Abstract: Climate change intensifies seasonal droughts in alpine meadows on the Qinghai-Tibet Plateau, impacting the adaptability of key plant functional groups. However, how plant functional groups with different flowering phenologies adjust their reproductive allocation through trait-based strategies remains poorly understood. This study examined the effects of spring, summer, and whole growing season drought on reproductive strategies of early-spring flowering (ESF) and mid-summer flowering (MSF) plant functional groups. Results revealed that spring drought significantly reduced the reproductive efficiency of MSF plants, whereas summer drought resulted in higher reproductive efficiency compared to spring drought. ESF plants exhibited greater resilience than MSF plants across all seasonal drought treatments, highlighting the advantage of their drought-avoidance strategy. Under whole growing season drought, ESF plants adopted conservative resource-use strategies, including decreased specific leaf area (SLA) and increased leaf carbon to nitrogen ratio (C/N), carbon to phosphorus ratio (C/P), and leaf dry matter content (LDMC). In contrast, MSF plants experienced phosphorus (P) limitation and height reductions. Notably, whole growing season drought induced interannual cumulative effects in MSF plants, such as increased LDMC and decreased SLA, indicating higher morphological plasticity. Furthermore, ESF plants enhanced flower allocation through both increased P availability and C/N (nutrient-sensitive strategy). For MSF plants, flowers allocation directly regulated by leaf nitrogen content (LNC), indirectly enhanced through resource reallocation form decreased plant height (morphology-integrated strategy). This study elucidates the covariation patterns between functional trait and reproductive allocation in ESF and MSF plants under seasonal drought, providing a mechanistic framework for predicting alpine ecosystems responses to future droughts.

Key words: Alpine grassland, Flowering functional group, Functional traits, Qinghai-Tibet Plateau, Reproductive allocation, Resource-use strategy, Seasonal drought