Belowground differences in the biotic structure of natural and managed coastal foredunes

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Drew Eugene White, Katheryne V. Ramos, Mary E. Goeke and Julie Zinnert, Department of Biology, Virginia Commonwealth University, Richmond, VA, Drew Eugene White and Nicholas Cohn, Engineer Research and Development Center, United States Army Corps of Engineers, Duck, NC, Elizabeth H. Davis and Christopher J. Hein, Department of Physical Sciences, Virginia Institute of Marine Science, Gloucester, VA

Background/Question/Methods
Coastal sand dunes are the first defense against storm surge and flooding of the communities they front. Recent studies demonstrate belowground biomass importance for resisting erosion. Although the value of dunes as a buffer to mitigate the impacts of coastal hazards is widely accepted, there is little quantitative understanding of how natural and managed dunes vary in response to storms and sea-level rise. Natural dunes form in part through feedbacks between plant growth and sediment accumulation. This process buries plant biomass and induces growth in burial-tolerant species, creating biotic structure that grows with and within naturally formed dunes. In constructed dunes, vegetation is planted on top of emplaced, unconsolidated sand, limiting internal structure. These differences may affect the protective function of dunes. Our objective is to characterize and quantify belowground composition and structure in natural, managed, and artificial dunes in the Outer Banks, North Carolina (USA).
Cross-shore transects of sediment cores (each ~ 120 cm long) were collected from the dune toe to the dune heel across seven dune sections with varying management histories . Cores were analyzed for sediment texture and segmented into 30-cm sections. Roots were removed by wet sieving, scanned for morphology, dried, and weighed for biomass.

Results/Conclusions
Rooting characteristics (i.e., surface area, length, biomass) were higher in natural sites than managed sites and more roots were present at greater depth; however, natural sites showed considerable variation. Natural sites with higher rates of erosion over the last few years exhibited less belowground biomass. Managed sites were commonly planted with Ammophila breviligulata and contained roots with high surface area in the upper 30-60 cm. Across all locations and depths, there was a higher presence of fine roots, <0.5 mm in diameter. Abundance of roots was highest on the dune face and decreased at the dune toe. Our results suggest that managed and natural dunes exhibit differences in the amount and distribution of roots, likely as a result of site history and species composition. We also demonstrate patterns of root presence according to depth, which may affect the interaction between roots and waves during erosional events. These results will be incorporated into erosion-prediction models and shared with community stakeholders and policymakers.