Belowground Decomposition Rate Thresholds and Their Relationship to Plant Community State Change Along a Virginia Barrier Island Dune/Swale Transect

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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. It is likely that the proximity of the freshwater free surface (groundwater) affects root decomposition, which intrinsically affects nutrient cycling. At the Virginia Coast Reserve-Long Term Ecological Research Site in Virginia, USA, we identified thresholds to belowground decomposition rates by measuring decay of native roots and rhizomes at 8 elevations (1.12 – 3.21 m) and four depths per elevation (10 cm increments), corresponding to 32 distances from groundwater. Decay thresholds by elevation and distance to groundwater were used to determine if there is a relationship between decay and the abrupt transitions in habitats. Linear decay rates (k=0.1237 – 0.3409 yr^-1) varied according to average depth to groundwater, with lowest decay occurring in low elevation/anoxic conditions (marsh sites), and 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 distance to the freshwater free surface (r² = 0.74). Elevations above 2.06 m appear substantially less affected by fluctuations in groundwater levels (r² = 0.07) than lower elevations (r² = 0.61). Within site decay only varied by depth at two elevations (p<0.05) where fluctuating groundwater levels likely had the largest impact (within wooded swales). Decay varied significantly among elevations and depths (p<0.005), but elevations above 2.26 m did not significantly differ at any depth. Three groups of similar levels of decay occurred at elevations that correspond to the three main habitats. These results suggest groundwater plays a large role in decay rates of belowground plant litter and that elevations from groundwater have decay thresholds associated with habitat type and transitioning ecological states.