1) So far, the principal force shaping local plant abundance patterns remains unclear. Rarity can result from poor competitive ability or from small vegetative or generative reproduction, but also from strong self-limitation. The same mechanisms can drive species-specific overyielding, i.e. increased species productivity at high community diversity. Rare species can then benefit more (i.e. overyield to a larger extent) from growing in species-rich communities because of altered competitive hierarchies or smaller conspecific frequencies. Here we test which mechanism is the most important determinant of species rarity and of species-specific productivity across a diversity gradient ranging from one- to 60-species plots. 2) For that we measured vegetative growth, competitive ability (competitive effect), and negative frequency dependence for 49 perennial grassland species from Central Europe. We then linked these characteristics with species abundance (measured as species biomass from 60-species plots) and with species-specific overyielding in The Jena Experiment. 3) Species with higher rates of vegetative growth (when grown without neighbors) were also more abundant in the Jena Experiment. Larger species-specific overyielding was then associated with a stronger negative frequency dependence. As species with greater vegetative growth were also more self-limited, larger overyielding in species-rich communities was characteristic for common rather than for rare species, refuting our initial hypothesis. Finally, path analysis indicated that species with poor capacity to suppress neighbors also profited more from growing in diverse communities. 4) Synthesis. Our results identify key mechanisms driving abundance and productivity of species in synthetic communities differing in species richness. While vegetative reproduction was closely associated with abundance, intraspecific interactions (strength of negative frequency dependence) shaped species productivity across a richness gradient. Our study sheds light on the abundance patterns of species and their influence on community functions, such as biomass production, of species-rich and -poor vegetation.