防护林是我国人工林的主体，然而，由于各种原因，目前部分引进树种出现严重衰退，尤其在我国防护林建设核心区-科尔沁沙地，引种的沙地樟子松、杂交杨大面积死亡现象引起了林学领域广泛关注；前期研究表明，水分亏缺是主要原因，而相同水分条件下的乡土树种并没有出现死亡现象。因此，明确外来树种和乡土树种对水分条件的适应机制极为重要。本项目以科尔沁沙地主要防护林树种（外来种：樟子松、杂交杨，乡土种：油松、白榆和锦鸡儿）为对象，以影响防护林树种存活和生长的水分因子为主线，利用稳定同位素技术（δD、δ18O和δ13C）和树木年轮学方法，重点研究主要防护林树种水分利用来源，重建历史时期水分利用效率；明确外来树种与乡土树种水分利用策略的差异，揭示科尔沁沙地外来树种衰退机制和乡土树种适应机理。研究结果将丰富防护林学，对指导防护林体系建设、实现植被恢复和重建具有重要的理论和现实意义。

Shelter/protective forests play an important role in establishment of forest plantations in China. However, some protective forest composed of exotic tree species have declined seriously due to various reasons, especially in the Keerqin sandy land, the key area of the shelter forest system project. The decline of the shelter forests composed by the introduced Pinus sylvestris var. mongolica and Populus spp. over large areas has been paid wide attentions by the people in forestry. Previous studies indicate that water deficit is the main reason of the decline, but the native tree species did not present the decline phenomenon in the same water condition. Therefore, it is vital to determine the acclimation mechanisms of both the exotic and native tree species to the current water conditions. On the basis of reported researches by our group, we select the main shelter forest tree species (the exotic species: Pinus sylvestris var. mongolica and Populus spp.; the native species: Pinus tabuliformis, Ulmus pumila and Caragana spp.) as the objects of this study. Water condition, as the most important factor influencing the survival and growth of the trees in the shelter forests in the Keerqin sandy land, is considered as the main topics in this project. Based on the stable isotope and tree ring approaches, water use sources will be mainly studied and the water use efficiency during historical period will be reconstructed for the main shelter forest tree species. The specific objectives of this project are as follows: 1) to determine the difference in water use strategies between the exotic and native tree species; 2) to reveal the mechanisms of the exotic tree species decline and the native tree species acclimation. Besides enriching shelter forestry and guiding the construction of shelter forests, the results obtained in this project certainly will also has both important theoretical and practical significances for promoting rehabilitation and reconstruction of vegetations in sandy land.