Barrier islands are highly dynamic landscapes, subject to frequent and severe disturbance. Recently, islands have been characterized based on vegetation and topography as exhibiting different disturbance regimes - reinforcing or resisting. Disturbance resisting locales typically have continuous, high elevation foredunes and wide stable inland areas supporting productive communities. These locales are commonly associated with the grass species, Ammophila breviligulata, which traps sediment and builds long dune ridges because of its growth strategy. This contrasts with disturbance reinforcing regions which are comprised of low, hummocky dunes, frequently overwashed, and as a result support a limited vegetation in the inland community. Spartina patens is commonly associated with these regions. To identify patterns of distribution, we collected elevation and species cover from several Virginia barrier islands that exhibit each of the disturbance regimes. To test the effect of repeated disturbance on plant growth, we performed a field bucket burial experiment in which each of the species was buried incrementally to total depths of 15cm, 30cm and 45cm of sand over one summer season. Height, biomass and several functional traits related to growth (i.e. specific leaf area (SLA), δ13C, and leaf nitrogen) were measured at the end of the experiment.
Results/Conclusions
Both species were distributed across multiple islands, but we found differences in the range of elevations. Spartina occupied lower elevations (0.94 - 3.51m) compared to Ammophila (1.05 - 4.76m). Cover of Spartina was also higher at the lower elevations. In the manipulative experiment, increased burial significantly reduced plant height at the higher levels (30cm, 45cm, p = 0.0038). Spartina was significantly taller than Ammophila in plots that received 45 cm burial (p = 0.0108). Spartina also had significantly higher biomass at the 45cm burial, although the response was weak (p = 0.0448). Our results show that Spartina occurs at lower elevations and outperformed Ammophila at higher levels of disturbance (i.e. burial). Spartina appears to be better adapted to the stressors associated with disturbance reinforcing regimes. Its ability to quickly respond to disturbance could select for this species that fails to rebuild high, continuous dunes and lead to feedback cycles that reinforce the disturbance regime. Quantifying the burial response and distribution patterns of these common dune species brings us one step closer to understanding the mechanisms by which vegetation influences disturbance regimes.