The invasive plant <i>Alliaria petiolata</i> (garlic mustard) inhibits ectomycorrhizal fungi in its introduced range

Summary Ectomycorrhizal (EM) fungi play key roles in forest ecosystems, but the potential effects of invasive plants on EM fungal communities have not been assessed. In this study, we tested whether the non‐mycorrhizal herbaceous plant Alliaria petiolata (garlic mustard) can alter the abundance of EM fungal communities in North America. In three forests in New England, USA, we compared EM root tip abundance in soils where A. petiolata had invaded to adjacent areas without a history of A. petiolata invasion. At one site, we also intensively sampled EM root tip abundance across the edges of A. petiolata patches to determine the spatial pattern of A. petiolata effects on EM fungi. In a glasshouse experiment, we experimentally invaded soils with A. petiolata and Impatiens capensis , a native species and compared EM fungal colonization of white pine ( Pinus strobus ) seedlings grown in both soils. We also measured the effect of the A. petiolata allelochemical benzyl isothiocyanate on the growth of three species of EM fungi in pure culture. In the field, EM fungal root tip biomass was lower in invaded soils, with the strongest reductions observed in forests dominated by conifers. Alliaria petiolata invasion did not have a significant effect on total root biomass. The influence of A. petiolata on EM fungal abundance in the field was localized, with the strongest inhibition observed within 10 cm of the edge of A. petiolata patches. Pine seedlings growing in soils that were experimentally invaded with A. petiolata also had lower EM fungal root tip biomass compared to uninvaded soils. The native species I. capensis caused similar reductions in EM fungal colonization. Growth of pure cultures of all three species of EM fungi was completely inhibited by benzyl isothiocyanate. Synthesis. Alliaria petiolata inhibits the growth of EM fungi in forests of its introduced range. Changes in EM fungal communities caused by the invasion of A. petiolata may influence tree seedling establishment and biogeochemical cycling.