LAUSR

Abstract Northern China’s plantations have long played a role in providing ecological services to the people of China. Reliable detection and attribution of ecosystem water and energy exchange is a precondition to the development of strategies for the sustainable management of these plantations. Here, the seasonal and interannual variability in surface energy exchange and evapotranspiration (ET) over a young pine plantation was investigated using eddy-covariance measurements collected over a four-year period (2012–2015). Seasonal patterns in net radiation (Rn), latent (LE) and sensible heat fluxes (H) were largely similar over the four years. H was the dominant energy component, with LE exceeding H only during the mid-growing season. A significant share of Rn was allocated to H during the 2014 summer drought. Energy exchange for the young pine plantation was characterized by high Bowen ratios (β = H/LE; 6.28, 6.42, 5.65, and 5.34 for the four years), high H/Rn (0.37, 0.36, 0.36, and 0.30), and low LE/Rn (0.15, 0.17, 0.17, and 0.16), with mean annual values of 5.92, 0.35, and 0.16 for β, H/Rn, and LE/Rn, respectively. Daily maximum and annual ET were 4.8, 4.5, 4.1, and 3.3 mm day−1 and 328, 371, 290, and 326 mm for the four years, with a mean annual value of 329 mm. Seasonal variation in ET was strongly controlled by biological factors (i.e., bulk surface conductance and vegetation greenness and density, as characterized by the normalized difference vegetation index) regulated by soil water availability and water vapor pressure deficit (VPD). Evapotranspiration varied interannually and the evapotranspiration-to-precipitation ratio (ET/P) ranged from 0.79 to 1.62. Soil water replenishment through precipitation during the non-growing period of the previous year and mid-growing season of the current year was responsible for the interannual variation observed in ET. These results clearly indicate the importance of precipitation timing and soil moisture carry-over from previous years in controlling ecosystem energy and water vapor exchange. Irrigation during the spring and dry periods of the year is highly favorable for growth of plantation trees in northern China.