Impact of high temperature in 2023 and 2024 on spring leaf flush phenology in Japan derived by GCOM-C satellite.

In Japan, the exceptionally high temperatures in 2023 and 2024 may have impacted vegetation phenology. We investigated the timing of the spring leaf flush (SOS) of non-evergreen plants using satellite remote sensing. We utilized data from the SGLI sensor on JAXA's GCOM-C satellite. Compared to NASA's MODIS sensors, which are widely used for phenology studies, GCOM-C/SGLI offers two advantages: more stable orbital control in recent years and a higher spatial resolution (250 m) in the green channel which allows to calculate the Chlorophyll Carotenoid Index (CCI), a more robust index than NDVI against snowmelt. We defined the onset of SOS as the point where CCI exceeds zero during spring, and estimated SOS over seven years (2018-2024) in Japan, validated with ground-based time-lapse camera data. On average (2018-2022), SOS tended to occur later in northern or inland mountainous areas. Compared to the average, SOS in 2023 and 2024 occurred earlier in most of northern Japan. Specifically, an earlier SOS by 3 to 7 days occurred in the southern Tohoku, Kanto, and Chubu in 2023, and across the entire Tohoku and Hokuriku in 2024. These responses of SOS to temperature anomalies align with previous studies on future phenology in Japan.