Seasonal Drought Timing Shapes Flowering Phenology Directly and Through Biotic Interactions.

Flowering time underpins plant fitness, species coexistence, and ecosystem functioning. While global warming consistently advances flowering, the influence of water availability remains unclear. We hypothesized that this inconsistency reflects the overlooked timing of drought. In 200 experimental Mediterranean annual-plant communities, we imposed autumn, winter, and spring dry periods and grew plants in monocultures and mixtures to disentangle physiological and competition-mediated responses. Dry autumn and spring shortened flowering duration: dry autumn delayed onset, dry spring advanced termination. Some shifts were direct; others emerged through competition. A new community-level index revealed greater phenological segregation in mixtures, showing that plasticity alone can generate niche separation under competition. Both dry autumn and spring tended to further enhance this segregation. Together, our results demonstrate that the seasonal timing of drought governs flowering responses through both direct physiological pathways and indirect biotic interactions, emphasizing rainfall seasonality as a key driver of ecological responses to climate change.