Ammophila arenaria as a Nurse Plant: Implications for Management of an Invasive Species

Ammophila arenaria (European beachgrass) was initially introduced to California from the coasts of Europe and North Africa to stabilize dunes and has since become a dominant invasive species on dune habitats along the west coast of the United States (Wiedemann and Pickart 1996). Ammophila arenaria primarily spreads by a rhizome network that is stimulated by active sand burial, allowing the species to rapidly stabilize shifting sand dunes (Buell et al. 1995). The rapid spread of dense stands of A. arenaria has caused a reduction in native plant richness and abundance (Wiedemann and Pickart 1996). However, at lower densities, A. arenaria may not have a negative impact on native plant species diversity and could even play a facilitative role (J. Solins, University of California, Davis, unpub. data). Tall vegetative structures such as shrubs often act as nurse species by providing a wind break, shade and soil stabilization for smaller plants and seedlings in dune systems (Shumway 2000, Rudgers and Maron 2003, Castanho et al. 2015). Due to a more favorable microclimate, herbaceous vegetation under shrubs and bunchgrasses are often larger, and have a higher reproductive output, compared to individuals in open areas (Shumway 2000). Given that A. arenaria is a tall species relative to most dune plants that stabilize soil, it has potential to act as a nurse plant.

Although considering the positive impacts of invasive species can make management decisions more complicated, it is necessary to fully understand potential consequences of invasive species removal. If invasive species have positive impacts on native diversity, then additional management may be required to mitigate negative impacts of invasive species removal. For example, if native shrubs or bunchgrasses provide favorable microclimates similar or superior to those created by A. arenaria (i.e., Rudgers and Maron 2003), then active planting of these native nurse species may be a beneficial practice. This study examines: 1) whether A. arenaria is facilitating a more diverse native community than areas in the absence of a tall vegetative structure; 2) whether understory communities differ under A. arenaria and Baccharis pilularis (coyote brush), a widespread native shrub; and 3) whether the presence of A. arenaria and B. pilularis have a similar effect on native plant diversity.

I conducted this study at the UC Davis Bodega Marine Reserve in Bodega Bay, California, US, on sand dunes with low (< 30%) A. arenaria cover. Ammophila arenaria was introduced to the property in the 1920s–1950s to stabilize naturally shifting sand dunes and has since spread through most of surrounding areas. I visually estimated the percent cover of each species in the 0.5 m² area where vegetation grew most densely under 12 A. arenaria and 12 B. pilularis individuals (hereafter, focal species), and within twelve 0.5 m² control plots where no shrubs or bunchgrasses were present. I randomly selected all sampling sites among focal species and open areas that were not within 0.5 m of another shrub or bunchgrass. To provide insight into characteristics of nurse species that may affect their understory communities, I recorded the focal species’ height, width at the widest diameter, percent canopy cover over the sampled understory vegetation and the cardinal direction of the sample area relative to the base of the focal species.

I calculated Shannon diversity in each plot using the species percent cover estimates. I then used a linear model and ANOVA to determine if there were differences in diversity among treatment groups, and performed mean comparisons using a Tukey test (p ≤ 0.05). To examine whether communities differ under different nurse species, I used Non-metric Multidimensional Scaling (NMDS) with the Bray-Curtis dissimilarity index to visualize differences among plant communities, and PERMANOVA to test for significant differences in the community composition among groups. I conducted all analyses using R (v.3.2.3, R Foundation, Vienna Austria), and used the R package “vegan” to calculate Shannon diversity and to perform NMDS and the PERMANOVA.

Comparing focal species characteristics indicated that, on average, A. arenaria was taller (height: 0.56 m ± 4.51 SE), narrower (width: 0.82 m ± 11.2 SE), and had a lower...