Implementing a Plant Hydraulics Parameterization in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) v.1.4

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Abstract Drought conditions cause stress to terrestrial ecosystems and make their accurate representation in models challenging. The Canadian LAnd Surface Scheme Including biogeochemical Cycles (CLASSIC) employs an empirical approach to link soil moisture stress with stomatal conductance. Such approaches typically perform poorly during drought. Here, we implemented an explicit plant hydraulics parameterization, that is, Stomatal Optimization based on Xylem hydraulics (SOX), in CLASSIC, thereby connecting the soil‐plant‐atmosphere continuum through plant hydraulic traits. The resulting was evaluated against the carbon and water fluxes measured with eddy covariance at eight North American boreal forest flux tower sites. Compared to CLASSIC, better simulated gross primary productivity (GPP) at all sites (increased 0.51 to 0.59; decreased RMSE 1.85 to 1.54 g C and bias −0.99 to −0.57 g C ). During droughts, identified using the Palmer drought severity index (PDSI), GPP simulated with improved compared to CLASSIC. In contrast, generally overestimated evapotranspiration (ET) compared to CLASSIC ( increased 0.61 to 0.64, but RMSE and bias worsened 0.54–0.78 mm and 0.09–0.32 mm , respectively). The overestimated ET may be due to evaporation parameterization, parameter uncertainties, or SOX limitations. Wood density was used to derive plant hydraulic parameters, enabling a parameter‐parsimonious implementation. This explicit plant hydraulics parameterization in CLASSIC will improve its ability to predict ecosystem responses to increasing frequency and severity of drought in boreal forests. Future work aims to refine the simulated evapotranspiration while retaining the improved GPP drought response.

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