Environmental and canopy stomatal control on ecosystem water use efficiency in a riparian poplar plantation

•Gc in spring and autumn primarily controlled EWUE.•FDR became the dominant biophysical factor regulating EWUE in summer.•FDR significantly enhanced EWUE by depressing ET and promoting GPP.•The stomatal optimization theory applied at VPD ≤ 3 kPa at the canopy level.•Large-scale poplar will exacerbate water source issues in northern China.