STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS IN A MATURE SCOTS PINE FOREST .3. VARIATION IN CANOPY CONDUCTANCE AND CANOPY PHOTOSYNTHESIS

Water loss from coniferous plantations may lead to water shortages in regions of scarce water supply. To model growth and transpiration from a forest canopy over a year or more, it is necessary to known how conductance and photosynthesis of the entire canopy changes with season and weather. The stomatal conductance and photosynthesis of current-year shoots of Scots pine (P. sylvestris) growing in eastern England [UK] were measured at different levels in the canopy throughout a year. Canopy conductance and canopy photosynthesis were calculated for the hours of measurement. The diurnal behavior of canopy conductance and canopy photosynthesis was substantially the same as that of the shoots. Maximum values of canopy conductance (1.6 cm s-1) occurred in Sept. and minimum values after mid-day in July and Aug. Maximum rates of canopy photosynthesis (> 1.2 mg m-2 s-1) occurred in Oct. The major determinant of canopy conductance during the growing season was water vapor saturation deficit, except during periods when air temperature was less than 10.degree. C or incident quantum flux density was low, particularly at dawn and dust. Canopy conductance declined sensitively with saturation deficit between 0.5 and 1.5 kPa [pascal] but then remained constant at larger deficits. Stomatal closure at large saturation deficits had a major influence on canopy conductance in July and Aug., in spite of increasing leaf area index as the new shoots elongated during this period. Rate of canopy photosynthesis was strongly dependent on the quantum flux density except at high temperatures and large saturation deficits when canopy photosynthesis was also limited by canopy conductance. The response of canopy photosynthesis to light incident above the canopy was linear except at both high light and large saturation deficits when hysteresis occurred. The relationship between canopy photosynthesis and canopy conductance was curvilinear at large saturation deficits but independent of conductance at small deficits.