Despite prolific literature on theory of plant community assembly, we still have little understanding of what drives plant community formation along stress gradients of different ecosystems. Recently, functional traits have proved useful in empirically testing community assembly and coexistence across environmental gradients. Barrier islands are ideal systems to investigate how plant communities form over stress gradients that are highly dynamic, spatially compressed, and experience multiple effects of climate change. Barrier islands are ubiquitous landforms occurring along 15% of all coastlines. Abiotic filters that are dominant in other ecosystems over long spatial gradients, such as soil chemistry, soil moisture, and temperature can vary rapidly over short spatial and temporal scales on barrier islands. When non-resource abiotic stressors stabilize in protected swales, biotic interactions play an important role in coastal plant communities, as competition for resources is critical in nutrient depleted soils. Our objective was to determine whether widely used functional traits (i.e. specific leaf area, plant height, leaf C and N) are more informative in explaining plant zonation than species abundance across a barrier island spanning beach, dune, swale, and marsh. We quantified species abundance, functional traits, and environmental variables across Hog Island, part of the Virginia Coast Reserve LTER.
Environmental variables ranged from high to low stress across the environmental gradient. We documented a range of species with varying life history traits, representing multiple functional groups. Despite variability of functional groups and life history strategies, community weighted functional traits were very low and had no variation across the different environments (SLA: 60.3 – 165.4 cm2g-1, plant height: 20.8 – 94.1 cm, leaf nitrogen: 0.96 – 2.92%). A Mantel test showed that functional traits did not correlate significantly with important environmental variables (soil chloride levels, distance from shoreline, and elevation) while community species had a positive relationship (r = 0.54, P < 0.001). Along the compressed stress gradient of a barrier island, widely used functional traits are not as informative as species composition in explaining plant zonation patterns. It is well documented that functional traits are related to drivers of community assembly across stress gradients. However, our data show that defining communities along compressed stress gradients that are dynamic in both time and space do not necessarily follow trait-based paradigms that have been recently proposed. This expresses the importance of understanding zonation patterns among species before defining assembly rules using trait-based approaches.