Abstract: This paper studied the photosynthesis characteristics and water use efficiency (WUE) of Fraxinus mandshurica under three years simulated water stresses 85%～100% (CK),65%～85% (MW) and 45%～65% (LW) of field water-holding capacity.The results showed that in comparing with CK,the photosynthesis rate and WUE were higher in treatment MW,but decreased by 7.26% and 1.13%,respectively in treatment LW,indicating that Fraxinus mandshurica could fully exert its photosynthesis potentiality under long-term medium water stress,and was more resistant to moderate than to critical water stress.In treatment CK,the diurnal change ofP_n and WUE was a single and a double peak curve,respectively,while in treatments MW and LW,the diurnal change of Pn was a double peak curve,but that of WUE had no significant peak,which correlated not only to the physiological characteristics of Fraxinus mandshurica under different water stress,but also to the diurnal change of environmental factors.

Key words: Surface runoff, Seasonal dynamics, Spatial distribution, Environmental conditions, Leaf area index, Changbai Mountain Natural Reserve