Diversity and identity of economics traits determine the extent of tree mixture effects on ecosystem productivity

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.