PS 22-52
Riparian vegetation recruitment along western slope of the central Sierra Nevada mountain streams
Riparian vegetation patterns along central Sierra Nevada mountain streams are largely controlled by geomorphic characteristics of the watershed and the hydrologic regime. Riparian establishment is limited by narrow valley bottoms with minimal floodplain development and alluvial deposits, and prevalence of bedrock and coarse substrate. Species-level recruitment patterns, survival, and growth are influenced by annual flow patterns (hot, dry summers and wetter winter/spring seasons), infrequent extreme precipitation events, and multiple years with low precipitation. Riparian species’ life history strategies influence recruitment success and survival under the dynamic and episodic, yet seasonally predictable, hydrology conditions. Regional climate change models project changes in inter- and intra-annual hydrology patterns that may affect riparian recruitment. The objectives of this study were to identify the primary determinants for successful riparian recruitment (hydrologic and geomorphic) and develop tools to guide flow management and restoration decisions for long-term sustainability of mountain riparian habitats. Riparian vegetation recruitment and survival were assessed and related to hydrologic and geomorphic processes in twelve reaches along eight streams (unimpaired and impaired rivers), and included: vegetation surveys; tree core analyses; topographic surveys of rooted elevations of the cored trees; reach-specific stage-discharge relationships and modeling; and annual hydrograph analyses.
Results/Conclusions
Specific hydrologic attributes were identified as important determinants for successful recruitment and survival of riparian vegetation along the study streams, including: (1) frequency of high magnitude scouring or “re-setting” flows; (2) frequency, magnitude, and timing of seed setting flows (recruitment flows); (3) spring flow hydrograph shape/recession rates; and (4) inter-annual flow variability. Distribution and rooted elevations of the riparian trees reflected influences of geomorphic and hydrologic processes on riparian recruitment and survival at varying temporal and spatial scales. Successful recruitment events in the studied reaches were preceded by large scouring flows that created open areas with fresh alluvial deposits for seed germination of riparian species. Channel morphology and site topography control wetted width, water depths, and inundation duration that influence recruitment and survival. Hydrologic conditions favorable for successful recruitment included peak snowmelt occurring in late-May to mid-June with reductions in stage during the spring hydrograph recession typically less than 10%/day (up to 16%/day). Survival was influenced by subsequent scouring flows and summer water availability. Results of this study provide insights into the key hydrology and geomorphic processes that maintain the distribution and complexity of riparian habitats along central Sierra Nevada mountain streams.