Assessing the wavelength-dependent ability of solar-induced chlorophyll fluorescence to estimate the GPP of winter wheat at the canopy level

Recent studies have demonstrated that solar-induced chlorophyll fluorescence (SIF) can offer a new way for directly estimating the terrestrial gross primary production (GPP). The main objective of this study is to investigate whether the red or far-red SIF is a better indicator of GPP using both simulations by the SCOPE model (Soil Canopy Observation, Photochemistry and Energy fluxes) and the observations of winter wheat at the canopy level. The results showed that: (1) both far-red SIF and GPP increased with leaf area index (LAI), whereas the red SIF quickly reached its saturation with an LAI value of 2 due to the strong reabsorption effect; (2) the diurnal GPP could be robustly estimated from the SIF spectra for winter wheat at each growth stage, whereas the correlation weakened greatly at the red band if all the observations made at different growth stages or all the simulations with different LAI values were pooled together - a situation that did not occur at the far-red band; (3) the SIF-based GPP models derived from the 2016 observations were well validated using the data set from 2015, with a root mean square error (RMSE) value of 0.128 and 0.133 (mg m(-2) s(-1)) at the oxygen-A (O-2-A) band and oxygen-B (O-2-B) band, respectively. Therefore, the far-red SIF may be more reliable for mapping GPP for remote-sensing applications with heterogeneous and diverse vegetation growth conditions.