A number of global change drivers, such as land use change, climate change, or air-borne eutrophication, have considerable impacts on the biological diversity of forest ecosystems. Understanding and forecasting the consequences of these impacts on ecological processes, functions and the delivery of ecosystem services is certainly one of the major challenges for ecological research. Current investigations on the functional role of forest biodiversity are based on two distinct, but complementary approaches: (i) comparative studies in established stands of differing tree diversity, and (ii) experiments manipulating diversity by planting new stands with various species numbers, including silvicultural trials with only low diversity levels. The talk reviews studies from such approaches, suggesting a positive relationship between tree diversity and functions related to productivity, associated biodiversity, and soil parameters. However, no and even negative effects were also documented, and many studies find stronger effects of species identity than diversity. In addition, disentangling the diversity signal from confounding environmental heterogeneity remains difficult. Comparisons of tree species performance in pure and mixed plantations imply that changes in light acquisition and plant nutrition may be important underlying mechanisms for the observed diversity effects. In sum, a clear general view of the functional role of tree diversity in forest ecosystems has not emerged so far, but different mixture effects on several functions and services are evident. The presence of certain functional traits, i.e. species identity, is certainly a major driver of ecosystem functioning; hence trait-based approaches may offer deeper insights for understanding the significance of biodiversity and for the management of forest ecosystems.