Evaluation of Leaf-To-Canopy Upscaling Approaches for Simulating Canopy Carbonyl Sulfide Uptake and Gross Primary Productivity

Carbonyl sulfide (COS) measurements provide an important mechanism for quantifying the terrestrial carbon and water cycles. Terrestrial photosynthesis can be inferred from vegetative uptake of COS due to the shared diffusion pathway of COS and CO2 into the chloroplasts. However, the efficacy of different leaf-to-canopy upscaling approaches for estimating plant COS uptake and photosynthesis at the canopy scale remains uncertain. In this study, the daytime canopy COS uptake rate (Fcosc) and gross primary productivity (GPP) were simulated using big leaf (BL), two big leaf (TBL) and two leaf (TL) upscaling approaches in the Boreal Ecosystem Productivity Simulator terrestrial biosphere model for two forest sites. We found that TL and TBL performed better than BL in estimating GPP as evaluated by the eddy covariance derived GPP at the studied sites. BL overestimated GPP and canopy stomatal conductance to COS (Gs_cos), resulting from the overestimation of canopy intercepted solar radiation. The positive biases of GPP by TBL could be attributed to the positive biases in GPP of shaded leaves by TBL. The results in this study demonstrate the improved accuracy in using TL for estimating GPP at the canopy scale. Improved leaf-to-canopy upscaling approach in terrestrial biosphere models will improve the accuracy of canopy COS uptake and GPP estimation on regional to global scales. Over the past two decades, measurements of carbonyl sulfide (COS) concentrations and fluxes have emerged as an important tool for investigating terrestrial carbon and water cycles. COS and CO2 diffuse from the atmosphere into chloroplasts in leaves through a shared pathway and terrestrial photosynthesis can be inferred from vegetative uptake of COS. However, the efficacy of different leaf-to-canopy upscaling approaches for estimating the vegetation COS uptake at the canopy scale remains uncertain. In this study, the daytime canopy COS uptake rate, gross primary productivity (GPP) and canopy stomatal conductance were simulated using big leaf (BL), two big leaf (TBL) and two leaf (TL) upscaling approaches in the Boreal Ecosystem Productivity Simulator terrestrial biosphere model for two forest sites. We found that TL and TBL performed better than BL in estimating GPP and canopy stomatal conductance to COS. Improved leaf-to-canopy upscaling approach in terrestrial biosphere models will improve the accuracy of canopy COS uptake and GPP estimation on regional to global scales. Big leaf (BL) overestimated gross primary productivity (GPP) and canopy conductance to carbonyl sulfide (COS) due to its overestimation of the instantaneous canopy intercepted solar radiation Two leaf and two big leaf performed better than BL in estimating GPP and canopy conductance to COS at the studied sites The biases in GPP estimates by BL decreased with increasing leaf area index