Improving GPP and SIF Simulation With a Mechanistic Photosynthesis Model Integrated Into the BEPS Framework

Accurate representation of plant photosynthesis in terrestrial biosphere models (TBMs) is critical for reliable carbon-cycle simulations. Most TBMs employ the Farquhar-von Caemmerer-Berry (FvCB) model, which uses an empirical representation of electron transport that limits the simulation of gross primary productivity (GPP) and solar-induced chlorophyll fluorescence (SIF) under variable conditions. Here, we present BEPS-CB6F, an improved model that incorporates the mechanistic cytochrome b₆f (Cyt b₆f) scheme (CB6F) of Johnson and Berry into the Biosphere-atmosphere Exchange Process Simulator (BEPS). This implementation replaces empirical formulations with a process-based energy-allocation framework and links GPP and SIF through shared physiological parameters, including the maximum Cyt b₆f activity (V_qmax) and the fraction of total leaf absorbance allocated to photosystem II (PSII) (β₂). BEPS-CB6F also integrates key photoprotective processes, including cyclic electron flow around photosystem I (CEF), non-photochemical quenching of photosystem II (NPQ), and photosynthetic control of Cyt b₆f, within a two-leaf canopy scheme that differentiates sunlit and shaded responses. The results show that across flux-tower sites, BEPS-CB6F substantially improves SIF simulations, with RMSE and rRMSE reductions at more than 90% of sites, and yields moderate but consistent improvements in GPP, including higher R² and reduced RMSE at over 80% of sites. The model alleviates GPP overestimation under low light, particularly in shaded leaves, and markedly reduces SIF overestimation under high irradiance in sunlit leaves. BEPS-CB6F further enhances performance during heat and high-VPD conditions. By explicitly representing temperature-responsive CEF and NPQ, it captures the strong midday suppression of GPP and SIF, including reductions in GPP and SIF during heatwaves. Sensitivity analyses indicate that V_qmax and β₂ strongly influence GPP simulations, while β₂ is the primary driver of SIF simulations. These results highlight the importance of mechanistic electron-transport representation and demonstrate the potential of CB6F to improve terrestrial biosphere model predictions of carbon uptake and fluorescence.