Coastal dune plants are subjected to numerous stressors including sea spray, burial by sand, high irradiance, water and nutrient limitation, and high soil salinity.  Global climate change may exacerbate these conditions because of increased temperatures, sea level rise, and increased storm frequency and intensity, leading to shifts in species distribution and composition on coastal dunes.  We compared the distribution of Cirsium horridulum, Conyza canadensis, and Rumex acetosella, three forbs with varying growth habits, on three dunes with increasing distance from the ocean on a Virginia barrier island.  For each species we obtained leaf C:N, %N, δ¹³C, δ¹⁵N, relative water content, and tissue chlorides to determine the relationships between spatial distribution and specific physiological strategies for survival in a high stress environment.  We also quantified environmental characteristics including depth to the water table and soil chlorides for the front, top, and back of each dune. 

Results/Conclusions

Cirsium had the highest δ¹³C (-27.1 ± 0.1 ‰) indicating greater water use efficiency (WUE) as compared to Conyza and Rumex.  Cirsium had the largest range of C:N values suggesting a more plastic nitrogen use efficiency (NUE).  Higher WUE and NUE may explain why Cirsium was the only species found at every site.  Rumex was absent from primary dunes where soil salinity was highest, 11,108 ± 1,389 µg/g.  The only species present on the top of secondary dunes where the water table was > 2 m from the soil surface was the relatively succulent Cirsium.  Conyza was not present on tertiary dunes where the water table was closest to the surface, indicating that Conyza is less competitive for water and/or nutrients.  Leaf δ¹⁵N of Cirsium became more negative with increased dune age due to the more N enriched older soils.  Rumex populations may decline as sea level rises and storms increase, resulting in higher salinity.  Conyza populations may decline if depth to the water table increases from higher temperatures.  Community composition may be drastically altered and dune stability jeopardized as climate changes unless plant adaptations and/or migrations keep pace with environmental change.