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. 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. 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. 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. 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.