Arbuscular mycorrhizal fungi and water table affect wetland plant community composition

1 Most studies of the community-level effects of arbuscular mycorrhizal fungi (AMF) have been conducted in upland grassland plant communities where a majority of the plant species are colonized by AMF. Here, we examine the effects of AMF on plant community composition in experimental wetland plant communities, where the dominant plant species are non-mycorrhizal and subordinate plant species are colonized by AMF. We also assess how an important abiotic soil variable, depth to water table (soil saturation), might mediate the community-level effects of AMF. 2 In the low water table (un-saturated) treatment, above-ground plant biomass increased in the presence of AMF relative to the controls, while in the high water table treatment, biomass decreased with the presence of AMF. Contrary to predictions, plant diversity was unaffected by the presence of AMF in the low water table treatment, but significantly decreased in the presence of AMF in the high water table treatment. Changes in biomass and composition were driven by the interactions between the dominant non-mycorrhizal species Carex hystercina, and the remaining mycorrhizal plant species. 3 Our results indicate that AMF have the potential to influence plant community composition in calcareous fens and that these effects can be mediated by soil saturation. 4 This study has implications for understanding how established principles of above-ground/below-ground interactions from upland communities translate to wetland plant communities and for understanding how AMF function can be mediated by abiotic soil properties. Contrary to previous thought, AMF may be important drivers of plant community composition in wetland plant communities.