Spatiotemporal differentiation and driving mechanism of vegetation phenology in a high-latitude region of China

Photosynthetic phenology describes the seasonal timing of vegetation photosynthetic activity. Using solar-induced chlorophyll fluorescence (SIF) data from 2001 to 2023, we extracted the start of the growing season (SOS), end of the growing season (EOS), and length of the growing season (LOS) in Hulunbuir, China, and compared forestland, grassland, and cropland. The results showed that SOS and EOS mainly occurred at 120–144 and 244–256 days of year (DOY), respectively, while LOS was mainly 100–130 d. Forestland SOS was earlier than that of grassland and cropland. Under the original thresholds, forestland LOS was slightly longer than grassland LOS, although their ranking was sensitive to threshold choice. Across the region, SOS advanced by 0.07 d yr -1 , EOS was delayed by 0.04 d yr -1 , and LOS lengthened by 0.10 d yr -1 . Earlier SOS occurred in central and northern Hulunbuir, whereas later SOS and shorter LOS characterized the southwestern forest–grassland transition zone. EOS advanced in the central Greater Khingan Mountains but was delayed to the east and west; LOS lengthened in the eastern and southeastern grassland but shortened in the mountains. Forestland SOS and LOS showed less interannual variation than those of grassland and cropland, and most valid pixels had Hurst estimates below 0.5. Land use type had the highest single-factor explanatory power for spatial SOS variation, whereas annual mean maximum temperature ranked highest for EOS and LOS. After detrending, positive correlations of EOS and LOS with temperature and negative correlations with precipitation remained predominant. The direction of SOS-temperature correlations depended on the temperature metric, while the dominant SOS-precipitation correlation differed between forestland and grassland. These findings inform vegetation monitoring, carbon-sink assessment, and ecological restoration in high-latitude cold temperate regions.