Propagule pressure and resource availability determine plant community invasibility in a temperate forest understorey

Abstract

Few field experiments have examined the effects of both resource availability and propagule pressure on plant community invasibility. Two non‐native forest species, a herb and a shrub (Hesperis matronalis and Rhamnus cathartica, respectively), were sown into 60 1‐m2 sub‐plots distributed across three plots. These contained reconstructed native plant communities in a replaced surface soil layer in a North American forest interior. Resource availability and propagule pressure were manipulated as follows: understorey light level (shaded/unshaded), nutrient availability (control/fertilized), and seed pressures of the two non‐native species (control/low/high). Hesperis and Rhamnus cover and the above‐ground biomass of Hesperis were significantly higher in shaded sub‐plots and at greater propagule pressures. Similarly, the above‐ground biomass of Rhamnus was significantly increased with propagule pressure, although this was a function of density. In contrast, of species that seeded into plots from the surrounding forest during the growing season, the non‐native species had significantly greater cover in unshaded sub‐plots. Plants in these unshaded sub‐plots were significantly taller than plants in shaded sub‐plots, suggesting a greater fitness. Total and non‐native species richness varied significantly among plots indicating the importance of fine‐scale dispersal patterns. None of the experimental treatments influenced native species. Since the forest seed bank in our study was colonized primarily by non‐native ruderal species that dominated understorey vegetation, the management of invasions by non‐native species in forest understoreys will have to address factors that influence light levels and dispersal pathways.