetation across broad spatial scales, it is often difficult to decouple variation in vegetation communities caused by local filters such as fluvial disturbance and channel form from variation that is attributable to larger scale filters such as geology, climate and local plant species pools. We used low-order stream riparian vegetation data from across the interior Columbia River (CR) and Missouri River (MR) basins to ask four questions: (1) are there distinct riparian plant communities within low-order streams of the interior CR and upper MR basins? (2) What environmental filters correspond to these identified plant communities? (3) Of these filters, are large-scale processes more responsible for shaping the composition of riparian communities than watershed and stream level filters? (4) Do identified riparian plant communities correspond to distinct channel conditions? Clustering methods, indicator species analysis, PERMANOVA and ordination techniques were used to quantify how vegetation communities were correlated to landscape- and reach-scale variables, including channel habitat, watershed land-use and climate attributes. Landscape scale variables such as elevation and climate and watershed management parameters such as grazing and forest cover were strongly correlated to vegetation community composition. A suite of instream habitat variables was correlated to vegetation community composition while unique vegetation communities generally corresponded to unique channel forms and instream habitat. These vegetation-stream habitat relationships may be a product of vegetation itself or vegetation and filters that also shape geomorphic process. Based on the observed relationships between riparian vegetation and environmental filters, we conclude that when monitoring riparian status and trend or setting riparian management and restoration objectives, managers should attempt to account for multiple interactions between regional species pools, regional environmental variability and stream physical habitat.