Background: Species-specific genotypic features, local neighbourhood interactions and resource supply stronglyinfluence the tree stature and growth rate. In mixed-species forests, diversity-mediated biomass allocation has beensuggested to be a fundamental mechanism underlying the positive biodiversity-productivity relationships. Empiricalevidence, however, is rare about the impact of local neighbourhood diversity on tree characteristics analysed at avery high level of detail. To address this issue we analysed these effects on the individual-tree crown architectureand tree productivity in a mature mixed forest in northern Germany. Methods: Our analysis considers multiple target tree species across a local neighbourhood species richnessgradient ranging from 1 to 4. We applied terrestrial laser scanning to quantify a large number of individual maturetrees (N= 920) at very high accuracy. We evaluated two different neighbour inclusion approaches by analysing botha fixed radius selection procedure and a selection based on overlapping crowns. Results and conclusions: We show that local neighbourhood species diversity significantly increases crowndimension and wood volume of target trees. Moreover, we found a size-dependency of diversity effects on treeproductivity (basal area and wood volume increment) with positive effects for large-sized trees (diameter at breastheight (DBH) > 40 cm) and negative effects for small-sized (DBH < 40 cm) trees. In our analysis, the neighbourinclusion approach has a significant impact on the outcome. For scientific studies and the validation of growthmodels we recommend a neighbour selection by overlapping crowns, because this seems to be the relevant scaleat which local neighbourhood interactions occur. Because local neighbourhood diversity promotes individual-treeproductivity in mature European mixed-species forests, we conclude that a small-scale species mixture should beconsidered in management plans.