Community phylogenetic diversity drives flowering synchrony among coexisting plant species even under drought conditions.

Flowering time is a critical event in the lifetime of angiosperms, being particularly sensitive to environmental conditions, although the range of flowering response should be ultimately constrained by evolutionary history. We hypothesize that (1) phylogenetically diverse assemblages exhibit more segregated flowering peaks due to lineage-specific constraints, while closely related assemblages show synchronized flowering, and (2) under drought conditions, phenotypic plasticity most likely increase flowering overlap, though this response may depend on species' phylogenetic relatedness in the assemblage. We designed assemblages with annual plants of semiarid systems of Spain, considering two contrasted levels of phylogenetic diversity (PD) and two water availability treatments in a common garden experiment, where we analysed the flowering segregation among species. High PD assemblages resulted in greater flowering overlap, while assemblages composed of close relatives segregated more their flowering peaks. Water stress triggered flowering synchronization both in high and low phylogenetic diversity assemblages. Our findings suggest that the high phylogenetic diversity characteristic of Iberian gypsophilous annual plant communities is compatible with substantial flowering overlap, which may, in turn, facilitate species coexistence and contribute to the remarkable species richness observed in these environmentally harsh habitats.