Interactive effects of invasiveness and community invasibility of grasses on US Pacific Northwest dunes

Non-native species have traits that indicate invasiveness (e.g., dispersal, fecundity, growth, or functional traits) and invaded communities differ in factors that influence invasibility (e.g., resource availability, disturbance and stress, or biotic interactions), but rarely are species and communities considered together. Here we examine both the invasiveness of three dune grass species and the invasibility of foredune plant communities dominated by each species across a 250 km stretch of coastline in the US Pacific Northwest. A field experiment was performed in which two non-native dune grass species (Ammophila breviligulata and A. arenaria) and one native species (Leymus mollis) were introduced into three community types across a range of sand deposition rates. Our results show that the two non-native beachgrasses were the most invasive, with higher survival, biomass, leaf area, and tiller abundance than the native dune grass species across varying community tiller densities and sand deposition rates. Of the three community types, the native L. mollis community was the most invasible, with higher colonist survival and biomass, especially under increasing tiller densities and sand deposition rates, suggesting facilitation. In contrast, in the two Ammophila-dominated communities, colonist survival decreased with increasing community tiller densities and sand deposition rates, indicating competition and/or sand burial as mechanisms mitigating invasibility. However, of the colonists that survived, increasing sand deposition generally had a positive effect on biomass, indicating sand fertilization. Our invasiveness and invasibility results match observations of species displacement since the introduction of the non-native beachgrasses to the region over a century ago.