Determinants of Riparian Vegetation along Western Sierra Nevada Mountain Streams and Management Implications

Abstract:

Riparian habitats represent a small fraction of lands in Sierra Nevada mountain watersheds, yet support a disproportionately high diversity and richness of biota. The presence of these habitats in the Sierras is largely controlled by geomorphic characteristics of the watershed and hydrologic regime. Riparian species’ life history strategies influence recruitment success and survival under the dynamic and episodic, yet seasonally predictable, hydrology conditions of the region, including infrequent extreme precipitation events and multiple years with low precipitation. However, dams and diversions affect flows throughout the Sierra Nevada and regional climate change models project changes in inter- and intra-annual hydrology patterns that may affect riparian recruitment and future distribution. Studies were conducted in twelve reaches on six rivers (impaired and unimpaired rivers) that directly link successful riparian recruitment years to hydrologic and channel geomorphic conditions, and included: vegetation surveys, tree core dating, topographic surveys, reach-specific stage-discharge relationships and modeling, and annual hydrograph analyses. Availability of suitable substrate for establishment created by scouring high flow events; timing of snowmelt recession that varied by water year and elevation; spring snowmelt stage recession rates typically less than 12 percent per day; and site-specific controls on inundated width and depth and water availability were identified as the primary determinants for successful recruitment years across all rivers and reaches. The results were used to develop tools to guide and communicate evaluations of flow management alternatives to managers and stakeholders and can also be used to assess potential riparian responses to flow management actions or climate change that may affect the future distribution and complexity of riparian habitats along Sierra Nevada mountain streams.