SPATIOTEMPORAL PATTERNS AND DRIVERS OF VEGETATION PHENOLOGY IN THE THREE-NORTH SHELTER FOREST PROGRAM REGION BASED ON SOLAR-INDUCED CHLOROPHYLL FLUORESCENCE

As an important ecological security barrier in China, studying the vegetation phenology in Three-North Shelter Forest Program (TNSFP) region is of great significance for monitoring the photosynthesis and carbon-water cycle of vegetation in northern terrestrial ecosystems under the background of global climate change.Based on the data of solar-induced chlorophyll fluorescence (SIF) from 2001 to 2023, the dynamic threshold method was used to extract the start time (SOS), the end time (EOS) and the length of growing season (LOS) of vegetation in the TNSFP region.Combined with linear trend analysis, the coefficient of variation (CV), the Hurst index (H), optimal parameter geographic detector, Pearson correlation analysis and other methods, the spatial-temporal variation characteristics and future trends of vegetation photosynthetic phenology in the TNSFP region were explored, and the driving force of spatial heterogeneity and temporal variations were analyzed.The results show that: (1) The spatial distribution of the SOS, EOS, and LOS in the western and southern parts of the study area showed early-late-long characteristics, respectively, while the eastern and northern parts showed late-early-short characteristics.(2)The SOS advanced at a rate of 0.16 d/a, the EOS delayed at a rate of 0.20 d/a, and the LOS extended at a rate of 0.26 d/a.(3) The SOS, EOS, and LOS in 23 years exhibited low fluctuation, and most areas will show a trend of delay, advanced, and shortening in the future.(4) In terms of spatial heterogeneity, elevation and precipitation are the dominant factors for the SOS, minimum temperature and maximum temperature are the dominant factors of the EOS, precipitation is the dominant factor of the LOS, and the interaction of each driving factor improves the explanatory power of spatial heterogeneity in vegetation photosynthetic phenology.(5) In terms of temporal variation, the correlations between vegetation phenology and temperature and precipitation showed obvious spatial differences.The EOS and LOS was mainly positively correlated with temperature.The results help to understand the spatiotemporal dynamic variations of vegetation photosynthetic phenology and the internal influencing mechanisms, and provide theoretical support for the construction and management of the ecological environment in the TNSFP region.