Groundwater Thresholds for Root Decomposition and the Relation to Barrier Island Plant Communities

Abstract

Thresholds for belowground decomposition rates based on nearness to groundwater were identified on a Virginia barrier island. Negative exponential decay rates (k = 0.310–0.915 yr.−1) varied according to average distance to the freshwater free surface, with lowest decay occurring in low elevations (marsh and deeper soils of a wooded swale), and highest decay occurring at mid to high elevations (surface soils in wooded swales and all dune sites). The majority of decay rate variances were explained by mean annual depth to the freshwater free surface (r2 = 0.78). Locations with mean annual groundwater depths greater than 0.95 m were substantially less affected by fluctuations in groundwater levels (r2 = 0.09) than where groundwater was near the soil surface (r2 = 0.83). Two vegetation-based decay thresholds were identified at mean annual groundwater depths of −0.041 m and 0.538 m, separating belowground decay into three groups (low, moderate, and high decay). These groups tended to correspond to the three interior barrier island plant communities (marsh, wooded swale, and dune), but with overlap. The groundwater free surface provides a useful metric that can be used in geospatial models to predict process rates and vegetation patterns in the dynamic barrier island landscape.