Humidity of air is a key environmental variable in controlling stomatal aperture of plant leaves. In current land surface schemes the stomatal conductance-humidity relationships are mostly based on the models proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus-Nijhoff Publ., pp. 221-224) and by Leuning (1995, Plant, Cell & Environment, 18, 339-355). Results of independent evaluations on the two models are in disagreement. In this study, we developed a quantitative approach that is based on canopy latent heat and CO₂ fluxes to assess the response of canopy stomatal conductance to humidity. Using the eddy-covariance flux measurements and the EALCO model simulations at three boreal forest sites in Canada, we critically examined the performance of the Ball-Woodrow-Berry model and the Leuning model.

Results/Conclusions

Our results show that the BWB model which employs linear relationship between stomatal conductance and relative humidity resulted in large bias when humidity was high. The Leuning model which employs hyperbolic function of water vapour pressure deficit (VPD) reduced this bias, but it still could not adequately capture the remarkable increase of stomatal conductance at high humidity. Two new models that are based on the power functions of relative humidity deficit and VPD were proposed to improve the accuracies in predicting the canopy stomatal response to humidity. Our results also indicate that models based on VPD unanimously performed better than those based on relative humidity, consistent with the hypothesis that stomatal aperture responds to the leaf water loss since VPD rather than relative humidity directly drives the transpiration rate of canopy leaves. The improvements of the new models are expected to be more significant to ecosystems with high humidity conditions.