Moisture availability is the main limiting factor of plant growth and biomass production in arid and semi-arid grasslands. The question whether plant responses to changing precipitation are species-specific, or change over entire plant communities is still controversial. Our study focussed on intraspecific changes in the plant traits canopy height, plant width, specific leaf area, chlorophyll fluorescence, performance index, and individual biomass of three congeneric species pairs with changing precipitation in Mongolian rangelands, covering a gradient from the desert to the forest steppes. Using this trait data set, we focussed on three questions: (i) Is the replacement of congeneric species along an environmental gradient also reflected in their trait values? (ii) Can intraspecific trait variation patterns be derived from patterns in species abundances, i.e., are trait values optimal where species are most abundant? (iii) Is the within-population trait variability lowest in populations growing under very dry conditions, i.e., under highest environmental stress, caused by stronger filtering? We tested the responses of the six traits to changing precipitation according to species’ identity and abundance. We found unimodal relationships between most of the species’ traits and precipitation, and strong associations between species abundances and trait values, but not for all investigated species. Trait variability did not significantly differ between populations from different positions along the precipitation gradient. Our results highlight that species show multiple or even opposite trait responses along the precipitation gradient. It thus remains challenging to predict how plant distributions will shift under changing environmental conditions based on their trait composition.