Root diameter predicts the extramatrical hyphal exploration distance of the ectomycorrhizal fungal community

Locally coexisting tree species of temperate forests often vary widely in the diameter of their absorptive roots, resulting in contrasting strategies of root foraging within soil nutrient hot spots. We hypothesized that root diameter would also influence the extramatrical hyphal exploration distance of the mycorrhizal fungal community due to coevolution of the plant and fungal partners leading to functional complementarity. We collected absorptive roots from mature trees of nine ectomycorrhizal tree species in replicated monoculture plots of a plantation where the soil is relatively homogeneous. The identities of ectomycorrhizal fungal taxa and their relative abundances were determined by DNA sequencing. The hyphal exploration type (i.e., contact, short-distance, medium-distance or long-distance) was assigned to each taxon, allowing us to calculate an index of the abundance-weighted mean extramatrical hyphal exploration distance of the fungal community. Overall, there was a significant, positive correlation between root diameter and our index of hyphal exploration distance such that tree species with the thicker roots were associated with fungi with the longer hyphal exploration distance. Moreover, we found that root diameter negatively correlated with the proportion of contact ectomycorrhizal fungal taxa and positively correlated with the proportion of medium-distance ectomycorrhizal fungal taxa. Our results suggest that among temperate ectomycorrhizal tree species, root diameter selects for communities of mycorrhizal fungi of specific exploration distance in a way that is consistent with root-fungal functional complementarity in nutrient foraging.