Divergent Performances of Vegetation Indices in Extracting Photosynthetic Phenology for Northern Deciduous Broadleaf Forests

Accurate estimation of photosynthetic phenology is of great importance for understanding carbon cycles. Most vegetation indices (VIs) calculated from remotely sensed reflectances represent the canopy structure and have high uncertainty in detecting the photosynthetic phenology. We compared the start/end of the photosynthetically active season (SOS/EOS) extracted from the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), the near-infrared reflectance of vegetation (NIRv), and the product of NIRv and solar incident radiation (NIRvP) over northern deciduous broadleaf forests (DBFs), and we used the metrics generated from solar-induced chlorophyll fluorescence (SIF), a proxy for photosynthesis, as reference. We found that the growing season extracted from the structural VIs was generally longer than the duration of photosynthetic activity retrieved from SIF: SOS derived from NDVI < NIRvP < EVI $\approx $ NIRv $\approx $ SIF and EOS from NDVI $>$ NIRv $\approx $ EVI $>$ NIRvP $\approx $ SIF. We investigated the mechanism underlying these phenological discrepancies using the paradigm of light-use efficiency (LUE). Our results show that the divergent performances of VIs were related to main factors limiting photosynthesis, which vary across different growth stages. The fraction of absorbed photosynthetically active radiation (FAPAR) absorbed by chlorophyll (FAPAR $_{\mathrm {chl}}$ ) that is well characterized by both EVI and NIRv, was the dominant factor of spring photosynthetic phenology, whilst NIRvP that is a proxy of the total amount of photosynthetically active radiation absorbed by chlorophyll (APAR $_{\mathrm {chl}}$ ) was the dominant factor in autumn when radiation determines photosynthetic phenology. As such, we suggest that these factors be accounted for when selecting VIs for the extraction of photosynthetic phenology, i.e., EVI and NIRv are more suitable for accurate retrieval of SOS, and NIRvP is more suitable for accurate retrieval of EOS.