Shifting interactions between ectomycorrhizae, plants and insect herbivores in a CO2-enriched world

1. Increasing atmospheric CO2 concentrations are changing how plants interact with their biotic mutualists and antagonists, but few syntheses consider how the three-way interactions between mycorrhizae, plants and herbivores will shift under rising CO2. 2. We summarise the mechanisms by which ectomycorrhizal (EcM)-associated plants, their mycorrhizae and insect herbivores interact with each other under current conditions and evaluate a set of expectations for how these interactions might shift under higher CO2. We then outline priorities for future work on EcM-plant-herbivore interactions as atmospheric CO2 continues to rise. 3. EcM colonisation has variable but often positive effects on herbivory, while herbivory has consistently negative impacts on EcM colonisation. Mechanistic evidence suggests that the positive EcM effect on herbivory will strengthen and the negative impact of herbivory on EcM will be ameliorated under higher CO2. 4. Synthesis: While more empirical evidence on fungal-plant-herbivore interactions is needed in EcM systems, our synthesis suggests that EcM associations may play an under-recognised role in dictating future terrestrial carbon capture by mediating herbivory and the ability of plants to compensate for herbivory as atmospheric CO2 continues to rise.