Seasonal timing of extreme drought events drives functional trait shifts and plasticity in dominant Inner Mongolian steppe grasses

Extreme droughts are intensifying, and their seasonal timing significantly influences plant functional traits in semi-arid grasslands. This study applied imposed extreme drought conditions in spring, summer, or autumn to three dominant steppe grasses (Leymus chinensis, Stipa krylovii, and Cleistogenes squarrosa) in a temperate steppe in Inner Mongolia, measuring 12 functional traits and soil moisture at four depths. Linear mixed-effects models and response coefficients (RC; trait change relative to control) indicated that summer drought caused the most significant deviations from ambient conditions, primarily affecting surface soils. Leaf-size traits (leaf area, width) showed the greatest responsiveness, with leaf nitrogen concentration consistently increasing under summer drought, while leaf C:N ratio declined. Species-specific strategies emerged: S. krylovii displayed substantial biomass and cover gains in spring and summer, followed by reductions in plant height and leaf size in autumn; C. squarrosa exhibited predominantly negative responses across traits, particularly in the summer and autumn; and L. chinensis showed minimal changes, with slight increases in acquisitive leaf traits during summer and small structural declines in spring and autumn. These findings highlight the importance of drought timing, species identity, and their interaction in shaping functional reconfiguration. Simple leaf-size and nitrogen metrics offer seasonally relevant indicators for monitoring and preserving steppe resilience amid escalating climatic extremes.