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

Abstract

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 ofRhamnuson four herbaceous species native to southern Wisconsin. In the greenhouse, we assessed howRhamnusleaves and fruit affected seed germination and seedling growth relative to sugar maple (Acer saccharum) leaves. Fewer seeds ofEurybia macrophyllaandThalictrum dasycarpumgerminated underRhamnusleaves, 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 inThalictrum, survival and flowering inEurybia, and flowering and growth inSymphyotrichum 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 (ThalictrumandEurybia) than the common species (Geranium maculatumandSymphyotrichum). 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.