Ecosystem water use efficiency (EWUE), defined as the ratio of ecosystem gross primary production (GPP) and evapotranspiration (ET), is regulated by multiple biophysical factors and their interactions. One particular unknown is the relative importance of environmental control and canopy stomatal control on EWUE for their direct and indirect roles in different seasons. We employed the eddy covariance technique and micrometeorological sensors to measure the carbon and water fluxes and environmental variables over a riparian poplar plantation in northern China from 2014 to 2017. We found that canopy conductance (Gc) was the dominant factor regulating EWUE in spring and autumn, while the fraction of diffuse radiation (FDR) was the primary controlling variable in summer. During the growing season, FDR depressed ET and promoted GPP primarily through its direct role rather than indirectly affecting other environmental factors. This plantation could utilize diffuse radiation efficiently by regulating stomatal function when FDR was lower than 0.6. More importantly, the stomatal optimization theory worked merely under certain conditions (i.e., VPD ≤ 3 kPa). Correspondingly, the poplar plantation tended to show less conservative water use strategy with decreasing surface soil water content (SWC) when the non-stomatal function was dominant. Our study highlighted the importance of FDR and Gc on EWUE and the potential of future water resource impact resulting from extensive poplar plantation establishment in northern China.
