Invasive Rosa rugosa Reduces the Species Richness of Yellow Dune Vegetation and Causes a Shift in the Species Composition of Grey Dune Vegetation ^†

This study aimed to determine the impact of invasive Rosa rugosa Thunb. on the plant communities of the Baltic coastal dunes. The study was carried out in 22 sites scattered along the Hel Peninsula (Poland). Each site consisted of a pair of plots: a plot with the invader vegetation and an adjacent plot with the resident vegetation (control plot). For each plot, botanical data (the abundance of individual species, total species richness and the richness of species belonging to different functional groups) were collected and basic soil parameters were measured. According to the analysis of the control plot species composition, the study sites can be divided into two groups, including the plant communities characteristic of the yellow dunes (Elymo-Ammophiletum arenariae association, EA; N = 11 sites) and those characteristic of the grey dunes (Helichryso-Jasionetum litoralis association, HJ; N = 11 sites). R. rugosa influenced these two communities differently. In the EA sites, it displaced many species, leading to a significant decrease in the species richness. In the HJ sites, it caused a shift in the species composition; although it outcompeted some species (e.g., zoochores), it also created conditions for the existence of others (e.g., graminoids, a plant group that, interestingly, was negatively affected by the invader in the EA sites). Changes in the plant communities probably resulted not only from the direct impacts of R. rugosa on the plants (by shading and occupying space) but also from invader-induced soil alternations (an increase in the organic layer thickness and an increase in pH and the content of some nutrients).