中文摘要：

北京山区长年干旱缺水，研究北京山区典型树种长期水分利用效率的特征对于筛选抗旱造林树种具有重要意义．为了探讨北京山区典型树种（侧柏、油松、刺槐、栓皮栎）长期水分利用效率特征，本文基于稳定碳同位素技术测算出4个树种的水分利用效率，并对结果进行了分析与研究．结果表明，叶片δ13C值和水分利用效率同向变化，4个树种的水分利用效率从大到小排列顺序为栓皮栎〉侧柏〉刺槐〉油松，说明4个树种中栓皮栎的抗旱性最强而油松最弱；阳生叶的水分利用效率高于阴生叶；植物叶片日内水分利用效率上午略大于下午，但区别不明显；另外，侧柏冠层不同高度叶片水分利用效率表现为上层〉中层〉下层．

英文摘要：

Researching the characteristics of long-term water use efficiency of typical tree species in Beijing mountain areas is very important for selecting drought-resistant tree species,because Beijing mountain areas have been drought for many years. In order to investigate the water use efficiency differences,this paper presents a method to calculate WUE based on stable carbon isotope,and the results were analyzed and studied. Results showed that ：WUE was proportional to the value 0f813C, and the order of these four species＇ WUE was Quercus variabilis 〉 Platycladus orientalis 〉 Robinia pseudoacacia 〉 Pinus tabulaeformis. All of these explain that the Q. variabilis has highest tolerance to drought,white P. tabulaeformis has lowest tolerance to drought among these four typical species. The value of δ13C and the water use efficiency of sun leaves is high than the shade leaves, and the value of δ13C in the forenoon is bigger than in the afternoon（not significant）. The value of δ13C and the water use efficiency will increase with the height of canopy.