Barrier islands constitute 10-15% of coastlines worldwide, are economically valuable, and protect lagoons and saltmarshes in back-barrier bays from wave attack. Two competing sets of processes influence island state: (1) dune-constructive processes in which vegetation induces sediment accretion; and (2) dune-destructive processes in which storms cause erosion and vegetation mortality. Based on the relative rates of constructive and destructive processes, barriers may exist in either a sparsely-vegetated, frequently overwashed, low state, or a high, well-vegetated dune state. Under some conditions, islands (or island segments) may exhibit bistability, in which both low and high elevation states are stable. It has been proposed that a salt- and burial-tolerant plant, Spartina patens, prevents overwash-intolerant dune-building grasses (Ammophila breviligulata) from establishing, thereby maintaining a low state (“maintainer hypothesis”). However, recent studies in the Virginia Coast Reserve (VCR) demonstrate that S. patens induces sediment accretion, potentially facilitating A. breviligulata establishment (“facilitator hypothesis”). We examine the effects of species growth characteristics and species interactions on barrier island state using a two-species spatial logistic growth module within the Coastal Dune Model, a physics-based eco-geomorphic model of dune development. We vary intrinsic growth rates, carrying capacity, and sensitivity to overwash to examine how vegetation parameters may influence state.
Results/Conclusions
We find that vegetation colonization rates, growth rates, and vegetation sensitivity to wave overwash alter the parameter space in which barriers exhibit low, bistable, or high stable states. When simulating foredune development with a single dune-forming species alone versus one with both a back-beach species and a dune-forming species, the presence of the back-beach species increases propensity of the system to exhibit bistability or enter a high elevation state. These results support the “facilitator hypothesis,” in which back-beach accretion by S. patens increases establishment by the dune-building A. breviligulata. To examine the viability of the “maintainer hypothesis,” we further examined how species interactions might alter bistability. Results suggest that, if S. patens exerts a strong negative effect on A. breviligulata growth, its impedance of Ammophila growth would inhibit dune development and thereby maintain the landscape in a frequently overwashed, low state. However, published greenhouse experiments do not support this negative interaction along VCR barriers. This potentially facilitative relationship between back-beach species and dune-forming species has broad implications for barrier-lagoon systems: facilitation of dune building by back-beach species may promote development of high barriers, and therefore reduce the frequency and magnitude of overwash and sedimentation on back-barrier swale and marsh ecosystems.