Comparison of the USO and CAP stomatal control approaches using JSBACH

The land surface model JSBACH may be modified to use different stomatal control functions to determine the exchange of carbon dioxide and water between vegetation and its environment. These different functions yield different predictions for gross photosynthetic production (GPP), evapotranspiration (ET) and canopy conductance on a given grid cell, which can then be compared to observations by hindcasting. In this study, prediction performances of several stomatal models based on the CAP optimisation hypothesis are compared to that of the more widely used Unified Stomatal Optimisation (USO) model. The CAP hypothesis implies a direct downregulation of photosynthesis with decreasing leaf water potential, thus coupling water transport in the xylem with carbon dioxide assimilation in the leaves. The hindcasting is done using eddy covariance data for boreal Scots pine (Pinus sylvestris) stands from two sites in Finland (Hyytiälä and Sodankylä). Preliminary results indicate that while performance regarding GPP and ET is similar, CAP-based models give a higher canopy conductance that is more sensitive to drought.