Controls of Evapotranspiration and CO2 Fluxes from Scots Pine by Surface Conductance and Abiotic Factors

Evapotranspiration (E) and CO2 flux (Fc) 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, Fc, surface conductance (gc), and decoupling coefficient (V), showing similar trends to those in radiation (PAR) and vapour pressure deficit (d). The maximum mean daily values (24-h average) for E, Fc, gc, and V were 1.78 mmol m 22 s21, 211.18 mmol m22 s21, 6.27 mm s21, and 0.31, respectively, with seasonal averages of 0.71 mmol m22 s21,24.61 mmol m22 s21, 3.3 mm s21, and 0.16. E and Fc were controlled by combined biological and environmental variables. There was curvilinear dependence of E on gc and Fc on gc. Among the environmental variables, PAR was the most important factor having a positive linear relationship to E and curvilinear relationship to Fc, while vapour pressure deficit was the most important environmental factor affecting gc. Water use efficiency was slightly higher in the dry season, with mean monthly values ranging from 6.67 to 7.48 mmol CO2 (mmol H2O) 21 and a seasonal average of 7.06 mmol CO2 (mmol H2O) 21. Low