Do mycorrhizal fungi alter plant tolerance to herbivory?

Because arbuscular mycorrhizal fungi (AMF) can facilitate nutrient uptake and increase host plant growth, these symbionts can potentially increase tolerance to herbivory. That is, they may reduce growth depression caused by herbivory.  However, AMF also depend on the host for food, and this demand on the host’s carbon budget could exacerbate the effects of foliar herbivory. Increased tolerance has been demonstrated in some factorial experiments in which AMF colonization and herbivory were manipulated, but so have other outcomes.  To determine whether there is a general pattern of altered tolerance to herbivory in mycorrhizal plants I conducted meta-analysis of results from studies involving foliar herbivory. As a main effect, herbivory strongly and consistently reduced plant growth.  AMF increased plant growth, but the magnitude and even direction of the effect varied as a function of the type of herbivory (artificial vs. insect) and plant characteristics (e.g., grass vs. legume vs. forb).  While the interaction of AMF and herbivory was generally positive, suggesting enhanced tolerance, this effect was usually very small, and never significant.  Consequently this analysis does not support the hypothesis that AMF alters plant tolerance to foliar herbivory.  Due to a small data set, the patterns are not robust.  Nonetheless, they suggest avenues for future research.