Mycorrhizal inoculum and performance of nonmycorrhizal Carex bigelowii and mycorrhizal Trientalis europaea

We studied the competition between mycorrhiza-forming Trientalis europaea L. and nonmycorrhizal Carex bigelowii Torrey ex Schweinitz in a climate chamber experiment. The plants were grown either singly or together with a conspecific or heterospecific individual, with arbuscular mycorrhizal inoculum present or absent. Inoculated T europaea formed abundant arbuscular mycorrhizal structures, but the mycorrhizae did not affect its biomass or the whole plant's relative growth rate (RGR). Carex bigelowii did not form mycorrhizae, but its shoot biomass and RGR were lower in the inoculated pots. The presence of a conspecific or heterospecific plant had no effect on the shoot biomasses or RGR cf either plant species. Mycorrhizal inoculation increased the root/shoot ratio of C. bigelowii in all competition treatment. The presence of C bigelowii decreased the root/shoot ratio of T europaea in both mycorrhizal and nonmycorrhizal state. Mycorrhizal inoculum thus had a direct negative effect on the growth of a nonmycorrhizal plant. The result suggests that although mycorrhizae may not always directly affect the performance of the host plant, they may possibly increase the host plant performance in relation to nonmycorrhizal neighbours. Mycorrhizal inoculum and mycorrhizal symbiosis may increase asymmetry of interspecific competition, which may facilitate the coexistence of plant species in cases when a larger individual is more negatively affected.