Although mycorrhizal fungi are often cited as textbook examples of mutualism, there is tremendous variation in plant response to arbuscular mycorrhizal fungi. Theory predicts that resource ‘mutualists’ may only provide fitness benefits to plants at low resource levels or at high resource loss rates. Furthermore, we know little about how mycorrhizal fungi alter herbivore resistance and tolerance in the field. We tested the hypothesis that mycorrhizal fungi mediate insect herbivory by increasing resource availability and promoting allocation to resistance and tolerance. We use this framework to help explain the dominance of Solidago canadensis in old-fields across the northeastern US. Specifically, we ask whether mycorrhizal fungi substantially increase herbivore resistance/tolerance of S. canadensis relative to other plant species. In replicated in situ monocultures (4x4 m) of 8 old-field species, we factorially manipulated insect herbivory and mycorrhizal fungi and monitored herbivory, growth rates and reproduction in four goldenrod, three aster and one grass species.
We found very species-specific responses to both herbivory and mycorrhizal fungi. Many plant species had weak to no significant response to herbivores or mycorrhizal associations, but had low herbivore loads. The dominant species, S. canadensis, had the greatest herbivore load and herbivore exclusion significantly increased growth rate and reproduction. Mycorrhizal individuals grew larger and had a higher probability of flowering when exposed to herbivory, relative to non-mycorrhizal plants. In contrast to our predictions, some herbivores increased rates of attack on mycorrhizal plants, potentially detecting higher nutrient concentrations. Although the dominant species suffered decreased resistance when associating with mycorrhizal fungi, it gained the greatest tolerance, potentially tempering the top-down pressures of over 40 insect herbivores. Our findings confirm that interactions between plants and mycorrhizal fungi can be context dependent, and mycorrhizal dependency can increase with herbivory.