Quantifying the sand capture efficiency of native and non-native dune plants with an in-situ experiment and numerical modeling during wind gust conditions.

Coastal dunes are natural landforms formed by complex positive feedbacks between wind driven sediment transport processes, the presence of vegetation, and existing dune morphology. To improve our knowledge of the sand capture efficiency of native and non-native vegetation species found on the Pacific Northwest coast, we designed a 3-day in-situ experiment using four mono-species vegetated plots (20 m by 10 m in size) during strong wind gust conditions on the coast of Oregon, USA. The dune grass species monitored during this experiment were the non-native Ammophila arenaria, Ammophila breviligulata, the new hybrid between A. arenaria x A. breviligulata, and the native Leymus mollis. Wind speed and direction sensors were set upwind and in the center of each plot during the experiment. Multi-directional sand traps were set up at the same position as the wind sensors during a 24-hour period to capture sediment transport coming into and moving through the plots during the stronger wind periods. Lidar surveys were performed before and after the wind event to compute morphological change and to estimate the vegetation cover for each plot. The vegetation cover of the non-native plots ranged between 37% and 47% while the native vegetation cover was 9%. During the wind gust, the plots were exposed to upwind speeds ranging from 3.6 to 4.5 m/s. Between the upwind positions and the plot center, wind speeds decreased by 16% to 28% for the non-native species, while wind speeds increased by 16% within the native species plot. The volume of sand in the A. arenaria plot increased by 6.4% (i.e., +10.8 m3) and the volume of sand withing the plot with A. breviligulata increased by 5.4% (i.e., +4.4 m3). Limited change was observed in the hybrid plot (+1.1% of its volume i.e., +1.3 m3), and the L. mollis (native vegetation) plot lost sand (-4.5% of its volume i.e., -0.1 m3). While the multi-directional sand traps facing the dominant wind direction were saturated during a last hours of overnight windy period, the traps indicate that wind from the south was the dominant transport direction with slightly less transport from the SE and the SW. Lidar and wind data were used in an aeolian sediment transport and dune building process-based model to simulate morphological changes during the wind gust event. The calibrated model will be used to explore the parameter space for drivers of aeolian transport through the vegetated plots. Although further field experiments are needed, this experimental design shows promising results for understanding the effect of different dune grass species on aeolian sand flux and associated morphological changes and encourages further work towards models that capture the magnitudes and relative differences among plots, depending on environmental and ecological boundary conditions.