TREE TRADE-OFFS IN STREAM RESTORATION PROJECTS: IMPACT ON RIPARIAN GROUNDWATER QUALITY A Report Submitted to the Chesapeake Bay Trust by

Riparian zones are considered one of the most important best management practices to reduce nitrogen (N) and phosphorus (P) losses from field to streams, and ultimately the Chesapeake Bay (Lowrance et al. 1997). For decades, there has been extensive research on variables regulating the efficiency of riparian zones in improving water quality with respect to nitrate in subsurface flow, and soluble and particulate phosphorus dynamics in riparian zones (e.g. Lowrance et al. 1997; Mayer et al. 2007). This research stressed the role of vegetation at regulating many key riparian functions, including groundwater quality (through N and P update), soil biogeochemistry, soil erosion, streambank stabilization and the ability of riparian zones to provide habitat for wildlife (see review in Dosskey et al. 2010). However, in many cases, riparian vegetation is removed during stream restoration projects to facilitate access to the stream during construction, remove legacy sediments, or to allow for stream bank regrading to facilitate stream-floodplain interaction. In this context, vegetation removal has both immediate effects (e.g. reduced transpiration, reduced stream shading) and longer-term effects (e.g. reduced soil organic carbon). However, the impacts of tree management (years since removal, current status / survival rate) on N and P in groundwater remains poorly understood (Dosskey et al. 2010). In spite of many years of research on riparian zone water quality functions in Maryland and elsewhere, extensive field data and models to estimate the water quality impacts of tree removal are still missing, especially within the context of stream-riparian restoration projects.