Mycorrhiza at the Cutting Edge: Trees at the Edges of Their Distribution Host Diverse and Distinct AMF Communities

Abiotic conditions deteriorate predictably at the edges of plant distribution ranges. Any adverse environmental settings could interfere with the functioning of nutritional mutualisms, such as arbuscular mycorrhiza. Documenting how transitioning from the kernel of a plant distribution towards the edges changes how mycorrhiza functions remains nonetheless underexplored. We questioned here whether arbuscular mycorrhizal fungal (AMF) richness and AMF community turnover decline towards the edges of the distribution of three AMF-associating tree species, Artocarpus hypargyreus, Machilus breviflora and Xanthophyllum hainanense. We assayed roots from 62 tree individuals over a representative fraction of their distribution range and used molecular tools to assay AMF. Contrary to our expectations, we observed increases in AMF richness across marginal populations of those tree species and minimal differences in community turnover. We observed many AMF indicator species of marginal populations of the three hosts, which nonetheless all represented rare AMF taxa. Remarkably, the distance from the kernel of the distribution explained an independent fraction of AMF community variance compared to abiotic parameters and host selectivity. We uncover a novel axis of AMF community variation, the relative distance between the kernel and the edges of the distribution for the plant host. Exploring the significance of this new axis could bring new insights into the functioning of arbuscular mycorrhizal fungi by addressing questions related to how mycorrhizal plants handle stress and what drives host selectivity in the arbuscular mycorrhiza.