Spatial Variation in Environment and Physiological Strategies for Forb Distribution on Coastal Dunes

Our objective was to characterize a barrier island dune chronosequence to determine the relationships between spatial distribution and specific physiological strategies for forb survival in a high stress environment. Specifically, we hypothesized that (1) forbs on dune crests, where water is less available, would exhibit higher water-use efficiency (WUE) with higher delta C-13 compared with the front and back of each dune, (2) forbs on the primary dune where soil N is lowest would exhibit higher N use efficiency (NUE) with lower C:N ratio compared with forbs on secondary and tertiary dunes, and (3) the taprooted annual, Conyza sp., would have a lower WUE and NUE compared with two shallow-rooted biennials. For each species, we determined leaf C:N, %N, delta C-13, relative water content, and total chlorides. Environmental characteristics quantified on each dune included depth to water table and soil chlorides, organic matter, and pH. No significant differences were found for delta C-13 across dune positions or C:N values across dune ages; therefore, these two hypotheses were not supported. Conyza sp. had the lowest delta C-13 and C:N ratio, which supports our hypothesis that Conyza sp. would have the lowest WUE and NUE and might have more access to resources compared with Cirsium and Rumex species. Cirsium sp. had the highest delta C-13 (-27.1 +/- 0.1 parts per thousand), indicating greater WUE. Cirsium sp. also had the largest range of C:N values, suggesting a more plastic NUE. Higher WUE and NUE may partially explain why Cirsium sp. was the only species found at every site.