Groundwater Thresholds for Root Decomposition and the Relation to Barrier Island Ecological State Changes
Barrier islands off the eastern shore of Virginia exhibit distinct habitats that abruptly transition between periodically brackish/freshwater marshes, wooded swales, and sparsely vegetated dunes. There is strong evidence that the plant communities and ecosystem processes occurring in each habitat are primarily influenced by nutrient availability and the distance between two of the three free surfaces: land and freshwater. At the Virginia Coast Reserve-Long Term Ecological Research Site in Virginia, USA, thresholds to belowground decomposition rates were identified by measuring decay of native roots and rhizomes at 32 elevations in relation to mean annual groundwater levels (-0.356 – 1.937 m). 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 elevation/anoxic conditions (marsh, and bottom soils of a wooded swale), and the highest decay occurring at mid to high elevations (upper soils in wooded swales and all dune sites). The majority of variance in decay rates can be explained by mean annual depth to the freshwater free surface (r2 = 0.78). Locations with mean annual groundwater depths greater than 1 m appear substantially less affected by fluctuations in groundwater levels (r2 = 0.09) than locations nearer to groundwater (r2 = 0.83). Belowground decay was more rapid from 0-20 cm compared to 20-40 cm (p < 0.05) and was divided into 3 groups (low, moderate, and high decay) that correspond to the three interior barrier island ecological states. Results from this study indicate a strong relationship between decay rate dependence on groundwater levels and state changes on a barrier island.
Barrier island, Decomposition, Groundwater, LTER, Long Term Ecological Research, State change, Thresholds, Ecosystems, Nutrient availability, Eastern Shore of Virginia, Biology, Botany, Plant Sciences
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