Urbanization has caused significant environmental changes, such as urban heat island. These changes can significantly affect terrestrial ecosystems. For example, vegetation phenology, a sensitive indicator of the biological impact of environmental changes, has been altered in cities. However, to what extent does urban warming affect spring phenology remains unclear. More importantly, urban heat island is characterized by a stronger magnitude at night than that during the daytime, and how does such asymmetric diurnal warming in cities affect spring phenology is unexplored. The objective of this study was to investigate the responses of spring phenology to temperature in 85 large cities in the United States. Based on impervious surface area and urban extent, buffers for each study city were derived to represent urban-rural gradients. The changes of spring leaf-out and temperature across buffers were analyzed and their causal relationships were investigated using partial correlation analysis. Phenological models were also used to explore the underlying mechanisms of the phenological responses.
Results/Conclusions
We found that temperature significantly increased and spring leaf-out prominently advanced from rural to urban areas for most study cities (P < 0.05). The changes in spring leaf-out were significantly correlated with temperature but the magnitude of correlation showed large regional variations. Moreover, the phenology showed distinct responses to daytime temperature and nighttime temperature. These findings provide the observational evidence of spring phenology changes in cities. It suggests that the divergent phenological responses to asymmetric warming need to be considered for an improved prediction of future phenology and better representation of land–atmosphere interactions in Earth system models.