Dynamic changes and driving factors of land surface phenology under the background of urbanization

Rapid urbanization would have significant impacts on vegetation phenology. However, the factors influencing the spatiotemporal changes in urban vegetation phenology are still unclear. We used five fitting methods to construct normalized difference vegetation index (NDVI) curves in the Beijing-Tianjin-Hebei urban agglomeration, and obtained the phenology characteristics of urban vegetation in this area from 2001 to 2019 by the threshold method. We compared the spring and autumn phenology in urban built-up areas and hilly areas, and analyzed the effects of precipitation, air temperature, and land surface temperature (LST) on vegetation phenology. The results showed that from 2001 to 2019, the start of the growing season (SOS) in urban built-up areas in the Beijing-Tianjin-Hebei agglomeration was on average 16.88 days earlier than that in hilly areas, and that the end of the growing season (EOS) in urban built-up areas was 12.22 days later than that in hilly areas. During the study period, the SOS of vegetation in urban built-up areas of the Beijing-Tianjin-Hebei region had been gradually delayed, while that in hilly areas was gradually advanced, and the rate of change of phenology in the urban built-up areas was faster than that in the hilly areas. The difference between the SOS of the two areas decreased significantly over time (-0.58 d·a^-1). As for the EOS, the urban built-up areas and hilly areas both showed a trend of delayed, but the differences between them was not significant with time (-0.10 d·a^-1). The contribution of LST in the urban built-up areas to SOS was close to that of air temperature, while the contribution of LST in hilly areas to SOS was only 1/2 of that of air temperature, indicating that the heat island effect and air temperature within the city jointly influenced urban vegetation phenology, and their contributions were almost equal. The results could help understand the role of urbanization in the variations of vegetation phenology and provide a reference for further assessment of carbon sink potential of urban vegetation.