Processes regulating the invasion of European buckthorn (Rhamnus cathartica) in three habitats of the northeastern United States

Invasion by exotic plants often is restricted by processes, such as seed predation, acting on early life-history stages; however, the relative importance of these processes might vary among habitats. Modern human land use has created a mosaic of habitats in many landscapes, including the landscape of the northeastern United States. European buckthorn (Rhamnus cathartica) is an exotic plant that has achieved varying success in North American habitats. We studied dispersal, seed survival, germination, and seedling survival of buckthorn populations at four plots in each of sugar maple (Acer saccharum) forests, old fields, and abandoned conifer plantations in central New York State. Dispersal was low in maple forests, as evidenced by low collection rates of R. cathartica seeds in seed traps. Rates of post-dispersal seed predation were highest in maple forests and lowest in old fields, suggesting greater use of maple forests by granivorous rodents. Germination rates did not vary among seeds planted in soils of these habitats when studied in the laboratory despite differences in soil pH. Survival of transplanted seedlings was low in maple forests relative to old fields and plantations. Buckthorn invasion of old fields and abandoned plantations was not strongly constrained by factors we considered, and the buckthorn populations in these habitats were large. A combination of low dispersal by frugivores, low seed survival due to predation, and low seedling survival due to dim light conditions apparently prevents R. cathartica from invasion of intact maple forests of our area. Native fauna and canopy closure may act synergistically to reduce success of invasive plants in natural habitats.