| Understanding how biotic and abiotic contexts modify the strength of species interactions is a key goal in ecology. Mutualism effectiveness is particularly sensitive to environmental conditions, and the invasion of non-native species is hypothesized to be one biotic factor that can drive mutualism disruption between native species and their partners. Using an ecophysiological approach, I tested this mutualism disruption hypothesis using the allelopathic invasive plant, Alliaria petiolata, and examined its impacts on the mutualism between symbiotic arbuscular mycorrhizal fungi (AMF) and Maianthemum racemosum, a common native herb in North America. To establish the potential for mutualism disruption in this system, I measured field concentrations of A. petiolata's allelochemicals and tested the toxicity of these levels on AMF spore germination in a bioassay. I found that field-detected levels of allyl isothiocyanate, a key component of A. petiolata's allelochemical profile, reduced spore germination by over 50% relative to controls. Additionally, by assessing fungal abundance in the field, I found that sites invaded by A. petiolata generally have reduced fungal hyphal lengths compared to uninvaded sites. In a separate common garden study, I demonstrated that A. petiolata allelochemicals significantly reduced soil respiration rates around M. racemosum plants, indicating active disruption of AMF associated with the plant roots. To investigate the impacts of mutualism disruption by A. petiolata on M. racemosum, I used a combination of field and greenhouse studies. First, in a short, 2-week field study, I found that A. petiolata allelochemicals reduced physiological function and carbon acquisition in M. racemosum. Second, data from a season-long greenhouse study demonstrated that the physiological declines induced by A. petiolata allelochemicals were persistent and translated into reductions in allocation to key traits, including carbohydrate storage, root growth, and asexual reproduction. Together, these studies indicate that A. petiolata allelochemicals disrupted AMF function, resulting in water stress and altered source-sink dynamics for the native plant, and drove declines in both physiology and allocation to competing functions. Overall, my results suggest that allelopathic invasion is one critical, yet underexplored, biotic context that can dictate the outcome of plant-AMF mutualisms. |
