Does resource transfer through mycorrhizal networks mediate plant-plant interactions?

Ian R. Mounts, Biology, University of Mississippi, University, MS and Jason D. Hoeksema, Department of Biology, University of Mississippi, University, MS

Background/Question/Methods
The observation that species of mycorrhizal fungi are often compatible with multiple host plant species, coupled with the ability of genetically compatible hyphae to anastomose, suggest that common mycorrhizal networks (CMNs) linking plants of the same and different species are likely ubiquitous in terrestrial ecosystems. What is less clear is the degree to which these CMN linkages have unique consequences for the population and community ecology of typical green plants, or whether mechanisms and consequences of plant-plant interactions are commonly altered by these linkages. Are physical mycelial linkages between plants per se, beyond just the presence of mycorrhizal fungi, generally important for plant performance, plant-plant interactions, or plant community dynamics? Or, alternatively, can apparent influences of CMNs be explained by more general theoretical frameworks of population dynamics, especially plant-soil feedbacks? Much of this debate has previously focused on whether consequential amounts of carbon (C) flow from one plant to another through CMNs. Instead, we summarized the evidence for flows of nitrogen (N), phosphorus (P), water, and potential signaling molecules through CMNs, and reviewed results from published field and laboratory experiments in which CMNs were manipulated experimentally.
Results/Conclusions
Previous studies demonstrate that flow of N, P, water, or signaling molecules through CMNs can be substantial, in amounts that likely affect performance of recipient plants. These resource flows are not captured by population dynamics, as in the plant-soil feedback framework, since plant-plant facilitation via this mechanism does not require density responses of the fungi. Rather, these results suggest the potential for a unique influence of CMNs per se on plant-plant interactions. Relatively few experiments have unequivocally tested whether CMNs have unique effects on plant ecology (e.g., plant-plant interactions); however, we do now have multiple examples from both the field (mainly in ectomycorrhizal seedlings) and laboratory (mainly with arbuscular mycorrhizal herbs) demonstrating such effects. Unfortunately, most of these experiments are not accompanied by data on resource transfers, fungal densities, or fungal community composition that would allow a clear determination of the mechanism. A small number of examples do support the idea that CMNs alter plant-plant interactions via plant-to-plant resource flows, and thus cannot be explained by the plant-soil feedback framework. However, substantial progress on this debate will only be made when we have many more tests of alternative mechanisms besides plant-to-plant C flow, especially coupled with experimental manipulations of CMNs to test for consequences on specific aspects of plant community ecological processes.