Mycorrhizae influence plant community structure and diversity in tallgrassprairie

In grassland ecosystems, symbiotic associations between plants and mycorrhi zal fungi are widespread and have important influences on the life historie s, demography, and species interactions of plants, and on belowground ecosy stem processes. To assess the consequences of the symbiosis at the plant co mmunity 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 mycorrhiza l control plots. Mycorrhizal colonization of roots in control plots was inv ersely related to annual precipitation. Suppression of mycorrhizae resulted in decreases in abundances of the dominant, obligately mycotrophic C-4 tal l grasses, compensatory increases in abundances of many subordinate faculta tively mycotrophic C-3 grasses and forbs, but no change in total abovegroun d biomass, as estimated from canopy density. Suppression of mycorrhizal sym biosis resulted in a large increase in plant species diversity. Two possibl e mechanisms for mycorrhizal mediation of plant species composition and div ersity are: (1) alterations in resource distribution among neighbors via hy phal connections, and (2) differential host species responses to mycorrhiza l 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 effect s on plant community structure, and that differential host species response to fungal colonization is a key factor explaining the dominance of warm-se ason C-4 grasses in tallgrass prairie and limiting plant species evenness a nd diversity. The results also underscore the importance of above- and belo wground linkages in tallgrass prairie and indicate that alterations in belo wground fungi and rhizosphere processes can have large effects on abovegrou nd floristic composition and diversity in grasslands.