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Watershed-scale controls on riparian vegetation distribution and dynamics: Impacts of geomorphology, climate, and disturbance
AuthorKnight, Anna Chapman
AdvisorWeisberg, Peter J.
Environmental and Natural Resource Sciences
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Riparian ecosystems provide critical services in semi-arid landscapes, but they are vulnerable to degradation. Fluvial landforms, depth to groundwater, and substrate type influence reach-scale riparian vegetation patterns. Watershed geomorphology and disturbance shape stream channel processes, groundwater conditions, and sediment characteristics, yet relationships between watershed characteristics and riparian vegetation distribution and dynamics are poorly understood. We investigated relationships between watershed geomorphology, climate, upland vegetation, and disturbance and (1) riparian vegetation extent and composition, and (2) multi-decadal riparian changes.Study sites consisted of 565 small mountain watersheds across the Great Basin ecoregion of the western United States. We mapped annual fractional cover of woody and herbaceous riparian vegetation from 1985 to 2017 from Landsat images. We used a GIS- based approach to derive watershed characteristics from digital data sets. We calculated 2017 riparian vegetation extent and composition and analyzed relationships with watershed characteristics using random forest models. Vegetation changes were classified based on trends in fractional cover over the 33-year time series. We related watershed characteristics to vegetation changes in burned and unburned sites using ordination and hierarchical cluster analyses.Riparian extent increased with watershed characteristics indicating greater water availability. Riparian composition was more woody in watersheds with high flood potential characteristics, including high relief ratio and ruggedness. These characteristics were associated with increasingly woody composition in unburned watersheds and loss of riparian vegetation in burned watersheds. Riparian vegetation extent and composition were relatively stable during the study period, but changes were highly variable among watersheds. Steep, rugged, high-elevation watersheds with low perenniality and high upland tree cover were prone to extensive riparian degradation, especially when burned. Upland vegetation was strongly associated with riparian vegetation dynamics, but further research is needed to identify causal linkages.This study provides new information on the distribution of herbaceous and woody riparian vegetation that can inform riparian condition assessments for land management and restoration. Watershed characteristics and disturbance regimes influence riparian vegetation distribution and changes through effects on flooding, sediment characteristics, and water availability. Understanding watershed conditions can improve management and restoration outcomes and provides important context for fine-scale studies of riparian vegetation patterns.