Above‐ and Belowground Impacts of European Buckthorn (Rhamnus cathartica) on Four Native Forbs

Fast‐growing European buckthorn (Rhamnus cathartica) shrubs are aggressively invading woodlands in eastern and midwestern North America. Buckthorn casts dense shade, alters soil conditions, and may be allelopathic. We used greenhouse and field experiments to investigate above‐ and belowground effects of Rhamnus on four herbaceous species native to southern Wisconsin. In the greenhouse, we assessed how Rhamnus leaves and fruit affected seed germination and seedling growth relative to sugar maple (Acer saccharum) leaves. Fewer seeds of Eurybia macrophylla and Thalictrum dasycarpum germinated under Rhamnus leaves, and those that germinated were slowed. We planted seedlings of the four natives into four treatments at three sites in the field to assess how their survival, growth, and flowering respond to the separate and combined effects of light levels, buckthorn canopies, and buckthorn soils. Buckthorn consistently reduced native plant performance by diminishing survival, flowering, and growth in Thalictrum, survival and flowering in Eurybia, and flowering and growth in Symphyotrichum lateriflorum. Removing buckthorn canopies enhanced growth of these native species, but buckthorn soils separately inhibited growth at least as much (despite being more fertile). Buckthorn's impacts on native plants exceed effects attributable to light levels and soil fertility, suggesting allelopathy. Buckthorn reduced performance more in the uncommon species (Thalictrum and Eurybia) than the common species (Geranium maculatum and Symphyotrichum). As we do not yet know how long these inhibitory soil effects persist, we need additional research to learn how best to control buckthorn's impacts on native plant communities.