Responsiveness of plants to mycorrhiza regulates coexistence

Abstract

Abstract Association of plants with arbuscular mycorrhizal (AM) fungi may act as either an equalizing or a stabilizing mechanism of coexistence, but the effect of the symbiosis likely depends on the responsiveness to fungi of plant species within a community, rather than simply as a binary metric based on presence or absence of AM fungi. Here, we test the hypothesis that communities consisting of plants that are unresponsive to mycorrhiza, or that differ widely in their response to mycorrhiza, would exhibit larger differences in their niche and fitness resulting to weaker plant–plant competition. We took two complementary approaches to test our hypotheses: first, we reanalysed data from the classic “BIODEPTH” grassland biodiversity manipulation experiment to assess niche and relative fitness differences of plant communities solely comprising species considered to be either obligate or facultative symbiotrophs. Second, we assembled replicate synthetic plant communities comprising all 70 possible four‐species mixtures from a pool of eight species in the presence of an AM fungus to assess pairwise competition coefficients and infer coexistence potential. The species were characteristic of pastures, had an identical CSR survival strategy, and in nature differ in their extent of colonization by AM fungi. We separately established monocultures to estimate competition coefficients and assess plant responsiveness to mycorrhiza. Analysis of data from BIODEPTH showed that plant communities comprising solely species with obligate associations with AM fungi had greater niche and fitness differences compared with communities comprising species without obligate associations with AM fungi. In the manipulation experiment, we found that plants that depended on AM fungi competed more strongly than plants with little dependence on AM fungi. The weakest competition was between AM‐responsive and AM‐unresponsive plant species. Coexistence was more likely in pairwise combinations in which at least one of the plant species responded negatively to mycorrhiza. Synthesis. We directly quantified variation in niche and fitness differences between plant species due to interaction with AM fungi. Integrating this knowledge into explicit competition models in plant community ecology could greatly enhance our ability to predict coexistence in nature.