1. Trait-based approaches are key to develop mechanistic understanding of differences in plant species performance under environmental change. While mean trait values have been widely used to link functional traits to species performance, the contribution of intraspecific trait variation and trait plasticity remains unclear. Moreover, environmentally induced changes in species biomass is caused by changes in the number of individuals and individual growth rate, both of which should be influenced by trait differences and plasticity. Our goal in this study is to use trait-based information to explain species performance via changes in species abundance and individual weight.

2. We measured the mean, intraspecific variation and plasticity of nine aboveground plant traits, and a further three mean root traits of ten common species in a precipitation manipulation experiment in semiarid grassland. We used this trait information to explain differences in the responses of species biomass, abundance and mean individual weight to changing precipitation. Species responses were calculated as the normalized slopes of the regressions between species biomass, abundance and individual weight with the manipulated precipitation amount.

3. We found strong differences in species responses to changing precipitation for species biomass, abundance and mean individual weight. Reduced precipitation decreased biomass, abundance and mean individual weight for some species, but increased them for others. Biomass and mean individual weight of species with resource-acquisitive traits, such as shallow rooted species, showed stronger positive responses to changing precipitation compared to resource-conservative traits, like those with deep roots. For aboveground traits, trait plasticity was the strongest predictor of species responses compared to mean traits and intraspecific trait variation. In addition, trait plasticity regulated changes in species biomass more via changes in species abundance than mean individual weight.

4. These results indicate that trait plasticity is a key driver for determining species specific responses to changing precipitation and needs more consideration for understanding and predicting ecosystem structure and functioning in future climate scenarios.