Coastal sand beaches and dunes are important but understudied arbiters of coastal ecosystem services. They form at low-lying coastal margins where sand transported by oceanic waves and wind combine with vegetation to produce dynamic geomorphic structures. They occur at most latitudes on Earth and cover roughly 34% of the world’s ice-free coastline. For centuries, due to their unique position between ocean and land, coastal beaches and dunes have provided humans with important services particularly those related to coastal protection, recreation, and raw materials. Sand dune vegetation along with sand supply plays an important role in dune shape and ultimately vulnerability to extreme storms and sea level rise. Here we synthesize field surveys and a suite of interdisciplinary laboratory, mesocosm, and computer modeling experiments that examine the relative role of vegetation in determining dune geomorphology in the U.S. Pacific Northwest. We focus on how vegetation shapes dunes and results in variable levels of exposure to coastal hazards and conservation management.
Results/Conclusions
Results show that PNW dune shape is primarily a function of sediment supply and the geographic distribution of two species of non-native beach grasses (Ammophila arenaria and A. breviligulata). Over recent decades, A. breviligulata (American beachgrass) has increased its dominance over A. arenaria (European beachgrass) on dunes where it was originally planted. A species-specific biophysical feedback occurs between sand deposition and beach grass growth habit, resulting in distinctly different dune geomorphologies in locations dominated by these different grass species. The dense, vertical growth habit of A. arenaria allows it to capture more sand, produce more vertical tillers, and build taller, narrower dunes, while the less dense, lateral growth habit of A. breviligulata builds shorter but wider dunes. The species-specific feedbacks, along with invasion dynamics, have a first order effect on the region’s exposure to coastal hazards, in the present day and under a range of climate change and invasion scenarios. In addition, we found that habitat restoration areas, where beachgrasses are removed for endangered species, vary in their exposure to coastal hazards across a number of sites that vary in their geomorphology. These findings provide insights into the management of coastal dunes in light of global change.