Simulating dune evolution on managed coastlines: Exploring management options with the Coastal Recovery from Storms Tool (CReST)

Despite the importance of coastal dunes to many low-lying coastal communities and ecosystems, our understanding of how both climatic and anthropogenic pressures affect foredune evolution on time scales of years to decades is relatively poor. However, recently developed coupled numerical modeling tools have allowed for the exploration of the erosion and growth of coastal foredunes on time scales of hours to years. For example, Windsurf is a new process-based numerical modeling system (Cohn et al. 2019a) that simulates the evolution of dune-backed sandy coastal systems in response to wave, wind, and water level forcings. CReST, developed as a front-end interface to Windsurf, aims to add the ability to incorporate beach nourishment and dune construction, beach and dune grading, dune grass planting scenarios, dune grass removal, and the presence of hard engineering structures into the model framework to better account for the complex dynamics of managed coastlines. Initial model sensitivity tests suggest that the model provides a flexible framework to investigate the complex interactions between beaches and dunes for a variety of exploratory and applied applications.

Species-Specific Functional Morphology of Four US Atlantic Coast Dune Grasses: Biogeographic Implications for Dune Shape and Coastal Protection

Coastal dunes arise from feedbacks between vegetation and sediment supply. Species-specific differences in plant functional morphology affect sand capture and dune shape. In this study, we build on research showing a relationship between dune grass species and dune geomorphology on the US central Atlantic Coast. This study seeks to determine the ways in which four co-occurring dune grass species (Ammophila breviligulata, Panicum amarum, Spartina patens, Uniola paniculata) differ in their functional morphology and sand accretion. We surveyed the biogeography, functional morphology, and associated change in sand elevation of the four dune grass species along a 320-kilometer distance across the Outer Banks. We found that A. breviligulata had dense and clumped shoots, which correlated with the greatest sand accretion. Coupled with fast lateral spread, it tends to build tall and wide foredunes. Uniola paniculata had fewer but taller shoots and was associated with ~42% lower sand accretion. Coupled with slow lateral spread, it tends to build steeper and narrower dunes. Panicum amarum had similar shoot densities and associated sand accretion to U. paniculata despite its shorter shoots, suggesting that shoot density is more important than morphology. Finally, we hypothesize, given the distributions of the grass species, that foredunes may be taller and wider and have better coastal protection properties in the north where A. breviligulata is dominant. If under a warming climate A. breviligulata experiences a range shift to the north, as appears to be occurring with U. paniculata, changes in grass dominance and foredune morphology could make for more vulnerable coastlines.

Literature-based latitudinal distribution and possible range shifts of two US east coast dune grass species (Uniola paniculataandAmmophila breviligulata)

Previous work on the US Atlantic coast has generally shown that coastal foredunes are dominated by two dune grass species,Ammophila breviligulata(American beachgrass) andUniola paniculata(sea oats). From Virginia northward,A. breviligulatadominates, whileU. paniculatais the dominant grass south of Virginia. Previous work suggests that these grasses influence the shape of coastal foredunes in species-specific ways, and that they respond differently to environmental stressors; thus, it is important to know which species dominates a given dune system. The range boundaries of these two species remains unclear given the lack of comprehensive surveys. In an attempt to determine these boundaries, we conducted a literature survey of 98 studies that either stated the range limits and/or included field-based studies/observations of the two grass species. We then produced an interactive map that summarizes the locations of the surveyed papers and books. The literature review suggests that the current southern range limit forA. breviligulatais Cape Fear, NC, and the northern range limit forU. paniculatais Assateague Island, on the Maryland and Virginia border. Our data suggest a northward expansion ofU. paniculata,possibly associated with warming trends observed near the northern range limit in Painter, VA. In contrast, the data regarding a range shift forA. breviligulataremain inconclusive. We also compare our literature-based map with geolocated records from the Global Biodiversity Information Facility and iNaturalist research grade crowd-sourced observations. We intend for our literature-based map to aid coastal researchers who are interested in the dynamics of these two species and the potential for their ranges to shift as a result of climate change.