Contrasted root trait responses between saplings of an arbuscular and an ectomycorrhizal tree species in open field compared to forest conditions

Abstract

Plant nutrient acquisition strategies range along a spectrum from autonomous foraging to investment in cooperative foraging through mycorrhizal associations. However, in temperate ecosystems, many plant species encounter contrasted levels of symbiont availability in open fields versus closed forests. Little is known about how fungal partner availability may be associated with intraspecific variation in other root foraging traits in natural settings. Here, we addressed this issue by sampling saplings from two tree species: the arbuscular mycorrhizal (AM) Acer rubrum and the ectomycorrhizal (ECM) Quercus rubra from open fields (AM dominated) and adjacent forest plots (ECM dominated). For each species and environment, we measured morphological, architectural and symbiotic root traits. For the open field, Quercus had greater specific root length (SRL) while Acer had higher AM colonization and root diameter. In the closed forest, the opposite pattern was observed, namely Quercus had higher ECM colonization and Acer greater SRL. Both species showed evidence of a shift towards autonomous root foraging in the habitat with low expected symbiont abundance (open field for Quercus and forest for Acer). Although the confounding effects of site abiotic properties could not be strictly controlled in this study, these results suggest that plants might adjust root foraging traits according to local habitat conditions. Synthesis: Our results shed new light on the intraspecific variation in plant position along the so-called ‘collaboration gradient’, and suggest that mycorrhizal symbiont availability, along with other factors such as competition and site properties, may contribute to this variation.