Urbanization and plant diversity influence different aspects of floral phenology

Aaron Sexton, Biology, University of Louisville, Louisville, KY, Sarah Benton, Olmsted Parks Conservancy and Sarah Emery, Biology Dept., University of Louisville, Louisville, KY

Background/Question/Methods
Shifts in plant and animal phenologies have been documented across the planet in recent decades as global temperatures rise and landscapes become human dominated. In urban areas, where surface temperatures can regularly reach 5-6°C warmer than in surrounding rural areas, phenologies are expected to shift significantly. However, small high-quality habitat patches within urban areas can hold diverse communities of plant and animal life and may have the potential to offset some of the detrimental effects of urbanization. From April-October 2018 and 2019, we studied how floral phenology—the onset, duration, and distribution of floral events—, shifted in small grassland restoration sites across an urbanization gradient in Louisville, Kentucky, a city with one of the most severe urban heat islands in the US. In addition to urbanization, we examined how habitat patch size, plant diversity, and soil water holding capacity influenced multiple parameters of floral phenology. Our objectives were to understand 1) how urbanization influences floral phenology and 2) if high-diversity local habitats could influence or offset some effects of urbanization.
Results/Conclusions
We found that first date of flowering and peak abundance date occurred 1-2 weeks earlier in urban compared to rural areas, likely due to Louisville’s intense urban heat island. However, we found that floral duration was longest in sites with high plant species diversity, regardless of urbanization. Flowering schedule shape was not significantly influenced by any of our predictors across the entire community, but we did see different distributions across seasons (spring, summer, fall). We found that summer species shifted their distribution along an urbanization gradient to decrease their dispersion, but spring and fall species did not. We found no consistent impact of patch size or soil water holding capacity on phenology, but we did see strong inter-annual effects, with stronger responses to urbanization in 2019, which was a moderate drought year in the region. Shifts in floral phenology due to urbanization have implications for pollinator populations, and it is encouraging that increasing plant diversity at a local scale may help counteract larger-scale environmental changes. These findings highlight the importance of understanding not only how urbanization influence floral phenology, but how we can develop ecologically sound habitats in urban areas to support robust plant and animal communities.