Barrier islands are considered sentinel landscapes for the detection of global change. The mid-Atlantic islands of the Virginia Coast Reserve LTER present unique opportunities to monitor plant community responses over the long term. As the island shorelines oscillate, transgression and overwash events provide a natural model system for observation of potential impacts of sea level rise. Vines and lianas have been predicted to demonstrate enhanced competitive abilities in an environment of elevated temperature and CO2 concentrations, and we may expect twining or climbing plants to affect the established plant communities in these relatively pristine habitats. We selected the most seaward-establishing vine, Strophostyles umbellata (Fabaceae), to evaluate spatial variations in physiology, leaf diurnal movements, and biomass allocation in response to differential exposure to sea spray. Photosynthesis, stomatal conductance, relative water content, leaflet angle, azimuth, and temperature, and tissue chloride concentration were measured to evaluate plant responses. Additionally, seeds were exposed to various representative stresses to simulate environmental conditions of importance in the natural habitat; we conducted viability analyses to compare the effects of heat and scarification on seed germination.
Seed germination of Strophostyles increased in response to scarification and heat, while previous studies demonstrated that non-scarified seeds remain viable following immersion in seawater. Abrasion by sand did not appear to stimulate germination, though physical scarification did. Heat treatments also stimulated germination. Differences in species composition among sites suggest that salinity exposure affects the ability of S. umbellata to compete with neighboring vegetation, but does not fully inhibit success. Strophostyles species are symbiotic nitrogen-fixers, so their presence on coastal margins may be an important factor influencing nutrient budgets. The N-fixing symbiosis, in concert with the vine growth form, could confer significant advantages to Strophostyles over competing species, particularly if predictions regarding rising temperatures and CO2 concentrations prove accurate.