Accelerating urban warming effects on the spring phenology in cold cities but decelerating in warm cities

Urban is regarded as the ideal natural laboratory for predicting vegetation growth response to future warming, as urban warming promotes the growth of urban vegetation and advances spring phenology. However, the effects of daytime and nighttime warming on the spring phenology of natural vegetation remain unclear under global warming. Here, we used urbanization intensity (UI) as a proxy for future warming, then investigated the start dates of the growing season (SOS) response to daytime and nighttime warming across different levels of UI. We found both nighttime and daytime temperature sensitivities increased significantly from rural to urban centers in cold cities, decreasing in warm cities. It indicated that the advancement in spring phenology of urban vegetation accelerates along with UI in cold cities but slows down in warm cities. Because in cold cities, urban warming and increased humidity accelerate vegetation growth, while in warm cities urban warming and decreased humidity decelerate growth. Therefore, in future we should consider not only the asymmetrical responses of urban vegetation spring phenology to daytime and nighttime warming, but also the disparate responses in different background climate regions under future warming.