The Role of Vegetation in Determining Dune Morphology, Exposure to Sea-Level Rise, and Storm-Induced Coastal Hazards: A U.S. Pacific Northwest Perspective

Abstract

Coastal foredunes are often the “first line of defense” for backshore infrastructure from the hazards of erosion and flooding, and they are key components of coastal ecosystems. 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 chapter, 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 (PNW). We focus on how dunes of different shapes result in variable levels of exposure to coastal hazards. Results suggest 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 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.