Global warming is altering seasonal temperature regimes. However, community outcomes tend to be species-specific and can vary greatly, making it difficult to predict vegetation responses. Plasticity in functional traits may be a key mechanism buffering species-level biomass decline under warming conditions, but which trait axes are most consequential and whether buffering operates consistently under seasonally asymmetric warming remain unclear. In this study, we aimed to test this hypothesis by manipulating year-round, summer, and winter warming in a semi-arid grassland in Mongolia. We quantified species-level biomass responses alongside plasticity in leaf height and economic traits (specific leaf area and leaf dry matter content) and assessed concurrent shifts in community-weighted mean (CWM) traits. Warming tended to alter community biomass, species richness, and CWM traits, but only CWM leaf height showed a significant overall treatment effect and pairwise differences among warming treatments, indicating a shift toward lower-stature community structure. Across species, biomass consistently responded to warming negatively, but responses were less negative in species in which leaf height and specific leaf area reduced the most. In contrast, plasticity in leaf dry matter content was not associated with biomass response, and plasticity-biomass relationships did not differ among seasonal warming treatments. Our results demonstrate that species-level plasticity along structural and leaf economic axes can buffer warming-induced biomass declines in a semi-arid grassland. In conclusion, not all plasticity is equally functional, and identifying key trait dimensions improves mechanistic predictions of plant response to climate warming.