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1. While positive effects of biodiversity on temporal stability of communities have been demonstrated in theoretical and empirical studies, diversity -stability relationships at the population level remain poorly understood. 2. We investigated temporal variability of plant populations in experimental grassland plots of varying species richness (1, 2, 4, 8, 16 - 60 species), functional group richness and composition (presence/absence of legumes x grasses x small herbs x tall herbs) in a long-term biodiversity experiment from 2003 to 2009 ( 'Jena Experiment'). 3. Average population stability, defined as the reciprocal of the coefficient of variation of above-ground biomass production over time, differed largely between species but was generally higher in grasses and small herbs than in legumes and tall herbs. Furthermore, population stability was positively related to a species' proportional contribution to community biomass. Thus, an increasing number of subordinate species explained lower average population stabilities at higher diversity levels. 4. A negative covariance (CV) across all species-richness levels suggested negatively correlated species dynamics. Species belonging to different functional groups fluctuated asynchronously, while species dynamics within functional groups were more synchronous. Community-wide species synchrony decreased with increasing species richness, and temporal stability at the community level increased. 5. Synthesis: Our results suggest that diversity -stability relationships are driven by fluctuations in the population biomass of individual species which are less synchronized in more diverse than in less diverse mixtures and monocultures. Dominant plant species tend to be more stabilized than subdominant species, independently of community species richness. However, asynchrony of population dynamics outweighs decreasing population stability with increasing species richness, resulting in higher temporal stability at the plant community level.