Urbanization and plant diversity influence different aspects of floral phenology

Plant phenologies can shift as global temperatures rise and landscapes become human-dominated. Some of the most pronounced shifts in phenologies have been documented in urban areas, where surface temperatures can reach 5–7℃ warmer than surrounding rural areas. In this study, we examined how floral phenology—the initiation, peak, and duration of floral events — shifted in grasslands across an urbanization gradient in Louisville, Kentucky, a city with one of the most severe urban heat islands in the US. Our objectives were to understand (1) how urbanization influences floral phenology (2) whether high-quality habitats with increased habitat patch size and plant richness could offset some effects of urbanization, and (3) whether species responses varied across seasons. We found that average first date of flowering and peak abundance date occurred 1–2 weeks earlier in urban compared to rural areas. However, we found that floral duration was longest in sites with high plant richness, regardless of urbanization. We also found that summer-flowering species increased their floral duration in urban areas while spring and fall-flowering species shortened theirs. These differences in seasonal responses lead to an “urban summer spillage” effect where summer-flowering species are able to move into the temporal niche of spring and fall species in urban habitat patches. These shifts could lead to a reshuffling of communities and novel plant-plant competitive interactions or plant-animal interactions in the urban core, with lasting implications for urban conservation.