1. Although both observational and experimental studies have shown that positive tree species diversity–productivity relationships are predominant in global forests, weak or the lack of tree species diversity and productivity relationships also exist. Growing evidence has revealed that ecosystem productivity depends more on the functional characteristics of species than on their number. However, exactly to what extent tree diversity effects on ecosystem productivity are influenced by the variability and composition of functional traits have rarely been tested both across and at given species richness (SR) levels.
2. Here, we employed a meta‐analysis of global‐scale data from 59 tree diversity experiments to examine how the diversity and community‐weighted means (CWMs) of economics traits determine the outcomes of tree mixture effects on productivity across and within SR levels.
3. We found that the positive effects of tree mixtures on productivity were strengthened by the increasing multidimensional functional dispersion (FDis) and the CWM of leaf nitrogen content both across and within two‐ and four‐species mixtures. Moreover, the multidimensional FDis and the CWM of leaf nitrogen content increased the complementarity effect rather than the selection effect.
4. Synthesis. Our findings suggest that both diversity in the leaf economics spectrum and trait concentration on the ‘fast’ end of the spectrum strengthen biodiversity–ecosystem functioning relationships. This study provides mechanistic insights into the potent roles of plant economics traits, especially leaf nitrogen content, in determining the magnitude (and even directionality) of the biodiversity–ecosystem functioning relationships in forest ecosystems.

中文翻译：

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经济特征的多样性和同一性决定了树木混交对生态系统生产力的影响程度

1. 尽管观察和实验研究均表明，在全球森林中，树种的多样性与生产力之间存在正相关关系，但树种的多样性与生产力之间的关系仍然较弱或缺乏。越来越多的证据表明，生态系统生产力更多地取决于物种的功能特征，而不是数量。但是，很少能在给定的物种丰富度（SR）水平上和在给定的物种丰富度（SR）水平上测试树木多样性对生态系统生产力的影响在多大程度上受功能性状的变异性和组成的影响。
2. 在这里，我们对来自59个树木多样性实验的全球规模数据进行了荟萃分析，以检验经济性状的多样性和社区加权均值（CWM）如何确定树木混交的结果对SR范围内和之内的生产力的影响。
3. 我们发现，通过在两种和四种物种混合物中以及内部和两种物种混合物中增加的多维功能性色散（FDis）和叶氮含量的CWM，树木混合物对生产力的积极作用得到了增强。此外，叶片氮含量的多维FDis和CWM增加了互补效应而不是选择效应。
4. 综合。我们的发现表明，叶片经济谱的多样性和谱“快速”端的特征集中都增强了生物多样性与生态系统的功能关系。这项研究提供了有关植物经济性状（尤其是叶氮含量）在确定森林生态系统中生物多样性与生态系统功能关系的大小（甚至方向性）方面的潜在作用的机械性见解。