中文摘要：

北京山区处于易于旱的生态脆弱地带，了解其典型树种在有限的水分输入条件下正常生长的水分利用机制具有重要意义．本研究利用稳定同位素技术，采集北京山区典型树种侧柏和栓皮栎的栓化枝条和林下枯落物／土壤，并对其氢氧同位素组成进行分析，进而研究北京山区典型树种土壤水分利用特征．结果表明，侧柏和栓皮栎的枯落物平均厚度分别为4．0em和8．5cm，枯落物现存量分别为3．67t／hm2和10．82t／hm2；侧柏根系主要分布在表层土壤，栓皮栎根系主要分布在深层土壤中；旱季中两种树种的栓化枝条和0—30cm土壤的6D值和6M0值明显高于雨季，30_70cm土壤的δD值和δ180值在旱季和雨季均保持相对稳定；利用二元混合模型计算显示，旱季中，栓皮栎利用深层土壤水分占总水分来源的60％以上，侧柏利用的表层土壤水分则达60％以上；雨季中，两个树种利用表层土壤水分比例均比旱季提高10％以上；树木根系分布是决定树木水分利用来源重要因素之一．研究结果将为北京山区生态建设提供科学支撑和理论依据

英文摘要：

Beijing mountainous area locates in the drought-prone and ecologically-fragile zone of China. To understand how the typical tree species survive under the limited water conditions is of great importance. In this study, to understand soil water use patterns of typical tree species in Beijing mountainous area, suberized plant stems and soil samples of Platycladus orientalis and Quercus variabilis ecosystems were collected for isotopic analysis. The results showed that theaverage litter thickness for P. orientalis and Q. variabilis was 4.0 and 8.5cm, respectively, and the litter reserve was 3. 67 and 10. 82t/hm2 ,respectively. Roots of P. orientalis mainly distributed in the upper soil layer, while the roots of Q. variabilis mainly distributed in the deeper soil layer. Both soil δD and δ18o values of 0-30cm were higher in the dry season than that in the wet season, while the soil 8D and Sts O values of 50-70cm remain relatively stable. By two- compartment linear mixing model, the results showed that over 60 percent of the water was derived from the deep soil layer by Q. variabilis, while P. orientalis mainly depended on the upper soil layer. However, both these two tree species obtained more than 10% of water from the shallow soil depth in the dry season than that in the wet season and the root distribution was one of key factors that affected water sources for tree species. The results will provide scientific support and theoretical basis for future ecological construction in Beijing mountainous area.