Phenological responses to urban heat and light in the Greater Seoul area 

Vegetation phenology is a critical indicator of terrestrial carbon and water dynamics; therefore, it is essential to understand its sensitivity to the changing climates, especially to rising temperatures. A wide range of phenological responses can be explored in urban areas where anthropogenic activities elevated temperatures as well as artificial lights. In this study, we analyzed one decade (2012 - 2021) of data on surface temperature, phenology, and land cover from the Moderate Resolution Imaging Spectroradiometer (MODIS) and artificial lights at night (ALAN) data from the Visible Infrared Imaging Radiometer Suite (VIIRS) over the capital of South Korea and its surroundings. We calculated urban cover fractions (UCF) and estimated the long-term trends of the start of the season and the end of the season (ΔSOS and ΔEOS) and the temperature during the SOS and EOS (ΔTSOS and ΔTEOS) and in the ALAN (ΔALAN). We then investigated how much of those factors (i.e., ΔT, ΔALAN, and UCF) contributed to the ΔSOS and ΔEOS and explored temperature sensitivities of the ΔSOS and ΔEOS under different conditions. We found that the SOS appeared to advance by three days per decade (p = 0.068), while the EOS was delayed significantly (p < 0.001) by four days per decade over most of the study region. We also noticed that 70% of ΔSOS was primarily attributed to UCF, while ΔTSOS drove the rest. For the ΔEOS, the influence of ΔALAN appeared to be substantial (26%), with ΔTEOS having a similar contribution level (31%). We show that the temperature sensitivity of ΔSOS is higher by 4.5% in the highly urbanized areas, and the temperature sensitivity of ΔEOS is 4.3% higher in the regions with increasing lights. Our results suggest that vegetation phenological response to the increasing temperature would become complicated with the increases of urban-like conditions (i.e., higher levels of CO2 concentration), therefore highlighting the importance of further studies for a better understanding of terrestrial vegetation responding to the changing climates. This study is supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (2020R1C1C1014886 and 2022R1C1C2009543), the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure, and Transport (22CTAP-C163540-02), and the Korea Environment Industry & Technology Institute (KEITI) funded by Korea Ministry of Environment (2022003640002).