MYCORRHIZAL INFLUENCE ON INTRASPECIFIC AND INTERSPECIFIC NEIGHBOR INTERACTIONS AMONG COOCCURRING PRAIRIE GRASSES

1 A strongly obligately mycorrhiza-dependent grass, Andropogon gerardi i, and a less dependent species, Elymus canadensis, were grown in intr a- and interspecific combination in a target-neighbour experiment with and without mycorrhizal fungi to examine their influence on competiti on. 2 Mycorrhizal fungi significantly influenced the competitive effec ts and responses of both plant species. Strong competitive effects of Andropogon disappeared in the absence of mycorrhizas indicating that i ts competitive dominance in tallgrass prairie is highly dependent upon its mycorrhizal associations. The influence of mycorrhizal fungi on A ndropogon responses to neighbours decreased with increasing neighbour density indicating reduced host plant benefit from mycorrhizas under c rowded conditions. 3 Effects of mycorrhizas on competition were genera lly smaller for the less mycorrhiza-dependent Elymus. Elymus effects o n target plants were not strongly affected by mycorrhizas. Elymus targ et plants in competition with Andropogon neighbours performed better w hen nonmycorrhizal, due to the lack of significant competitive suppres sion by Andropogon in the absence of mycorrhizas. The influence of myc orrhizal fungi on Elymus responses to Andropogon neighbours increased with increasing neighbour density. Neither mycorrhizas nor phosphorus fertilization had a significant effect on intraspecific competition am ong Elymus. 4 Patterns of tiller production by target plants were simi lar to patterns in their total dry weight, indicating that competitive and mycorrhizal effects on target plant size were primarily a result of effects on tiller numbers rather than individual tiller size. 5 The results show that mycorrhizal symbiosis can strongly influence the pa tterns and intensity of both intraspecific density effects and intersp ecific competition between co-occurring prairie grasses and that the d egree of host-plant benefit derived from mycorrhizas is density depend ent.