Water use efficiency (WUE) at the ecosystem level is an important ecophysiological index reflecting the
coupling relationship between water and carbon cycles. CO2 and water vapor fluxes were measured by
the eddy covariance method during the period 2006–2010 over a warm-temperate mixed plantation in
the North China. The seasonal and interannual variations of gross primary productivity (GPP), evapotranspiration
(ET) and ecosystem WUE were analyzed, and the impacts of climatic variables and soil
moisture on GPP, ET and WUE were discussed. At the monthly scale, GPP and ET had similar relations
with solar radiation, air temperature, vapor pressure deficit (VPD) and precipitation. It is suggested that
photosynthesis and evapotranspiration were driven by climatic variables at the approximately equal
strength. During the growing season, WUE decreased significantly with the increase of VPD and solar
radiation. Cloudiness can improve photosynthesis and enhance WUE. GPP was 9–39% greater but ET
was 8–26% lower under cloudy sky conditions than that under sunny sky conditions. Therefore, WUE
was 29–72% higher under cloudy skies in comparison with that under sunny skies. Annual average
WUE ranged from 1.76 to 2.41 g C kg1 H2O. The major driver of interannual variability in WUE was soil
water content in May.