Arbuscular and ectomycorrhizal tree seedling growth is inhibited by competition from neighboring roots and associated fungal hyphae

Tree seedling establishment in herbaceous plant communities can be influenced by belowground interactions via both roots and mycorrhizal fungi. The interplay between these mechanisms may shift according to the mycorrhizal guild of the tree, which could lead to morphological adjustments in the seedling root traits, but little is known in this regard. Here, we evaluated the impact of dense arbuscular mycorrhizal (AM) herbaceous vegetation on the growth and root morphology of AM and ectomycorrhizal (ECM) tree seedlings.

Two parallel experiments were conducted using tubes to restrict passage of roots and/or fungal hyphae, so as to obtain tree seedlings growing either isolated from surrounding vegetation or interacting with neighboring roots and/or their associated fungal hyphae.

The growth of all tree species (AM and ECM) decreased when in contact with neighboring roots and hyphae, indicating competitive/antagonistic interactions. Root mycorrhizal colonization followed the same pattern, with highest colonization rates when seedling grew in isolation. Root morphological traits, however, were largely unresponsive to neighboring roots and/or hyphae.

Our study provides evidence for competitive interaction between tree seedlings and neighboring herbaceous vegetation, via both roots and root-associated fungal hyphae. These results call for a reappraisal of belowground competition as a major component of plant-plant interactions, including those mediated by fungal hyphae, even for plants not sharing the same mycorrhizal guild.