Timing is everything: seasonal drought alters flowering phenology and increases niche partitioning

Flowering phenology is central to plant fitness, species coexistence, and ecosystem functioning. Numerous studies have shown that global warming advances phenology, with wide-ranging ecological consequences. However, the effects of water availability remain unclear and inconsistent across ecosystems. We hypothesized that part of this inconsistency arises from the overlooked temporal aspect of water stress. We therefore tested how drought timing influences species-level flowering phenology and community-level phenological niche partitioning. For this purpose, we established a system with 200 experimental annual-plant communities characteristic of a Mediterranean climate. We manipulated the wet season by imposing dry periods at the beginning, middle, or end. Plants were grown in monocultures and mixtures to separate direct physiological responses from competition-mediated effects. Early- and late-season droughts shortened the species’ flowering duration: early drought delayed the onset of flowering, and late drought caused it to end sooner. Some of these shifts were direct responses to drought, while others were competition-mediated. Our new community-level index of niche partitioning revealed higher phenological segregation in mixtures than in monocultures. This indicates that phenotypic plasticity alone, without evolutionary adaptation, can promote niche separation under competition. Both early- and late-season droughts further increased niche-partitioning, with early drought also enhancing species diversity. Together, these results demonstrate that the seasonal timing of drought, not only its magnitude, governs plastic shifts in flowering phenology and reshapes competitive outcomes. Thus, our work identifies intra-annual rainfall patterns as a critical yet underappreciated driver of plant phenology and community structure under climate change.