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 plays an important role in shaping plant functional traits in semi-arid grasslands. This study experimentally imposed extreme drought in spring, summer, or autumn on three dominant steppe grasses ( Leymus chinensis , Stipa krylovii , and Cleistogenes squarrosa ) in a temperate steppe of Inner Mongolia, measuring 12 functional traits and soil moisture at four depths. Linear mixed-effects models and response coefficients (RC; trait deviation relative to control) indicated that summer drought induced the strongest trait shifts, against a background of consistently reduced surface soil moisture across all seasonal drought treatments. Leaf-size traits (leaf area, width) were the most responsive across species, accompanied by consistent increases in leaf nitrogen concentration and declines in leaf C:N ratio under summer drought. Species-specific trait-based response strategies emerged: S. krylovii showed early-season biomass and cover gains followed by later reductions in height and leaf size; C. squarrosa exhibited predominantly negative trait responses under mid- to late-season drought; and L. chinensis showed minimal changes, with slight increases in acquisitive leaf traits during summer and small structural declines outside the peak season. These findings highlight that drought timing governs functional reconfiguration, and that complementary species strategies, together with leaf-size and LNC indicators, provide season-aware metrics for assessing steppe resilience. • Rainout shelters imposed consistent surface-soil drying (0–10 cm) across drought seasons, with 10–20 cm effects detected only in autumn. • Seasonal drought drove contrasting whole-plant strategies: buffering ( L. chinensis ), opportunity-tracking with late-season retraction ( S. krylovii ), and contraction-dominated decline ( C. squarrosa ). • Summer drought produced the most coherent cross-species functional shifts, particularly in leaf size and nutrient status. • Leaf size traits and leaf nitrogen (and C:N) emerged as practical indicators of severe seasonal drought across species. • Species identity and drought season jointly structured multivariate trait syndromes, implying season-dependent pathways of drought adaptation and competitive rebalancing.