Arbuscular mycorrhizal fungi associate with roots of the majority of land plants and supply up to 80% and 25% of their P and N requirements, respectively. These fungi do not form on individual plants in isolation, the way they often have been investigated. Instead, arbuscular mycorrhizas form when hyphae connected to one root system branch throughout the soil while foraging for mineral nutrients, encounter, and colonize the roots of another plant, forming common mycorrhizal networks (CMNs). The importance of CMNs is their influence on the distribution of limiting mineral nutrients among plants, affecting the symmetry of competition among interconnected individuals. It is likely that most research on plant interactions may have incorporated CMNs unwittingly, but until recently, few investigators have attempted to distinguish the effects of CMNs. In a factorial experiment, we investigated if CMNs affect intraspecific and interspecific belowground interactions between populations of Andropogon gerardii, a highly mycorrhiza-dependent, dominant prairie grass and Elymus canadensis, a weakly dependent, subordinate prairie species. We tested the following hypotheses: 1) CMNs amplify intraspecific competition for both species, resulting in skewed size hierarchies and 2) CMNs contribute to an increased competitive ability of A. gerardii over E. canadensis in mixture.
Results/Conclusions
We found CMNs may have different roles in plant performance depending upon species’ physiological requirements. CMNs improved growth and survival of both A. gerardii and E. canadensis overall, but intensified intraspecific competition for A. gerardii. When grown in mixture with E. canadensis, A. gerardii overyielded total aboveground biomass, suggesting CMNs contributed to stronger intraspecific than interspecific interactions for this species. We found that severing CMNs increased mortality in E. canadensis populations, and this was likely caused by a reduced ability in water uptake. Our findings suggest that it is not just the presence of AM fungi which has implications for grasslands, but it is essentially the interconnecting hyphae of CMNs that mediate plant interactions and consequently affect population structure and community composition. This work begins to address if an increased competitive ability of dominant species may have resulted in selection to be highly dependent on arbuscular mycorrhizal fungi.