Vegetation spring green-up has shifted earlier as a response to recent warming trends in the Northern Hemisphere. Changes in the timing and duration of spring green-up (i.e., start of season, (SOS)) have significant influences on forest ecosystems through changes in biogeochemical processes and land surface properties. Together, these changes influence the regional and global-scale cycling of carbon, water, and nutrients of terrestrial ecosystems. Besides climate warming, spring green-up of urban trees is also influenced by the modified urban environment. For example, urban heat island effect, characterized by elevated air temperature in urban areas compared to surrounding rural areas, has significant impacts on vegetation phenology. The objective of this study was to characterize spatiotemporal changes of spring green-up in major cities in the United States (U.S.) to understand the influence of climate change and urbanization on spring green-up.
Based on impervious surface area (ISA) dataset and nighttime light derived urban extent at U.S. cities, buffers within each study city were designed to represent urbanization levels. ISA, SOS, and temperature within each buffer were explored for understanding the spatial pattern of SOS and the influence of temperature on SOS along urban-rural gradients, using Moderate Resolution Imaging Spectroradiometer (MODIS) phenology products and climate datasets from TopoWx (Topography Weather).
Results/Conclusions
We found a significant negative correlation between SOS and spring temperature (January-April) along urban-rural gradients for most study cities. Compared to rural areas, spring temperature became warmer at urban areas and SOS shifted earlier. Spring temperature and SOS showed rising and earlier trends during the period 2001-2014. Furthermore, the magnitude of the SOS responses to urbanization and warming temperature showed large regional variations. These findings provide the observational evidence of spring phenology changes in cities. Although cities are often used as natural “laboratories” for evaluating the response of phenology to warming, our study suggests that the urban phenology responses to warming are highly uncertain due to the complex urban environments, and an improved understanding of the mechanisms of spring air temperature on phenology is needed in future studies.