Stomatal Conductance and Photosynthesis in a Mature Scots Pine Forest. II. Dependence on Environmental Variables of Single Shoots

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Abstract
(1) Water use by coniferous plantations may lead to water shortages in regions where water is a scarce resource. To be able to estimate transpiration in relation to assimilation of carbon over a year, it is necessary to know how stomatal conductance and CO2 uptake change with environmental variables. (2) The stomatal conductance and photosynthesis of current-year shoots of Scots pine (Pinus sylvestris) growing in eastern England were measured at different levels in the canopy and correlated with local values of the main environmental variables. (3) The major determinant of stomatal conductance during the growing season was atmospheric water vapour saturation deficit, except during periods when air temperature was less than 10 ⚬C or incident quantum flux density was low, particularly at dawn and dusk. Stomatal conductance declined sensitively with water vapour saturation deficit between 0.5 and 1.5 kPa but then remained constant at larger deficits. (4) Light-saturated values of stomatal conductance decreased with depth in the canopy but it was not possible to determine the response curve between stomatal conductance and light adequately from the data. (5) The optimum temperature for stomatal conductance was c. 20 ⚬C. (6) Stomatal conductance remained independent of water potential throughout the period of measurement, in spite of the drought in 1976. (7) Rates of shoot photosynthesis were strongly dependent on quantum flux density except at high temperatures and large deficits when photosynthesis was also limited by conductance. The response to light was curvilinear except at leaf temperatures above 25 ⚬C when the response was approximately linear. (8) Shoot photosynthesis was light-saturated below, but not above, leaf temperatures of 15 ⚬C in the upper canopy while shoots within the middle and lower levels of the canopy did not receive sufficient light to become light-saturated. (9) The relationship between shoot photosynthesis and stomatal conductance was curvilinear at large deficits but photosynthesis was independent of conductance at small deficits. (10) The optimum leaf temperature for shoot photosynthesis was between 18 and 22 ⚬C. (11) Variation of conductance and photosynthesis at particular levels of each environmental variable suggest that each may act independently. This emphasizes the need for a mathematical model to provide an adequate description of the responses to environment.