1. 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.
2. 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.
3. 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.
4. 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.

中文翻译：

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树种混合导致欧洲森林细根土壤开发策略的转变

1. 混交林经常被证明可以增强地上生态系统的特性和过程。尽管细根对树木和生态系统功能具有重要意义，但树种多样性在细根驱动的地下过程中的作用仍然未知。此前，据报道，欧洲四种主要森林类型的树木混合物中细根生物量 (FRB) 产量不足。为了解释这一现象，我们在这里测试了树种混合对与土壤开发效率相关的细根性状的影响，包括来自外生菌根真菌 (EcM) 的生物反馈，并评估了根性状差异的作用。
2. 我们分析了来自欧洲主要成熟半天然林分的 315 个混合和单一特定树社区的土壤样本中吸收性细根（即前三个最远侧顺序）的形态和化学特征以及外生菌根定植强度。此外，我们量化了来自同一林分的土壤样品中的菌根丰度和多样性。
3. 在群落水平上，树木混合物中细根的特点是比根长和根氮浓度更高，直径和根组织密度更低，表明其生长速度更快。与单一特定林分相比的资源获取策略。与单一特定林分相比，混合物中更高的根 EcM 定植强度和土壤 EcM 多样性可能进一步为正生物反馈提供证据。此外，细根性状的多样性影响了 FRB，因为具有较高性状差异的混合物与 FRB 的较低减少有关。在系统发育组水平上，细根被子植物比粗根裸子植物表现出更强的混合反应，尤其是在根形态和 EcM 定植方面，表明对树木混合的反应策略不同。
4. 我们的结果表明较低的 FRB 可以反映土壤资源获取策略的转变，而不是混合物中树木的较低性能。他们表明，几种非排他性机制可以同时解释混合对 FRB 的负面净影响。这项研究揭示了使用综合方法（包括地上和地下生物量和性状）研究多样性对植物生产力的影响的重要性。