Despite decades of research on biological invasions, ecologists have not generally explained the “paradox” that occurs when a species reaches extraordinarily high densities where it is exotic, while remaining at lower densities in its home range where it is presumably best adapted. Increasing evidence points to biotic mechanisms in the proliferation of exotics, and recently, biogeographic differences in plant-microbial interactions have been invoked to explain the invasion of many plant species. Recent research also indicates that plant invasions have major impacts on microbial community assembly, leading to novel feedbacks between a reorganizing suite of species and the environment. At Harvard Forest, research is focused the suppression of soil fungi in Northeastern forests by secondary plant metabolites from invasive plants, with resulting implications for the growth of key native canopy tree species and plant community structure. A series of comparative studies, molecular analyses, and experiments are addressing microbial responses to invasive plant species and subsequent consequences for forest community structure.
Our results to date focus on the invasive plant, Alliaria petiolata (garlic mustard). This species exudes phytochemicals that are toxic to fungi and other microbes in forest soils. Our research indicates that A. petiolata’s presence corresponds with severe inhibition of growth and fungal colonization of native tree seedlings and their arbuscular and ectomycorrhizae mutualists. The impacts on the growth and performance of North American plants that rely on these microbes is profound, whereas congeneric European plants are only weakly affected. These findings indicate that antifungal phytochemicals, benign to mycorrhizal symbionts in the home range, impose a novel threat to native North American plant species. New and ongoing work further investigates the generality of A. petiolata's as well as other invasive species' phytotoxicity to native microbes. Microbial response includes community level shifts that vary across space and time. The decline of mycorrhizal fungi due to garlic mustard invasion not only affects tree seedling establishment and plant community structure, but also has the potential to alter bio geochemical cycling in forest soils. Thus the role of invasive plant suppression of forest soil microbes may extend to atmosphere-biosphere feedbacks accompanying global change.