Above- and below-ground complementarity rather than selection drive tree diversity-productivity relationships in European forests

1. Biodiversity experiments have identified both complementarity and selection as important drivers of the relationship between biodiversity and ecosystem functioning. However, their relative importance in above- and below-ground ecosystem compartments of mature forests remains yet to be explored. 2. We adopted a trait-based approach to partition biodiversity effects in above- and below-ground complementarity and selection. This approach was based on canopy and root traits measured in single- and mixed-species plots in mature forests across a European latitudinal gradient. 3. We assessed the relative importance of above- and below-ground selection and complementarity in driving the relationship between tree species diversity and above-ground wood production. We used the expected values (based on the values measured in monocultures) of leaf area index (LAI) and fine root biomass as proxies for above- and below-ground selection, whereas canopy packing and rooting depth variability were used as proxies for above- and below-ground complementarity. 4. Our results showed that tree species richness-wood production relationships were driven by above- and below-ground complementarity (i.e. canopy packing and rooting depth variability), rather than selection. The proxies for selection were found to have a positive effect on wood production but were not affected by tree species richness. 5. We concluded that above-ground-but also the largely neglected below-ground complementarity drives biodiversity-productivity relationships in mature forests. Our findings suggest that choosing tree species with complementary above- and below-ground traits should be considered in afforestation and forest management to promote tree diversity and productivity in European forests