EFFECTS OF MYCORRHIZAE, PHOSPHORUS AVAILABILITY, AND PLANT-DENSITY ONYIELD RELATIONSHIPS AMONG COMPETING TALLGRASS PRAIRIE GRASSES

A replacement series experiment was used to investigate the effects of mycorrhizae, phosphorus availability, and plant density on competitiv e relationships between three tallgrass prairie species of varying myc orrhizal dependencies. Under mycorrhizal conditions, the obligately my corrhizal dependent warm-season grass Andropogon gerardii (big blueste m) was a better competitor in mixture with the nonmycorrhiza-dependent cool-season grass Koeleria pyramidata (Junegrass). In the absence of mycorrhizae, however, competitive effects of big bluestem were greatly reduced and Junegrass experienced competitive release. Relative yield totals increased when mycorrhizae were suppressed, suggesting greater intensity of interspecific competition in the presence of mycorrhizae . Thus, the competitive dominance of big bluestem in tallgrass prairie is strongly related to its mycorrhizal status. Elymus canadensis (Can ada wild rye) outcompeted big bluestem both with and without mycorrhiz ae. Relative yield totals of this species mixture were also lower unde r mycorrhizal conditions, indicating that mycorrhizae increase the int ensity of interspecific competition between them. Relative yields of w ild rye competing with big bluestem increased in the absence of mycorr hizae, suggesting that it also experiences competitive release when bi g bluestem are not mycorrhizal. The outcomes of competition were gener ally similar among the three total plant density treatments and betwee n P-fertilized and nonfertilized treatments. However, interactions bet ween mycorrhizal effects and plant density confirm that outcomes of in terspecific competitive interactions may be density dependent in some cases.