Barrier islands are the dominant coastal feature of the Gulf of Mexico and along much of the eastern coast of the US and have significant ecological and economic value. These dynamic systems are maintained by a variety of abiotic factors, including the regular actions of winds and waves, along with stochastic disturbance from tropical storms. The sand dunes on barrier islands are created by interactions between these abiotic forces and the plants themselves, as the vegetation can actively build and maintain dune topographies. Permanent plots on St. George Island, Florida have been censused annually for 18 years, quantifying both vegetation and dune structure across the very different foredune, interdune, and backdune habitats that make up the island. Percent cover of all plant species, along with dune height, has been determined annually in 441 plots, with soil moisture and percent organic material quantified at irregular intervals. Correlations between vegetation patterns, dune morphology, and climate variables have been determined to quantify the effects of both short- and long-term weather events on individual plant species and plant community structure. In particular, we are interested in the effects of storms on successional processes and patterns across the three habitats.
Results/Conclusions
As expected, foredunes near the ocean shoreline exhibited the greatest sand movement, with less vegetation cover and lower species diversity. The lower lying interdunes experienced irregular flooding, with high cover of clonal grasses and forbs and somewhat higher diversity. Backdunes at the bay side of the island experienced the least disturbance and the highest species diversity. Storms with high surge had large effects on dunes and many individual species, but long drought periods also had significant effects, generally involving a different suite of species. Individual species patterns were highly variable; some species were relatively stable in all habitats, while salt-intolerant species were strongly suppressed by storms, especially in low-lying areas. The successional patterns also differed among habitats, with foredune plant communities maintained as early successional communities by storms, while interdunes shift to vegetation associated with low, wet soils, while backdunes undergo succession to include longer-lived and some woodier species. Larger storms, such as hurricanes, created large shifts in the species composition, but the plant communities responded by returning to original successional pathways. There was no evidence of alternate stable communities or alternate pathways to the same stable community.