Biotic or abiotic control? Assessing the mechanisms and implications of coastal dune bioengineering

Background/Question/Methods   Biotic and abiotic forces often interact to produce distinct physical features. The US Pacific Northwest coastal dune system provides an excellent template to study these interactions, due to the dramatic changes in dune geomorphology resulting from the invasion and subsequent dominance of two beach grass species over the last 100 years. The introduction of two Ammophila sand binding species replaced much of the native beach grass (Elymus mollis), leading to enhanced coastal protection with the formation of large, continuous, and relatively stable foredunes, where open, low-lying and dynamic dunes had previously dominated. Field data demonstrate a correlation between dominant beach grass species and foredune geomorphology, suggesting the potential for an ecological control of coastal vulnerability. However, sand binding species distributions are potentially confounded by the distribution of sediment supply, a primary control for dune geomorphology. In a 3-species interaction experiment, we investigated how sediment supply influences the dominance of three beach grasses found across the US Pacific Northwest coast. We then combined these results with field-based coastal distributions of dominant beach grass species, dune topography, and sediment supply models across 20 years to assess the relative roles of biotic vs. abiotic forces controlling dune geomorphology.

Results/Conclusions   Results from the species interaction experiment and field surveys indicate that A. breviligulata is a superior competitor across sediment supply regimes. However, field data show that A. breviligulata is associated with smaller foredunes than A. arenaria for a given sand supply regime. Further, our data suggest that the expansion of A. breviligulata is concurrent with lowered dune heights. The important combination of species growth habit and sediment supply regime indicates that if A. breviligulata continues to spread and out-compete A. arenaria, the topography of coastal foredunes may change. Results of this assessment are fundamental to understanding the mechanisms involved in coastal dune geomorphology and for informing coastal management decisions given climate change predictions of sea level rise and increased wave heights.