Quantifying carbon storage ecosystem services in US Atlantic coast dunes

Katya R. Jay, John S. Stepanek and Sally D. Hacker, Department of Integrative Biology, Oregon State University, Corvallis, OR, Cedric J. Hagen and Peter Ruggiero, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR

Background/Question/Methods
Recent work has highlighted the important, but previously overlooked role of blue carbon in global carbon cycling. While most blue carbon measurements have been made in seagrass beds, mangroves, and salt marshes, little is known about carbon storage in coastal dunes, which comprise 34% of Earth’s ice-free coastlines. Dunes form via biogeomorphic feedbacks between sediment and burial-tolerant vegetation. As land-use change and sea-level rise (SLR) decrease coastal habitat area, dune systems may lose carbon stocks both in the dunes themselves and in their vegetation. Carbon storage has only been measured in European dunes, where sequestration rates are lower than salt marshes and mangroves but higher than terrestrial forests. Here, we measure sand carbon density along the North Carolina (NC) Outer Banks with an aim to estimate carbon stocks and determine how they are influenced by measures of sand supply, dune habitat, and dominant grass species (Ammophila breviligulata and Uniola paniculata). We collected 1m long sediment cores at the dune toe (the foredune seaward extent), crest, and heel (the foredune landward extent) and extruded sediment in 2-cm increments. Vegetation community and dune topographic surveys were conducted concurrently. Samples were oven dried, homogenized, and analyzed for organic carbon content via elemental analysis.
Results/Conclusions
Average sand carbon density was 0.91 ± 0.13 kg/m3 (sand % carbon was 0.06 ± 0.01), but values varied between barrier islands, habitats, and grass species. Differences in beach/dune geomorphology (a sand supply proxy) also contributed to differences in carbon density. Kruskal-Wallis and Mann-Whitney pairwise tests showed higher carbon density at the dune heel than the crest (p<0.01), but differences between the heel and the toe were not significant. Areas dominated by U. paniculata, a C4 grass found throughout the southeastern US, had higher carbon density on average than areas dominated by A. breviligulata, a C3 grass found from NC northward (p<0.001). Regression models showed that sand carbon density was positively correlated with U. paniculata stem density and biomass and negatively correlated with beach slope and beach width. In comparison, sand carbon density values observed in European dunes are approximately 2-4 times higher. One possible explanation for these differences is the dynamic nature of NC barrier islands and the frequency of disturbance due to extreme storms and overwash events. It is critical to quantify carbon storage in Atlantic coast dunes to understand how this ecosystem service may change with SLR, warming, and shifts in dune grass species ranges.