Previous work demonstrated diversity effects on plant community productivity, stability and assembly. However, underlying mechanisms are still largely unknown. Therefore, we address such mechanisms based on the long time series of plant performance data provided by the Jena Experiment, on new experimental manipulations and on complementary data about aboveground plant-plant and plant-pathogen interactions.In work package 1, we test whether complementarity increases over time in the main and dominance experiments of the Jena Experiment. Furthermore, we compare community dynamics without and with invasion to test how community stability is related to fluctuations in plant species abundances. In a new trait-based experiment we test whether deliberately large trait differences maximize complementarity. In work package 2, we derive and compare matrices of pair-wise plant interaction coefficients from glasshouse experiments and the main and dominance experiments. We use these interaction matrices to predict diversity effects in mixtures. In work package 3, we use the invasion treatment to test whether community assembly under invasion leads to convergence of diversity and productivity of all communities, but different species composition of initially different communities.