Tree mortality reduces stemflow inputs and alters groundwater biogeochemistry in ghost forests

Sea-level rise is transforming coastal forest ecosystems by inducing tree mortality, changing hydrologic connectivity, and altering carbon cycling. This transformation creates coastal woodlands experiencing widespread tree mortality, defined by landscapes of dead or dying trees called ghost forests. While recent studies have highlighted hydrological consequences of tree loss through reduced transpiration, trees also represent localized sources of dissolved organic carbon (DOC) to subsurface waters through stemflow, yet changes in this pathway during mortality remain underexplored. In these transitioning coastal forests, the presence or absence of stemflow emerges as a key factor governing near-trunk recharge and vertical DOM inputs. We investigated how tree mortality alters stemflow contributions to DOM fluxes and groundwater composition in a mid-Atlantic coastal forest experiencing sea-level rise. Using field-deployed stemflow collectors, lysimeters, and groundwater wells across a mortality gradient, we assessed how the loss of stemflow influences water and carbon fluxes. Precipitation-groundwater relationships revealed a threshold-like response near healthy trees: for precipitation events <16mm, groundwater levels rose more sharply in areas receiving stemflow (no-collar) than in adjacent no-stemflow (collared) areas, whereas beyond this threshold the disparity diminished as both zones became similarly saturated. This nonlinear behavior aligns with enhanced infiltration near tree bases. In contrast, areas near moribund trees showed more linear precipitation-groundwater responses, indicating a weaker role of stemflow in regulating recharge. Tree mortality significantly reduced stemflow volume and DOM flux per unit basal area, even though DOC concentrations were elevated in dead-tree stemflow. Groundwater DOM composition differed from stemflow inputs, reflecting additional processing and sources beyond canopy-derived water. In mortality zones, fluorescence spectroscopy and PARAFAC modeling of groundwater DOM indicated reduced microbial and protein-like components typically associated with stemflow and increased terrestrial humic-like signals. These patterns are consistent with reduced vertical carbon inputs and altered recharge dynamics after transpiration loss. Overall, our findings show that tree mortality disrupts canopy-to-groundwater linkages, diminishing carbon inputs and altering DOM composition in ways that may reduce carbon retention in coastal forests under sea-level rise.