Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie

In tallgrass prairie, plant species interactions regulated by their associa ted mycorrhizal fungi may be important forces that influence species coexis tence and community structure; however, the mechanisms and magnitude of the se interactions remain unknown. The objective of this study was to determin e how interspecific competition, mycorrhizal symbiosis, and their interacti ons influence plant community structure. We conducted a factorial experimen t, which incorporated manipulations of abundance of dominant competitors, A ndropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal s ymbiosis using the fungicide benomyl under two fire regimes (annual and 4-y ear burn intervals). Removal of the two dominant C-4 grass species altered the community structure, increased plant species richness, diversity, and e venness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community s tructure, although plant species richness and diversity increased. Response s of individual plant species were associated with their degree of mycorrhi zal responsiveness: highly mycorrhizal responsive species decreased in abun dance and less mycorrhizal responsive species increased in abundance. The c ombination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, i ndicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that myco rrhizal fungi affect plant communities indirectly by influencing the patter n and strength of plant competitive interactions. Burning strongly influenc ed the outcome of these interactions, which suggests that plant species div ersity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis.