Climate-dependent effects of urbanization on plant phenology

Daijiang Li, Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, Brian Stuky, University of Florida, Benjamin Baiser, Wildlife Ecology and Conservation, University of Florida, Gainesville, FL and Robert Guralnick, Florida Museum of Natural History, University of Florida, Gainesville, FL

Background/Question/Methods
With 57% of the human population now lives in urban areas, urbanization is one of the most important drivers of global environmental change. This trend towards urban dwelling is forecast to accelerate and around 70% of the increasing human population to be expected to live in urban areas by 2050. Urbanization and its associated environmental consequences such as habitat fragmentation, urban heat island effects, pollutions, and altered biogeochemical and hydrologic cycling are affecting all living individuals in urban areas, forming novel ecosystems. However, we know little about how and why urbanization affect species, limiting our ability to have better urban planning and management. Plant phenology – the timing of plant life-cycle events, such as leaf-out and flowering – can serve as a “canary in the coal mine” for impending system shifts because phenology shifts are often the most immediate and visible ecological response to environmental change. We investigated the effects of urbanization on plant phenology across the United States and most of the Europe using both in situ observatory and remote sensing data.
Results/Conclusions
Separately, increasing population density and warmer regional temperature both advanced plant flowering and leaf-out, delayed leaf senescence and thus extended growing season duration. However, the influence of human population density on plant phenology depends on the regional temperature: high population density advanced plant flowering and leaf-out, delayed leaf senescence, and extended growing season duration in cold areas; however, high population density delayed plant flowering and leaf-out, advanced leaf senescence, and shortened growing season duration in cold areas. UHI effects (as measured by daily land surface temperature) alone cannot explain the overall influence of urbanization on plant phenology, suggesting that urbanization also affects plant phenology via other mechanisms. Our study provides strong empirical evidence that the influence of urbanization on plant phenology varies with regional temperature. Therefore, robust understanding and accurate prediction of phenological changes must take this interaction into account.