Land-use changes in cities, including an increase in structures with large thermal mass and decrease in albedo, together with waste heat generated by intensive energy use, leads to “urban heat islands” that are warmer than surrounding areas that have lower population densities and are less developed. The urban heat-island effect combined with broader global warming has resulted in cities warming much faster than surrounding suburban, exurban, and rural areas. At the same time, the urban heat-island effect can cause an increase in precipitation downwind of cities. Recent efforts have indicated that phenology is being altered in urban environments, but field observational records to assess these changes are limited in temporal, spatial, and taxonomic scopes. To assess more broadly the spatiotemporal and taxonomic effects of warming on phenology in urban environments, we used digitized herbarium images and data derived from our ongoing effort to characterize phenology of species from the eastern United Sates. We hypothesized that phenological advancement (as peak flowering time) of plants growing in urban areas would be more rapid than that observed in surrounding non-urban areas.
Results/Conclusions
Urban and suburban counties (population densities > 100 people / km2) have peak flowering times (averaged across all species) of about 0.5-2.4 days earlier than in rural counties with lower population densities. Uncertainty in the estimate results in part from: general increase in spring temperatures that are associated with phenological advancement; positive effects of spring precipitation on phenological advancement; and confounding of urbanization with elevation and proximity to the coast. Interactions among these factors include: a significant positive interaction between spring temperature and population density (a proxy for degree of urbanization); and a significant negative interaction between population density and spring precipitation. We conclude that urbanization may lead to earlier peak flowering, especially in cooler and wetter areas.