The role of vegetation in determining dune morphology, exposure to

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
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University
Department of Integrative Biology, Oregon State University
Department of Ecology, Evolution, and Behavior, University of Minnesota
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.