Plant functional traits, especially leaf traits, are accepted proxies for ecosystem properties. Typically, they are measured at the
species level, neglecting within-species variation. While there is extensive knowledge about functional trait changes (both
within and across species) along abiotic gradients, little is known about biotic influences, in particular at local scales. Here, we
used a large biodiversity-ecosystem functioning experiment in subtropical China to investigate intra-specific trait changes of 16
tree species as a response to species richness of the local neighbourhood. We hypothesized that because of positive complemen-
tarity effects, species shift their leaf traits towards a more acquisitive growth strategy, when species richness of the local neigh-
bourhood is higher. The trait shift should be most pronounced, when a focal tree’s closest neighbour is from a different species,
but should still be detectable as a response to species richness of the directly surrounding tree community. Consequently, we
expected that trees with a con-specific closest neighbour have the strongest response to species richness of the surrounding tree
community, i.e., the steepest increase of acquisitive traits. Our results indicate that species diversity promoted reduced competi-
tion and complementarity in resource use at both spatial scales considered. In addition, the closest neighbour had considerably
stronger effects than the surrounding tree community. As expected, trees with a con-specific nearest neighbour showed the
strongest trait shifts. However, the predicted positive effect of local hetero-specificity disappeared at the highest diversity levels
of the surrounding tree community, potentially resulting from a higher probability to meet a strong competitor in a diverse envi-
ronment. Our findings show that leaf traits within the same species vary not only in response to changing abiotic conditions, but
also in response to local species richness. This highlights the benefit of including within-species trait variation when analysing
relationships between plant functional traits and ecosystem functions.