Identifying failure mechanisms of native riparian forest regeneration in a variable-width floodplain using a spatially-distributed riparian forest recruitment model

Abstract

Native riparian forests provide the physical structure, nutrient inputs, and habitat elements necessary to support ecosystem function for many aquatic and terrestrial species. However, regeneration of these important ecotones is being negatively impacted by anthropogenic and climatic pressures, driving managers to consider creative ways to increase native forest recruitment. Decision-making tools currently have limited capacity to assist native forest management and restoration since most cannot identify which recruitment mechanisms fail or where those failures occur on the floodplain. In response, we reconfigured a validated spatially-distributed riparian forest recruitment model to identify the limiting recruitment mechanisms and delineate their spatial extent on the floodplain to guide and prioritize management and restoration efforts. Here we focus on native Salicaceae species, whose seed dispersal mechanism is primarily driven by hydrochory.Results indicate that the success of individual recruitment parameters in semi-confined river systems changes with both hydrologic condition and relative floodplain width (Wfp∗ = Wfloodplain/Wchannel). Overall, the disturbance mechanism (shear stress), required to create bare-soil conditions, was the most significant limitation to native forest recruitment in the system. Greater potential riparian recruitment occurs in wet hydrologic years than in average hydrologic years owing to greater floodplain inundation, but inundation alone is a poor success metric since recruitment requires an appropriate sequence of geomorphic disturbance, seed delivery, and water availability over the growing season. Consequently, narrow floodplains (Wfp∗< ∼3) have greater recruitment efficiency, potential recruitment per available floodplain area, than wide floodplains (Wfp∗> ∼3) but wide floodplains provide greater total potential recruitment area during wet hydrologic conditions. Model output maps that delineated individual and combined failure mechanisms during average and wet hydrologic years in the managed South Fork Boise River, USA were used to identify areas on the floodplain for potential restoration actions.