Spatial variations in the response of spring onset of photosynthesis of evergreen vegetation to climate factors across the Tibetan Plateau: The roles of interactions between temperature, precipitation, and solar radiation

Phenological changes of evergreen vegetation affect ecosystem functions and land-atmosphere exchanges. Although recent studies have documented changes in the spring onset of photosynthesis derived from satellite solar-induced chlorophyll fluorescence (SOSSIF) of evergreen vegetation in the Northern Hemisphere, spatial variations in the response of the onset of spring photosynthesis to climate change remain poorly characterized. Using a continuous solar-induced chlorophyll fluorescence dataset, we found that SOSSIF advanced in more than 70.0% of surveyed areas with evergreen vegetation, represented as pixels and spread widely across the Tibetan Plateau. Warming temperatures contributed to advanced SOSSIF in more than 42.0% of surveyed areas, mainly in the southeast and areas between 90 and 97 °E. Increasing precipitation contributed to advanced SOSSIF in about 15.0% of surveyed areas, mainly along the southeastern edge of the plateau. A negative partial correlation between SOSSIF and temperature (RP(SOSSIF,T)) was observed in more than 65.0% of surveyed areas, and stronger negative RP(SOSSIF,T) was found in areas with a larger positive interannual correlation between preseason temperature and precipitation, likely due to the better match between favorable temperature and water conditions in these areas. The positive RP(SOSSIF,T) was likely associated with low water availability. The partial correlation between SOSSIF and precipitation (RP(SOSSIF,P)) was spatially diverse, with negative RP(SOSSIF,P) in about half of surveyed areas. Stronger negative RP(SOSSIF,P) was observed in areas with a stronger negative interannual correlation between preseason precipitation and solar radiation, which was probably caused by the trade-off between water and light availability to maximize the benefits from precipitation. The positive RP(SOSSIF,P) was likely due to the lack of solar radiation. This study provides new explanations for spatial variations in the response of SOSSIF to temperature and precipitation, contributing to assessments of vegetation phenology and global carbon cycle modeling.