Controls of Evapotranspiration and CO(2) Fluxes from Scots Pine by Surface Conductance and Abiotic Factors

Evapotranspiration (E) and CO(2) flux (F(c)) in the growing season of an unusual dry year were measured continuously over a Scots pine forest in eastern Finland, by eddy covariance techniques. The aims were to gain an understanding of their biological and environmental control processes. As a result, there were obvious diurnal and seasonal changes in E, F(c), surface conductance (g(c)), and decoupling coefficient (Ω), showing similar trends to those in radiation (PAR) and vapour pressure deficit (δ). The maximum mean daily values (24-h average) for E, F(c), g(c), and Ω were 1.78 mmol m(−2) s(−1), −11.18 µmol m(−2) s(−1), 6.27 mm s(−1), and 0.31, respectively, with seasonal averages of 0.71 mmol m(−2) s(−1), −4.61 µmol m(−2) s(−1), 3.3 mm s(−1), and 0.16. E and F(c) were controlled by combined biological and environmental variables. There was curvilinear dependence of E on g(c) and F(c) on g(c). Among the environmental variables, PAR was the most important factor having a positive linear relationship to E and curvilinear relationship to F(c), while vapour pressure deficit was the most important environmental factor affecting g(c). Water use efficiency was slightly higher in the dry season, with mean monthly values ranging from 6.67 to 7.48 μmol CO(2) (mmol H(2)O)(−1) and a seasonal average of 7.06 μmol CO(2) (μmol H(2)O)(−1). Low Ω and its close positive relationship with g(c) indicate that evapotranspiration was sensitive to surface conductance. Mid summer drought reduced surface conductance and decoupling coefficient, suggesting a more biotic control of evapotranspiration and a physiological acclimation to dry air. Surface conductance remained low and constant under dry condition, supporting that a constant value of surface constant can be used for modelling transpiration under drought condition.