Arbuscular mycorrhizae alter intraspecific competition

By taking advantage of facultative mycotrophs' ability to grow with or without mycorrhizae at moderate soil nutrient levels, I investigated the role arbuscular mycorrhizae play in the growth, survivorship and size inequality development of even-aged monocultures (i.e., populations) of eight plant species grown over a range of densities.; Mycorrhizae influence plant response to density in several ways. For Sunflower grown at very high density (160 plants/100 cm{dollar}sp2{dollar}), mycorrhizae reduced shoot biomass, but improved survival by delaying self-thinning. When grown at low density (23 plants/100 cm{dollar}sp2{dollar}) in a regular planting pattern, mycorrhizae retarded the growth of individuals in the large-size classes as compared to the large-size classes of controls grown without mycorrhizae. While hyphae may aid in the uptake of nutrients at a greater rate than control individuals of the same size, this uptake comes at the cost of photosynthate. This photosynthate cost decreases the overall competitive ability of large-sized individuals, and so results in reduced growth of individuals of large size. The net result is greater size uniformity in mycorrhizal populations than control populations.; Species respond to mycorrhizae and density in a predictable fashion as shown by the six response types that I have recognized. Response type I plants exhibit a mycorrhiza growth advantage at low density going to mycorrhiza growth disadvantage at high density. Response type II plants exhibit a mycorrhiza growth disadvantage at low density going to mycorrhiza growth advantage at high density. Response type III plants exhibit a consistent mycorrhiza advantage over the densities examined. Response type IV plants exhibit a consistent mycorrhiza disadvantage over the densities examined. Response type V plants exhibit no affect of mycorrhiza. Response type VI plants do not exhibit an affect of density. Response types demonstrate that the outcome of mycorrhiza and density interactions is dependent on the plant host-fungus-soil combination used.; Mycorrhizal impact on size inequality appears to be a function of size. Slopes of regression lines for mean shoot mass indicate that at high mean shoot mass, mycorrhizal populations exhibit smaller coefficients of variation and less skewing than for the control populations.; Although species may respond to density and mycorrhizae with any of several response types, there seems to be a common, possibly allometrically-based plant size inequality response to mycorrhizae.