Rethinking ectomycorrhizal succession: Are root density and hyphal exploration types drivers of spatial and temporal zonation?

Ectomycorrhizal exploration types have become an increasingly popular functional explanation for observed patterns of fungal community structure. In this study, we examined the relationship between exploration types of ectomycorrhizal fungi and root density. We did so by sampling across a root density gradient formed by the edge-interior transition on 'tree islands', patches of ectomycorrhizal forest in a non-ectomycorrhizal vegetation matrix. We found evidence that long-distance exploration types were more prevalent in areas of low root density while short-distance exploration types were more common in areas of high root density. Gradients in root density are common in ectomycorrhizal forests and change predictably at forest edges, within a soil profile, or over early succession. Based on these results, we propose a general model using the concept of exploration types that could explain some of the spatial or temporal patterns commonly observed in ectomycorrhizal assemblages.