EFFECTS OF SHORT-TERM VARIATION IN WEATHER ON DIURNAL CANOPY CO2 FLUX AND EVAPOTRANSPIRATION OF A JUVENILE DOUGLAS-FIR STAND

Using a modified Bowen ratio method, measurements of canopy energy balances and CO2, flux densities (Fc) of a 22-year-old low productivity Douglas-fir stand, were made successfully at a remote site, during wet and dry growing seasons. Canopy conductances (gc) were calculated from canopy evapotranspiration rates (E) using the Penman-Monteith equation, and used to estimate mean sink level intercellular CO2 concentration (ci). Canopy responses of gc, E1 Fc and ci were related to variations in weather conditions. As observed in other work, gc was found to be strongly influenced by variation in vapour pressure deficit (D) caused by changes in air temperature. Night-time Fc measurements indicated that canopy respiration rates were often very high and also strongly dependent on air temperature. Hence air temperature was concluded to be a major factor limiting overall forest productivity. Variation in solar irradiance (S) appeared to have little direct effect on gc, while no simple relationship was apparent between S and Fc at the canopy level. In general, Fc was not limited by gc although these two variables were often correlated. Highest Fc and canopy water use efficiency ratios were observed on cloudy days with low air temperature and low D, suggesting maximum productivity occurred under these conditions. © 1990.