Biodiversity is declining world-wide due to land-use change, urbanization, global warming and other anthropogenic transformations of the environment. Accumulating empirical evidence suggests that this ongoing pauperization of ecosystems impairs ecosystem functioning and thereby threatens human well-being. For assessing the consequences of species extinctions as well as for a prioritization of conservation efforts, a thorough understanding of the relationships between biodiversity and ecosystem functioning is required. In the past, numerous experiments have shown that an increase in biodiversity usually enhances community productivity but we are only beginning to understand why. In this thesis, I used data from a large-scale grassland biodiversity experiment (Jena Experiment) to explore mechanisms underlying positive relationships between plant diversity and aboveground primary productivity.