Interannual controls on riparian plant health in a dryland river

Riparian zones in drylands provide important refugia for plants but depend on groundwater and thus are subject to local temporal and spatial variability in abiotic controls. In lieu of costly field-based sampling, we used readily available data to establish site-scale interannual relationships among riparian plant health and the abiotic factors that control their water balance for a historically persistent wetland adjoining the Santa Clara River in southern California, USA. Non-linear generalized additive model (GAM) analysis of plant health, represented using the normalized difference vegetation index (NDVI), confirmed robust relationships among plant health and various geomorphological and hydrological factors over multi-decadal timeframes, including years since last high-flow event, intra-year groundwater elevation changes and magnitude of 2-year cumulative surface water inflows. Geomorphic controls are related to years with high flows that cause extensive scour and deposition that re-set riparian plant communities. Relationships with dry-season groundwater declines reflect direct plant access to sub-surface moisture. Hydrological dependence via cumulative inflow magnitude indicates the dependency of groundwater elevations on sufficient winter recharge to prevent precipitous groundwater decline. GAMs-based inflection point analysis of surface water inflows versus groundwater elevations confirmed that the cumulative magnitude of multi-year inflows is critical in avoiding catastrophic groundwater declines and that large flood events drive groundwater recovery. We show that abiotic controls on plant health can be derived from readily available data and that non-linear analysis better represents the complexity of these scalar controls. Our analysis has relevance for ecosystem management of human-altered rivers and climate change adaptation.