Comparison of the capability and performance of “photosynthesis” and “structure” indices in retrieving vegetation phenology in the Northern Hemisphere

Phenology refers to the natural responses of vegetation to environmental factors and disturbances throughout its life cycle. Focusing on the mid-high latitudes vegetation in the Northern Hemisphere, this study reported the differences in phenology extracted using “photosynthesis” and “structure” indices, specifically solar-induced chlorophyll fluorescence (SIF) and leaf area index (LAI), respectively. We analyzed the dynamics of phenology inverted by different indices and validated the inversion accuracy using data from FLUXNET2015 gross primary productivity (GPP). The results revealed that: (1) Both the start of the growing seasons (SOS) and the end of the growing seasons (EOS) showed significant advancing trend. On average, the SOS derived from SIF (143 days) was later than that derived from LAI (136 days), whereas the EOS derived from SIF (236 days) occurred earlier than that from LAI (245 days); (2) Across all four climatic zones, phenology was dominated by an advancing trend, with the rate of phenological change derived from SIF generally faster than that from LAI. Distinct differences in phenological trends were observed among various vegetation types; (3) Phenological parameters extracted using SIF were closer to actual observations than those derived from LAI, with the accuracy of SOS inversion generally higher than that of EOS for both indices. The study demonstrates that “photosynthesis” indices can produce more accurate photosynthetic phenology than “structure” indices, which provided a reference for the subsequent vegetation phenology studies.