Carbon and Water Fluxes of the Boreal Evergreen Needleleaf Forest Biome Constrained by Assimilating Ecosystem Carbonyl Sulfide Flux Observations

Gross primary production (GPP) by boreal forests is highly sensitive to environmental changes. However, GPP simulated by land surface models (LSMs) remains highly uncertain due to the lack of direct photosynthesis observations at large scales. Carbonyl sulfide (COS) has emerged as a promising proxy to improve the representation of GPP in LSMs. Because COS is absorbed by vegetation following the same diffusion pathway as CO2 during photosynthesis and not emitted back to the atmosphere, incorporating a mechanistic representation of vegetation COS uptake in LSMs allows using COS observations to refine GPP representation. Here, we perform ecosystem COS flux and GPP data assimilations to constrain the COS- and GPP-related parameters in the ORCHIDEE LSM for boreal evergreen needleleaf forests (BorENF). Assimilating ecosystem COS fluxes at Hyytiala forest increases the simulated net ecosystem COS uptake by 14%. This increase largely results from changes in the internal conductance to COS, highlighting the need to improve the representation of COS internal diffusion and consumption. Moreover, joint assimilation of ecosystem COS flux and GPP at Hyytiala improves the simulated latent heat flux, contrary to the GPP-only data assimilation, which fails to do so. Finally, we scaled this assimilation framework up to the boreal region and find that the joint assimilation of COS at Hyytiala and GPP fluxes at 10 BorENF sites increases the modeled vegetation COS uptake up to 18%, but not GPP. Therefore, this study encourages the use of COS flux observations to inform GPP and latent heat flux representations in LSMs. Plain Language Summary Carbon uptake by boreal forests is highly sensitive to environmental changes. There is large uncertainty about how much carbon dioxide (CO2) boreal forests absorb through photosynthesis, as represented by land surface models. Carbonyl sulfide (COS), a trace gas that tracks photosynthesis, can help improve the representation of simulated plant CO2 uptake because COS and CO2 share a common pathway during leaf uptake. Using a mechanistic model of biospheric COS processes implemented in the ORCHIDEE land surface model, we assimilated ecosystem COS flux and plant CO2 uptake measured at Hyytiala boreal forest. We find that this joint assimilation improves the simulated plant CO2 uptake, as well as transpiration because of the strong link between COS, CO2 and H2O fluxes through stomatal diffusion. Scaling up this assimilation framework to evergreen needleleaf boreal forests, we find that assimilating ecosystem COS flux and plant CO2 uptake data increases the vegetation COS uptake for this biome, but not plant CO2 uptake. Our results imply that COS has the potential to constrain both plant carbon uptake and transpiration in land surface models, which should be further investigated, especially during drought events.