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Climate models predict large climatic variability and more frequent, prolonged droughts in northern China. Still, few studies have examined the seasonal and interannual variation in water use efficiency (WUE) in northern China's young plantations predisposed to drought. Here, variation in WUE was examined in a young plantation in Beijing over a 6-year period (2012-2017) at several timescales. The study was conducted by examining eddy-covariance measurements of gross ecosystem production (GEP) and evapotranspiration (ET). Results showed that the multi-year mean WUE for this young plantation was 1.71 +/- 0.22 g C per kg H2O. Drought reduced WUE in summer, but increased it in autumn, with no apparent effect in spring. Long-term droughts usually had a greater effect on the annual GEP, ET and WUE than short-term droughts. Intense drought was observed to have a long-term legacy effect on ecosystem GEP, ET and WUE of about 3 years. Monthly WUE had a strong positive linear relationship with normalized difference vegetation index (NDVI) and soil volumetric water content (VWC) during the growing season. Seasonal variation in WUE was mainly regulated by the biophysical factors controlling GEP. In contrast, interannual variation in WUE was controlled to a greater extent by vegetation cover (i.e., NDVI), which in turn responded to variation in VWC. These results clearly show the importance of timing and duration of drought in controlling ecosystem WUE. Collectively, WUE was more strongly affected by factors affecting carbon uptake than water consumption. Our findings suggest that future drought, when coupled with increased climate variability, irrigation-support planning and selection of appropriate tree species should be central to afforestation programs in northern China.