The role of vegetation in determining dune morphology, exposure to sea level rise, and storm-induced coastal hazards: A U.S. Pacific Northwest perspective Peter Ruggiero1, Sally Hacker2, Eric Seabloom3, and Phoebe Zarnetske4 1 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University 2 Department of Integrative Biology, Oregon State University 3 Department of Ecology, Evolution, and Behavior, University of Minnesota 4 Department of Forestry, Michigan State University Abstract: Coastal foredunes are often the ‘first line of defense’ for coastal ecosystems and backshore infrastructure from erosion and flooding. The shape and growth characteristics of coastal foredunes, typically characterized by simple morphometrics such as dune toe and crest elevations, and dune volume, are a product of both physical and biological forces. By influencing foredune shape, these forces ultimately affect the exposure of human populations and ecosystems to extreme storms and sea level rise. In this talk we synthesize a suite of interdisciplinary field, laboratory, mesocosm, and computer modeling experiments that have examined the relative role of vegetation in determining dune geomorphology in the U.S. Pacific Northwest (PNW), with particular attention to how dunes of different shapes result in variable levels of exposure to coastal hazards. PNW dune shape is primarily a function of sediment supply and two species of non-native beach grasses (Ammophila arenaria and A. breviligulata). Over recent decades, A. breviligulata (American Beach grass) has increased its dominance over A. arenaria (European Beach grass) on dunes where it was originally planted and has actively spread to new sites formerly dominated by A. arenaria. 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 is more suited for building 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. These findings draw on insights from geomorphology, ecology, and coastal engineering to assess coastal barrier vulnerability in light of global change.