Estimating maximum mean canopy stomatal conductance for use in models
2001
Ewers, B.E. | Oren, R. | Johnsen, K.H. | Landsberg, J.J.
Fertilized (F) and irrigated and fertilized (IF) stands of Pinus taeda L. produced twice the leaf area index of irrigated (I) and control (C) stands. Based on sap flux-scaled mean stomatal conductance (G(S)), we found that stomatal conductance in F was half that in other treatments. During the growing season, G(S) was related to vapor pressure deficit (D) and soil moisture. During the cooler season, soil moisture was high and light accompanied D in controlling G(S). Under all conditions and treatments, the rate of decrease in G(S) with D was proportional to G(S) at low D (= 1 kPa). We evaluated whether G(S) can be used as an input to growth models and used a simple growth model (3-PG), which also predicts stand transpiration (E(C)), to compare with direct E(C) measurements in the four stands. Model predictions of monthly E(C) based on Penman-Montieth equation parameterized with maximum G(S) (G(Smax)) estimated under highest "native" soil moisture (0.07 m3.m-3) produced long-term values within 10% of measured E(C). When the model was parameterized with G(Smax) estimated under experimentally raised soil moisture, or with porometrically measured conductance E(C) values were consistently overpredicted from 12 to 33%. Thus, sap-flux scaled mean canopy stomatal conductance obtained under non limiting light conditions, low D, and highest native soil moisture, is the most appropriate parameter value for certain single-leaf type of models.
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