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 partic...