Species interactions are often categorized as strictly positive or negative despite evidence that environmental factors can shift relationships along a continuum from mutualism to parasitism.  For example, plants and arbuscular mycorrhizal fungi (AMF) are commonly cited as strict mutualists.  Plants benefit from AMF by receiving soil nutrients, but in return they must provide the AMF with carbon.  Indeed, many temperate forest understory herbs rely on this mutualistic exchange of resources for survival and reproduction.  However, conflict may arise between the plant and AMF when one organism experiences an increase in costs or a decrease in benefits.  The recent invasion of forests in eastern North America by Alliaria petiolata (garlic mustard, hereafter GM) could alter the conditions favoring native plant-AMF mutualisms.  GM produces allelochemicals that can kill AMF’s nutrient-mining hyphae, yet leave the AMF’s internal carbon-absorbing arbuscules intact.  We hypothesized that exposure to GM allelochemicals diminishes AMF nutrient uptake and causes plant-AMF conflict that is detectable as decreased plant physiological rates.  To test our hypothesis, we conducted a field experiment in which one Maianthemum racemosum plant from each of nine pairs received a treatment of fresh GM litter – the paired plant was a control. 

Results/Conclusions

Prior to treatment, leaf gas exchange rates did not differ within the pairs.  Two weeks after treatment, GM treated plants showed a significant decline in leaf gas exchange (MANOVA on all 3 response variables, p<0.05).  Stomatal conductance decreased by 36% in treated plants compared to controls with similar decreases in photosynthetic and transpiration rates (ANCOVA p=0.003; p=0.05; p=0.01, respectively).  Interestingly, we found no difference in the number of AMF internal structures between the two treatments after the two week treatment period (paired t-test, p=0.81).  These results, together with our phospholipid fatty acid analysis of AMF hyphal biomass in the soil after treatment, indicate that GM can disrupt native plant-AMF mutualisms, apparently by affecting AMF uptake of soil resources without reducing the size of the AMF carbon sink.  We suggest that the creation of conflict within native plant-AMF mutualisms is an important indirect effect of allelopathic invaders that may allow them to succeed in novel habitats.