Direct estimation of photosynthetic CO2 assimilation from solar-induced chlorophyll fluorescence (SIF)

Much progress has been made in predicting terrestrial gross primary productivity (GPP) from solar-induced chlorophyll fluorescence (SIF). However, SIF-GPP relationships are mostly built by statistically relating top-of-canopy (TOC) SIF observations (SIFTOC) to eddy covariance flux-tower GPP estimates. We developed a process-based model, based on the mechanistic light response (MLR) model, to mechanistically link SIFTOC with the photosynthetic activity of vegetation. To apply this mechanistic model at the canopy scale, we 1) reformulate the equations by replacing the fraction of open PSII reaction centers (qL) and the maximum quantum yield of photosystem II (ФPmax) with nonphotochemical quenching (NPQ) and the quantum yield of photosystem II (ΦP); 2) reconstruct hemispherical broadband SIF fluxes at photosystem II (PSII) from the directional observed SIFTOC that is contributed from photosystem I and II; 3) estimate other key parameters including KDF (ratio between the rate constants for constitutive heat loss and fluorescence), Cc (chloroplastic CO2 partial pressure), and Γ* (chloroplastic compensation point of CO2) at the canopy scale based on assumptions and in-situ measurements. A comparison against flux-tower based GPP at a winter-wheat study site, demonstrates that the modeled GPP, driven by SIFTOC at 760 nm, air temperature, incoming photosynthetically active radiation (PAR), and directional reflectance in the red and near-infrared region, is able to quantify canopy photosynthesis with good accuracy at both half-hourly (R2 = 0.85, RMSE = 5.62 μmol m−2 s−1, rRMSE = 9.10%) and daily (R2 > 0.90, RMSE = 3.25 μmol m−2 s−1 and rRMSE = 8.69%) scales. The present model enhances our ability to mechanistically estimate GPP with SIF at the canopy scale, an essential step to model carbon uptake using satellite SIF at regional and global scales.