Barrier island morphology and dune vegetation pattern and process in the georgia bight
Stallins, Jon Anthony
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Vegetation patterns reflect the interplay of two processes: the imposition of site factors along gradients and disturbance exposure and recovery. I investigated how these processes interact to generate patterns in dune plant species composition, diversity, and soil-vegetation relationships on two barrier islands in the Georgia Bight: South Core Banks, North Carolina, a frequently stormoverwashed barrier island, and Sapelo Island, Georgia, an infrequently overwashed barrier. A single transverse compositional gradient characterized South Core versus two intersecting gradients on Sapelo. Dune ridge-and-swale topography on Sapelo restricts the spread of overwash, thus disturbance-structured vegetation adjoins the prominent transverse vegetation zonation expressed along gradients. On South Core, topography reinforces the spread of disturbance across the landscape, thus folding disturbance and transverse vegetation zonation into a single overlapping gradient. These contrasting gradient structures contributed to Sapelo's higher longitudinal and transverse compositional variability.|Biogeomorphic feedback and threshold processes that mold compositional patterns decouple disturbance exposure and effect on Sapelo, while this coupling is reinforced on South Core. The Intermediate Disturbance Hypothesis (IDH) presupposes a geographically-consistent direct coupling between disturbance exposure and effect, and thus was applicable at the landscape-scale only on South Core. Here, disturbance is frequent, and biogeomorphic processes reinforced an inhibition-structuring of diversity patterns. On Sapelo, diversity patterns did not conform to IDH, largely due to a decrease in the spatial scale of the processes regulating species patterns. The Dynamic Equilibrium Model (DEM) was a more robust framework for examining the geographic patterns of species diversity in this study given its ability to incorporate direct and indirect structuring effects of disturbance, and changes in the scale expression of diversity patterns.|Infrequent overwash enhanced edaphic organization on Sapelo, while frequent overwashdampened edaphic organization on South Core. Relatively stronger associations between soil and total vegetation cover were expressed on Sapelo, however, this may not necessarily be due to edaphic organization. Individual species on both islands were responsive to variability in edaphic properties. This association between the dominant dune-building and sediment-stabilizing taxa and edaphic variability on both islands reinforces their importance as regulators of topography and the spread of disturbance.