Linking Stemflow to Groundwater in Ghost Forests: Assessing Tracers and the Impact of Tree Health on Dissolved Organic Carbon Composition

Abstract

Coastal forests along the Atlantic seaboard are undergoing profound transformations due to rising air temperatures, sea level rise, and saltwater intrusion. One consequence is the emergence of “ghost forests”, characterized by moribund and dead trees that potentially disrupt biogeochemical cycling. Such disruptions would include altering stemflow (the intercepted precipitation that is funneled down tree stems), which serves a critical yet understudied role in carbon cycling. In coastal forests, where the groundwater table is at or near the surface, stemflow may interact with groundwater more directly, possibly introducing previously unaccounted-for dissolved organic matter (DOM). This study explores the interactions between stemflow and groundwater from moribund and healthy sweetgum trees (Liquidambar styraciflua L.) within a coastal forest experiencing the effects of sea level rise. Groundwater wells were placed near tree trunks and measured water levels for trees with stemflow collection systems, where groundwater is unaffected by stemflow, and trees without collection systems, where stemflow can infiltrate the soil and affect groundwater. Following each rain event, groundwater collected from stemflow-influenced and non-influenced areas showed little difference for dissolved organic carbon (DOC) concentrations, suggesting that any stemflow signal was lost or altered during subsurface transport. However, in examining the DOC composition (via optical indices), stemflow-influenced groundwater exhibited higher biological index (BIX) and E2:E3 values, hinting at more freshly produced DOC with lower molecular weights. Tracing lignin to examine the relationship between stemflow and groundwater was also evaluated; however, small differences between groundwater sample groups did not demonstrate lignin to be a feasible tracer. These findings indicate that even though stemflow is a transporter of organic matter to the forest floor, subsurface processes can prevent detection of these inputs in the groundwater. While lignin may not serve as an effective tracer for these systems, the use of optical indices can provide insight into stemflow-driven biogeochemical cycling and its subsequent effects on groundwater.