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

Area of Science:

• Earth System Science
• Plant Physiology
• Ecosystem Modeling

Background:

• Terrestrial biosphere models (TBMs) are crucial for carbon-cycle simulations.
• Current TBMs often use empirical electron transport models (e.g., FvCB), limiting accuracy in simulating gross primary productivity (GPP) and solar-induced chlorophyll fluorescence (SIF).
• Accurate representation of plant photosynthesis is vital for understanding global carbon dynamics.

Purpose of the Study:

• To introduce BEPS-CB6F, an improved TBM incorporating a mechanistic cytochrome b6f (Cyt b6f) scheme (CB6F).
• To enhance the simulation of GPP and SIF by replacing empirical formulations with a process-based energy-allocation framework.
• To improve the representation of photoprotective processes and canopy light responses.

Main Methods:

• Integrated the Johnson and Berry CB6F scheme into the Biosphere-atmosphere Exchange Process Simulator (BEPS), creating BEPS-CB6F.
• Implemented a process-based energy-allocation framework linking GPP and SIF through shared physiological parameters (Vqmax, β2).
• Incorporated cyclic electron flow (CEF), non-photochemical quenching (NPQ), and photosynthetic control within a two-leaf canopy scheme.

Main Results:

• BEPS-CB6F significantly improved SIF simulations, with RMSE and rRMSE reductions at over 90% of flux-tower sites.
• Moderate but consistent improvements in GPP simulations were observed, with higher R2 and reduced RMSE at over 80% of sites.
• The model accurately captured midday suppression of GPP and SIF under heat and high vapor pressure deficit (VPD) conditions.

Conclusions:

• The mechanistic representation of electron transport via CB6F substantially enhances TBM performance in simulating GPP and SIF.
• BEPS-CB6F demonstrates improved accuracy under varying light, temperature, and humidity conditions, particularly during heatwaves.
• This mechanistic approach offers a promising pathway for more reliable predictions of terrestrial carbon uptake and fluorescence.