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

ABSTRACT

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 b ₆ f (Cyt b ₆ f ) 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 b ₆ f activity ( V _{q max} ) and the fraction of total leaf absorbance allocated to photosystem II (PSII) ( β ₂). 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 b ₆ f , 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 R ² 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 _{q max} and β ₂ strongly influence GPP simulations, while β ₂ 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.