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 been rarely tested both across and at given species richness 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 of economics traits determine the outcomes of tree mixture effects on productivity across and within species richness levels. 3. We found that the positive effects of tree mixtures on productivity were strengthened by the increasing multidimensional functional dispersion and the community-weighted mean of leaf nitrogen content both across and within two- and four-species mixtures. Moreover, the multidimensional functional dispersion and the community-weighted mean 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.

The dataset includes data from studies up to June 01, 2020 which focus on tree diversity effects on ecocystem productivity. The studies for this meta-analyses meet the following criteria: (1) studies contained at least one mixture treatment with corresponding monocultures, (2) all productivity and names of the species in each mixture and corresponding monocultures could be directly extracted from the text, tables, and/or figures, (3) the proportion of constituent species in each mixture could be extracted or calculated, (4) studies were specifically implemented to isolate the effects of tree diversity from other factors, such as soil conditions and topographic features.