Glossy buckthorn (Frangula alnus), an exotic shrub from Eurasia, colonizes upland and mesic sites in New England, U.S.A., reducing the growth and survival of native tree saplings and lowering species richness. Although a generalist, buckthorn thrives particularly well along river, pond, and wetland margins, which are traditional habitat for speckled alder (Alnus incana ssp. rugosa), a native nitrogen-fixing shrub. Because buckthorn’s dense, monospecific growth is typical of allelopaths such as garlic mustard (Alliaria petiolata), we tested whether buckthorn chemically suppresses the growth or germination of alder and other native shrubs or disrupts Frankia, alder’s N-fixing symbiont. Our four assays were: 1.) germination of alder seeds in extracts of buckthorn and native shrubs, as well as in water 2.) growth of alder seedlings in extracts of buckthorn, native shrubs, and water 3.) growth of indigenous shrubs, including alder, in soil mixed with buckthorn and native root and leaf mulch, or no mulch 4.) growth of Frankia in buckthorn extract and water.
Results/Conclusions

All plant assays revealed negative effects of buckthorn on alder but mixed impacts on other species. The mulch trial pinpointed buckthorn root to be significantly more inhibitory than buckthorn leaf, suggesting that allelopathy occurs primarily through root exudation as opposed to passive leaf drop. This response, combined with buckthorn’s effects on Frankia, indicate primary belowground interactions. The mixed impacts on native species and potential inhibition of N-fixers in this nutrient-poor environment point to chemical suppression as a mechanism by which buckthorn changes the structure of native plant communities.