EFFECTS OF SUMMERTIME CHANGES IN WEATHER AND ROOT-ZONE SOIL-WATER STORAGE ON CANOPY CO2 FLUX AND EVAPOTRANSPIRATION OF 2 JUVENILE DOUGLAS-FIR STANDS

Variations in soil water storage (W), canopy evapotranspiration (E) and net CO2 flux density (F(c)) of two low productivity stands of Douglas-fir (12 and 22 years of a age), were studied during two consecutive summers. Measurements of E and F(c) were obtained using a modified Bowen ratio/energy balance method. The factors influencing F(c) were considered in the light of recent work by Farquhar and coworkers, while those affecting E were examined in the context of the McNaughton-Jarvis OMEGA parameter. The findings were as follows. (1) Daytime E and F(c) were generally lower in the younger stand, even though W was higher. These results were attributed mainly to lower leaf area index in the younger stand. (2) In the older stand, F(c) was significantly reduced by low soil water potential (PSI(s)) within the range of soil water storages at which measurement were made, while canopy conductance was less dependent on PSI(s). Following Farquhar and Sharkey, consideration of changes in estimated intercellular CO2 concentration at the canopy mean sink level (C(i)), indicated that reductions in F(c) were non-stomatal in origin, i.e. due to reduced foliar photosynthetic capacity rather than reduced stomatal conductance. (3) Evapotranspiration in the older stand was less related to net irradiance and more dependent on the atmospheric saturation deficit (D) than in the young stand, in accordance with the predictions of McNaughton and Jarvis. As W decreased, calculated values of OMEGA and of the Priestley and Taylor alpha parameter also declined, showing greater stomatal sensitivity at low PSI(s).