Symbiosis facilitates life under novel and stressful conditions. Most plants form root partnerships with mycorrhizal fungi, trading carbon for nutrients, water and pathogen protection. These mutualisms are thought to have aided the earliest plants in colonizing land and remain central to plant performance today. Yet their ecological roles are difficult to disentangle in natural systems, particularly in forests. Trees are long-lived, most forest mycorrhizal fungi are unculturable, and manipulating mycorrhizae impacts other critical forest parameters, making it hard to disentangle how symbiotic interactions shape aboveground biodiversity and functioning.

As a researcher, one can occasionally turn misfortune to advantage. Stinson et al. do just this, demonstrating that invasion by the non-native herb Alliaria petiolata (garlic mustard) suppresses native arbuscular mycorrhizal fungi (AMF), thereby reducing growth of AMF-dependent native plants, especially tree seedlings. By targeting the mutualism rather than the hosts directly, garlic mustard alters forest understory composition and threatens canopy regeneration.