title: Controls of evapotranspiration and CO2 fluxes from scots pine by surface conductance and abiotic factors.
abstract: 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 (Ω), showing similar trends to those in radiation (PAR) and vapour pressure deficit (δ). The maximum mean daily values (24-h average) for E, Fc , gc , 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 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 μmol CO2 (mmol H2O)(-1) and a seasonal average of 7.06 μmol CO2 (μmol H2O)(-1). Low Ω and its close positive relationship with gc 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.
authors.affiliation: The School of Soil and Water Conservation, Beijing Forestry University, Beijing, China. tianshanzha@bjfu.edu.cn
authors.forename: Tianshan
authors.initials: T
authors.lastname: Zha
authors.forename: Chunyi
authors.initials: C
authors.lastname: Li
authors.forename: Seppo
authors.initials: S
authors.lastname: Kellomäki
authors.forename: Heli
authors.initials: H
authors.lastname: Peltola
authors.forename: Kai-Yun
authors.initials: KY
authors.lastname: Wang
authors.forename: Yuqing
authors.initials: Y
authors.lastname: Zhang
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D055864:Droughts; D041605:Pinus sylvestris / Q000378:metabolism* / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide
references.citation: Tree Physiol. 2008 Feb;28(2):265-76
references.pmid: 18055437
references.citation: Trends Ecol Evol. 1993 Aug;8(8):289-94
references.pmid: 21236171
references.citation: Ann Bot. 2010 Mar;105(3):431-42
references.pmid: 19995810
references.citation: Tree Physiol. 2004 Jan;24(1):19-34
references.pmid: 14652211
references.citation: New Phytol. 2002 Mar;153(3):485-496
references.pmid: 33863220
references.citation: Tree Physiol. 2003 Feb;23(3):145-55
references.pmid: 12566265
references.citation: Tree Physiol. 2004 May;24(5):561-9
references.pmid: 14996660