The long term water use efficiency (WUE),of key species in deciduous broad-leaved forest in Beijing Mountain area,is strongly related to relative humidity,solar radiation,vapor pressure deficit and temperature. The ranges of water use efficiencies of Quercus liaotungensis,Prunus armeniac var. ansu, Fraxinus rhychophylla, Vitex negundo, Syringa pekingensis, and Juglans mandshurica are from 3 76 mmolCO 2·mol -1H 2O to 4 95mmolCO 2·mol -1H 2O in all growing season,with the mean water use efficiency b...