Dynamic restoration and the impact of native versus invasive vegetation on coastal foredune morphodynamics, Lanphere Dunes, California, USA

The Lanphere Dunes, part of the Humboldt Bay National Wildlife Refuge, has been the focus of foredune restoration efforts since the 1980s. Efforts have centred around removal of an invasive European beach grass species, Ammophila arenaria, introduced in the early 1900s to stabilize the dunes to protect landward communities from coastal flooding and storm surges. Despite effectively stabilizing the foredune, A. arenaria forms monotypic vegetation stands, with highly dense roots, rhizomes, and above-ground biomass that can lead to pronounced scarping of the seaward slope, alongshore steering of wind and sediment, a lack of landward transfer of sand, and a steeper, more peaked profile. Effective foredune restoration must consider the coupled interactions between dominant plant type and the geomorphic processes that influence dune form. A 5 ha reach of recently restored foredune was monitored biannually with terrestrial laser scanner and uncrewed aerial systems platforms between 2015 and 2021 to characterize the impacts of dynamic restoration on foredune form and resiliency. This reach included two control plots: (1) native, non-restored and (2) invasive, and three restored plots revegetated with native species: (3) a native grass (Elymus mollis), (4) a low-lying herb and subshrub assemblage, and (5) a mixture of the native grass, herbs, and subshrubs. After five growing seasons, restored plots exhibited distinct geomorphic and sediment budget differences. Natively vegetated plots recovered from extensive scarping 2 years faster than the invasive plot. Restored plots saw foredune height (0.5-0.7 m) and width increase, landward extension (1 m) while maintaining a similar seaward position, and positive lee-slope sediment budgets that exceeded both control plots (up to 0.015 m(3) m(-2) month(-1)). These results suggest that the native vegetation plots allowed increased landward sand transport across the foredune, and increased the capacity of the foredune to recover more quickly following dune scarping.