Neglecting tree size and stand structure dynamics might bias the interpretation of the diversity-productivity relationship in forests. Here we show evidence that complementarity is contingent on tree size across large-scale climatic gradients in Europe. We compiled growth data of the 14 most dominant tree species in 32,628 permanent plots covering boreal, temperate and Mediterranean forest biomes. Niche complementarity is expected to result in significant growth increments of trees surrounded by a larger proportion of functionally dissimilar neighbours. Functional dissimilarity at the tree level was assessed using four functional types: i.e. broad-leaved deciduous, broad-leaved evergreen, needle-leaved deciduous and needle-leaved evergreen. Using Linear Mixed Models we show that, complementarity effects depend on tree size along an energy availability gradient across Europe. Specifically: (i) complementarity effects at low and intermediate positions of the gradient (coldest-temperate areas) were stronger for small than for large trees; (ii) in contrast, at the upper end of the gradient (warmer regions), complementarity is more widespread in larger than smaller trees, which in turn showed negative growth responses to increased functional dissimilarity. Our findings suggest that the outcome of species mixing on stand productivity might critically depend on individual size distribution structure along gradients of environmental variation. ; This manuscript was funded by the European Union Seventh Framework Program (FP7/2007–2013) under grant agreement no. 265171, project FUNDIV-Europe, and from project FUNDIVER (MINECO, Spain; CGL2015-69186-C2-2-R). ; J.C. holds a Spanish FPU PhD Grant (FPU12/00575). ; PR-B was granted by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 265171 (project FunDivEUROPE) and No. PCOFUND-GA-2010-267243 (Plant Fellows). ; Peer reviewed