COS 93-6 - Plant community feedbacks on barrier island geomorphology in response to climate change

Thursday, August 11, 2011: 9:50 AM
9AB, Austin Convention Center
Spencer N. Bissett, Department of Biology, Virginia Commonwealth University, Richmond, VA, Steven Brantley, Coweeta Hydrologic Laboratory, USDA Forest Service Southern Research Station, Otto, NC, Donald R. Young, Alaska Department of Fish and Game, Catherine W. V. Wolner, Department of Environmental Sciences, University of Virginia, Charlottesville, VA and Laura J. Moore, Geological Sciences, University of North Carolina-Chapel Hill, Chapel Hill, NC

Evidence suggests that rising temperatures will increase the destructive potential of hurricanes, and past records attribute increased wave energy in the Atlantic Ocean to these storms.  Barrier islands may suffer increasing damage by hurricanes and northeasters, and can respond to increased storm frequency and sea-level rise (SLR) in two ways.  With gradual SLR, barriers become narrow, enabling cross-island sediment transport and island “rollover” towards the mainland.  Rapid SLR, particularly when compounded with storms, causes overwash events, submergence, and eventual disintegration if island volume and elevation are insufficient to prevent inundation.  We hypothesized that non-linear feedbacks between geomorphic and biological processes reinforce and expand overwash zone extent on barrier islands.  Specifically, we suggest that dune plant species and geomorphology interact, advancing islands’ physical evolution and impacting their future stability.  We tested this hypothesis at 12 representative sites on two islands of the Virginia Coast Reserve LTER site.  Hog Island is larger and dominated by intact primary dunes, indicating a history of recovery following overwash events.  Metompkin Island is narrower, lower in elevation, and dominated by overwash areas, indicating progress towards rollover.  Methods included field transect surveys, high-resolution GPS data collection, aerial photo analyses, and plant community analyses.


Plant percent cover values across transects supported hypotheses regarding the two most prevalent grass species, Ammophila breviligulata and Spartina patens.  The dune-building species A. breviligulata was dominant in intact dune systems and in intermediate or recovering areas on both islands.  On Hog Island, where large-scale overwash is limited to one major site, A. breviligulata indicator species (IS) value was significantly higher for all habitat types except swale and relict overwash areas, indicating recovery of dunes following overwash events on this wider, more topographically complex island.  On Metompkin Island, IS value of A. breviligulata was higher in intermediate and dune sites, while dominance of S. patens in overwash areas supports the maintenance feedback hypothesis.  Based on analysis of historic aerial photographs and ordinations of species abundance across the measured environmental gradients, we predict that dominance of S. patens will continue to increase as overwash areas spread, leading to narrowing of Metompkin Island.  Our results support the hypotheses that coastlines of infrequently-disturbed islands experience recovery driven by dune-building grass species, while frequent overwash disturbance tends to be reinforced by species maintaining low elevation.

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