Abstract: [Objective]To explore the influence of precipitation on water sources for Platycladus orientalis in Beijing mountainous areas and to provide a theoretical basis for understanding the response of P. orientalis to soil drought stress.[Method]We determined the hydrogen and oxygen values of xylem and investigated the water sources of P. orientalis in different precipitation conditions(A:no precipitation, 0.0 mm; B:1/2 precipitation, 12.7 mm; C:natural precipitation, 25.4 mm, D:two times of precipitation, 50.8 mm) by Iso-Source model during the vigorous growing season.[Result]P. orientalis showed a "plasticity root system" structure. It absorbed water from deep to surface soil layer with the precipitation gradients. In treatment A, P. orientalis mainly absorbed water from 60-100 cm and groundwater; the utilization rate was 31.5% and 32.8%, respectively. Similarly, in treatment B, it obtained water from 60-100 cm and groundwater; the utilization rate was 28.1% and 25.7%, respectively. But in treatment C, P. orientalis absorbed water from layer of 60-100 cm and groundwater; the utilization rate was 19.8% and 22.8%, respectively. At the same time, it absorbed water from the surface 0-20 and 20-40 cm soil layer; the utilization rate was17.8% and 22.9%, respectively. The water source of P. orientalis in treatment D was 0-20, 20-40, and 60-100 cm soil layer and groundwater with the utilization rate in the range of 20%-25.2%.[Conclusion]P. orientalis can adjust water source and its utilization ratio according to water conditions.

Key words: Platycladus orientalis, precipitation gradient, water source, utilization strategy, hydrogen and oxygen isotopes, Beijing mountain area