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 (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.

本数据集针对芬兰东部一处欧洲赤松（Scots pine）林分，采用涡度协方差技术（eddy covariance techniques）连续监测了异常干旱年份生长季内的蒸散发（Evapotranspiration, E）与二氧化碳通量（CO₂ flux, Fc），研究旨在明晰二者的生物与环境调控过程。
监测结果显示，蒸散发E、二氧化碳通量Fc、表面导度（surface conductance, g_c）以及解耦系数（decoupling coefficient, Ω）均呈现显著的昼夜与季节变化特征，其变化趋势与光合有效辐射（Photosynthetically Active Radiation, PAR）及水汽压亏缺（vapour pressure deficit, δ）的变化趋势高度一致。
蒸散发E、二氧化碳通量Fc、表面导度g_c与解耦系数Ω的24小时平均日最大值分别为1.78 mmol·m⁻²·s⁻¹、-11.18 μmol·m⁻²·s⁻¹、6.27 mm·s⁻¹与0.31；其生长季季均数值则分别为0.71 mmol·m⁻²·s⁻¹、-4.61 μmol·m⁻²·s⁻¹、3.3 mm·s⁻¹与0.16。
蒸散发E与二氧化碳通量Fc受生物与环境因子的共同调控，二者均与表面导度g_c呈曲线相关关系。在各类环境因子中，光合有效辐射PAR是最为关键的调控因子：其与蒸散发E呈显著正线性相关，与二氧化碳通量Fc则呈曲线相关关系；而水汽压亏缺δ则是影响表面导度g_c的首要环境因子。
干旱季的水分利用效率（water use efficiency, WUE）略高，其月均数值介于6.67~7.48 μmol CO₂·(mmol H₂O)⁻¹之间，生长季季均数值为7.06 μmol CO₂·(μmol H₂O)⁻¹。
解耦系数Ω取值较低且与表面导度g_c呈显著正相关，表明蒸散发E对表面导度变化较为敏感。仲夏干旱事件会降低表面导度与解耦系数Ω，这意味着蒸散发的生物调控作用更强，同时也反映了植被对干燥空气的生理适应。干旱条件下表面导度维持在较低且稳定的水平，这一结果支持“干旱条件下可采用恒定表面导度值进行蒸腾模拟”的建模思路。