Validation of a canopy gas exchange model and derivation of a soil water modifier for transpiration for sugar maple (Acer saccharum Marsh.) using sap flow density measurements

Hourly plot-level transpiration measurements were carried out in a stand of sugar maple (Acer saccharum Marsh.) near Quebec City, Canada, during one summer using radial flow meters installed in selected trees. The measurements were used to validate transpiration estimates obtained from a multi-layer gas exchange model that included the modeling of the within-canopy radiation regime and the thermal balance of the leaves. The comparison between modeled and measured hourly transpiration showed no bias in the predictions, and an r(2) value of 0.80. Because of the close coupling between transpiration and photosynthesis built in the model, these results suggest that modeled estimates of photosynthesis should also be well-related to actual rates at the stand-level. The transpiration data were also used to adjust an empirical transpiration model based on the Penman-Monteith equation in which the canopy conductance term was replaced by a function of global radiation, vapor pressure deficit and soil water depletion. This empirically-fitted model captured 85% of the variation observed in the data, including the effect of soil water depletion during a late-season drought. The soil water modifier included within this equation was compared with other soil water modifier functions obtained from the literature. The comparison highlights two difficulties in the derivation of soil water modifiers applicable outside experimental areas. The first is in the determination of rooting depth so that mass balances of soil water content can be carried out. The second is in the determination of soil physical properties so that absolute values of moisture contents can be translated into relative values of water availability. (C) 2002 Elsevier Science B.V. All rights reserved.