Data from: Overyielding in young tree plantations is driven by local complementarity and selection effects related to shade tolerance

1. Overyielding in mixed-species forests has been demonstrated in a vast body of literature, and the focus of functional biodiversity research is now shifting towards a mechanistic understanding of these observations. 2. We explored diversity-productivity relationships (DPRs) at two sites of a large-scale tree diversity experiment, with benign (Zed) and harsh (Ged) environmental conditions for plantation establishment. Additive partitioning methodologies were adopted to detect phenomenological patterns in the productivity data, and the trait structure of mixed communities was used to advance insights into compositional effects. 3. After six years of plantation development, biomass productivity was significantly higher in mixtures compared to the monocultures of component species. We observed that processes operated through direct tree-tree interactions, since the diversity signal disappeared where trees in mixed stands were surrounded by conspecific neighbors only. This result is particularly relevant for mixed-species planation systems, as trees are commonly planted in monospecific patches to simplify the management. Partitioning unveiled strong selection effects at both plantation sites. However, at the Ged-site this was caused by competitive dominance of species with fast young growth whereas at the Zed-site, species with slow young growth improved their performances but not at the expense of others (i.e. trait-dependent complementarity). Species tolerance to shading is an influential trait to predict biodiversity effects, with community-weighted means in shade tolerance mediating dominance effects (Ged) and functional diversity in shade tolerance mediating (trait-dependent) complementarity effects (Zed). 4. Synthesis. This study highlights that biodiversity effects in young tree plantations could be explained by the functional composition of mixed communities, with a key role for species levels of shade tolerance. As contrasting results between plantation sites were observed, future research should target the context-dependency of DPRs.1. 混交林的超产现象已在大量文献中得到证实，当前功能生物多样性研究的焦点正转向对这些观测结果的机制性解析。2. 我们在一项大型树木多样性试验的两个样地开展了多样性-生产力关系（diversity-productivity relationships, DPRs）研究，这两个样地分别为人工林建立提供了适宜（Zed）与严苛（Ged）的环境条件。本研究采用加性分解方法解析生产力数据中的现象学模式，并借助混交群落的性状结构深化对群落组成效应的认知。3. 人工林发育6年后，混交林的生物量生产力显著高于组成物种的单优林分。我们观测到，相关过程通过树木间的直接相互作用进行：当混交林分中的树木仅被同种邻木环绕时，多样性信号便会消失。该结果对混交人工林系统尤为重要，因为当前人工林常以单优斑块的方式进行种植以简化经营管理。分解分析揭示了两个人工林样地均存在显著的选择效应。然而，Ged样地的选择效应由幼龄生长快速的物种的竞争优势所主导；而在Zed样地，幼龄生长缓慢的物种提升了自身表现，且并未以牺牲其他物种为代价（即性状依赖的互补效应）。物种耐荫性是预测生物多样性效应的关键性状：耐荫性的群落加权均值介导了Ged样地的选择效应，而耐荫性的功能多样性则介导了Zed样地的（性状依赖型）互补效应。4. 综合与结论。本研究表明，幼龄人工林的生物多样性效应可通过混交群落的功能组成进行解释，其中物种耐荫性水平发挥关键作用。鉴于不同人工林样地的研究结果存在差异，未来研究应聚焦于多样性-生产力关系的情境依赖性。