Tree species mixing causes a shift in fine-root soil exploitation strategies across European forests

Abstract

Mixed-species forests have often been shown to enhance above-ground ecosystem properties and processes. Despite the significance of fine roots for tree and ecosystem functioning, the role of tree species diversity for below-ground processes driven by fine roots remains largely unknown. Previously, an underyielding of fine-root biomass (FRB) in tree mixtures across four major European forest types has been reported. To explain this phenomenon, we tested here the effect of tree species mixing on fine-root traits related to soil exploitation efficiency, including biotic feedbacks from ectomycorrhizal fungi (EcM), and assessed the role of root trait dissimilarity. We analysed morphological and chemical traits as well as ectomycorrhizal colonisation intensity of absorptive fine roots (i.e. first three most distal orders) in soil samples from 315 mixed and mono-specific tree neighbourhoods in mainly mature, semi-natural forest stands across Europe. Additionally, we quantified mycorrhizal abundance and diversity in soil samples from the same stands. At the community level, fine roots in tree mixtures were characterised by higher specific root lengths and root nitrogen concentrations, lower diameters and root tissue densities indicating a faster resource acquisition strategy compared to mono-specific stands. The higher root EcM colonisation intensity and soil EcM diversity in mixtures compared to mono-specific stands may further provide evidence for positive biotic feedbacks. Moreover, the diversity of fine-root traits influenced FRB, as mixtures characterised by a higher trait dissimilarity were linked to a lower reduction in FRB. At the level of phylogenetic groups, thin-rooted angiosperm species showed stronger responses to mixing than thick-rooted gymnosperms, especially in terms of root morphology and EcM colonisation, indicating different strategies of response to tree mixing. Our results indicate that a lower FRB can reflect a shift in soil resource acquisition strategies, rather than a lower performance of trees in mixtures. They show that several non-exclusive mechanisms can simultaneously explain negative net effects of mixing on FRB. This study sheds new light on the importance of using integrative approaches including both above- and below-ground biomass and traits to study diversity effects on plant productivity. A free Plain Language Summary can be found within the Supporting Information of this article.