Aboveground-belowground connections: Interactions among plants, insects and mycorrhizal fungi

Background/Question/Methods Mycorrhizal fungi and insects are both known to influence individual plants, plant communities and ecosystems, yet we still know comparatively little about their interactions with one another as they co-occur on host plants. In this presentation, we highlight recent studies demonstrating that consideration of functional variation at a variety of scales (genotype, species, community) is critical to understanding how plants, mycorrhizal fungi and insects interact to influence the fitness of their host plants and each other.  We also describe the results of long-term research, including insect removal experiments, on the relationships between two species of herbivorous insects and the ectomycorrhizal fungi (EMF) associated with their common host plant, pinyon pine, (Pinus edulis), to assess the importance of plant genotype and drought stress to the dynamics of plant-insect-mycorrhizal fungal interactions.  Results/Conclusions Recent studies demonstrate that interactions among plants, insects and mycorrhizal fungi are strongly dependent upon the species and genotype of the herbivores, plants and fungi involved.  Furthermore, interactions observed when insects, plants and mycorrhizal fungi occur in isolation may not be representative of their interactions in a complex community of organisms, both above- and belowground.  Our work with pinyon pine emphasizes these complexities.  The EMF communities of juvenile pinyons genetically resistant versus susceptible to a needle-feeding scale insect differ significantly from one another, and these shifts are predictably related to scale-induced foliage loss from trees during a year of average precipitation.  However, during drought, the EMF community of scale resistant and susceptible trees converges and there is no significant relationship with foliage loss.  In contrast, although the EMF communities of mature trees genetically resistant versus susceptible to a stem-boring moth also differ dramatically from one another, this difference is not explained by extent of moth damage in either a wet or dry year.  Instead, moth removal and reciprocal transplant experiments demonstrate that genetically-based resistance traits to the moth, an important aboveground herbivore, are strongly associated with the community composition of EMF, an important group of belowground mutualists.  Moth resistance and susceptibility are also associated with drought tolerance, suggesting unexpected but important linkages between resistance to insect herbivory, soil fungal community composition and response to climate change.