Mycorrhizae influence plant community structure and diversity in tallgrass prairie

In grassland ecosystems, symbiotic associations between plants and mycorrhizal fungi are widespread and have important influences on the life histories, demography, and species interactions of plants, and on belowground ecosystem processes. To assess the consequences of the symbiosis at the plant community level, we conducted a 5-yr field experiment in tallgrass prairie to investigate the influence of arbuscular mycorrhizal fungi on plant species composition, relative abundances, and diversity. Replicate plots in which mycorrhizal fungi were suppressed with benomyl application every two weeks during each growing season, were compared to nontreated mycorrhizal control plots on six watershed units at the Konza Prairie in northeastern Kansas. Benomyl successfully reduced mycorrhizal colonization to <25% of mycorrhizal control plots. Mycorrhizal colonization of roots in control plots was inversely related to annual precipitation. Suppression of mycorrhizae resulted in decreases in abundances of the dominant, obligately mycotrophic C-4 tall grasses, compensatory increases in abundances of many subordinate facultatively mycotrophic C-3 grasses and forbs, but no change in total aboveground biomass, as estimated from canopy density. Suppression of mycorrhizal symbiosis resulted in a large increase in plant species diversity. Two possible mechanisms for mycorrhizal mediation of plant species composition and diversity are: (1) alterations in resource distribution among neighbors via hyphal connections, and (2) differential host species responses to mycorrhizal fungal colonization in communities in which the competitive dominants are more strongly or more weakly mycotrophic than their neighbors. The results of this study demonstrate that mycorrhizal symbiosis can have large effects on plant community structure, and that differential host species response to fungal colonization is a key factor explaining the dominance of warm-season C-4 grasses in tallgrass prairie and limiting plant species evenness and diversity. The results also underscore the importance of above- and belowground linkages in tallgrass prairie and indicate that alterations in belowground fungi and rhizosphere processes can have large effects on aboveground floristic composition and diversity in grasslands.