Warming and precipitation change alter flowering phenology and coflowering networks in California serpentine grasslands

Coflowering, the temporal overlap in flowering among plant species, can influence plant fitness through its effects on heterospecific pollen transfer and competition for pollinators and resources. This temporal overlap is changing as plant species respond differently to climate change. However, difficulties in quantifying and comparing patterns of coflowering across diverse, multispecies communities have limited progress in understanding climate-driven shifts in coflowering. Network-based approaches offer a promising solution to this limitation. Here, we investigate how warming and altered precipitation influence coflowering network structure, flowering phenology, and seed production in 12 annual California serpentine grassland plant species in a mesocosm experiment, using coflowering network analysis and structural equation modeling. Communities consistently grouped into two phenological modules corresponding to early- and late-season annuals. Warming reduced the overall amount of coflowering in the community, driven primarily by weakened and less diverse coflowering among late-season species. Soil moisture moderated these effects: late-season coflowering was strongest under cool-wet conditions and weakest under warm conditions regardless of soil moisture. In contrast, early-season species had a more fixed phenological window, and maintained stable coflowering relationships across climates. Growth form (grass or forb) and origin (native vs. non-native) did not predict coflowering responses. We additionally found that temperature and soil moisture influenced seed production largely through their effects on flowering phenology, with early- and late-season species exhibiting distinct phenological responses and sensitivities. Our results demonstrate that warming and precipitation change can spread out flowering times within functional groups, reshaping community-wide coflowering networks through species-specific responses to altered climate conditions.