Decoupling Trends between Peak Photosynthesis Timing and Peak Greenness Timing

The peak photosynthesis timing (PPT) is a key factor that affects the seasonality of the terrestrial carbon uptake. Carbon phenology derived from gross primary production (GPP) has been used to validate the peak greenness timing (PGT) from satellite-based vegetation indices (VIs) in phenology research. However, PPT, derived from GPP, has not been comprehensively analyzed, especially taking different GPP estimates, fitting methods, and biomes into account. Moreover, whether or not the PPT trend is consistent with the reported PGT trend still unclear. We explored above questions at widely used flux sites and found that .no significant differences in PPT derived from GPP using different carbon flux partition methods. Moreover, fitting methods performed well in grassland, cropland, wetland, and wood savannas compared with evergreen needleleaf forest, deciduous broadleaf forest, and mixed forest. Unexpectedly, we did not find an advancing trend in PPT derived from GPP compared with PGT from SPOT-VGT NDVI. Our study suggests that the principle of the fitting method and physiological property of the biome should be taken into account when predicting PPT. More importantly, PGT is not a good proxy of the PPT. Therefore, PPT trends based on VIs should be viewed with caution. In general, this study is meaningful for better understanding photosynthesis and carbon cycling in the context of changing climate