防护林在应对生态脆弱区自然灾害和生态问题具有十分重要的地位，但人工营建的防护林出现了衰退现象，尤其是半干旱沙区防护林衰退更为严重。前期研究表明，水分失衡是衰退主要原因；但是，相同降水环境下乡土树种并未出现明显衰退。因而，明确引进树种和乡土树种水分利用机制便成为进一步揭示衰退机理的重要科学问题。本项目以科尔沁沙地南缘引进树种樟子松、杂交杨和乡土树种油松、小叶杨为对象，以影响防护林存活、生长的水分因子为主线，通过野外监测和定位试验相结合手段，利用高压流速仪和热扩散等方法，重点研究不同年龄引进树种和乡土树种水力结构动态特征、树干液流特性及其与环境因子关系，比较土壤水和地下水对蒸腾耗水的贡献；基于此，阐明不同树种水分传输有效性和安全性权衡机制、蒸腾耗水规律和蒸腾耗水来源，揭示不同树种水分利用及种间差异的水力学与生理生态学机理。研究结果对造林树种选择、林分结构配置和林地水分管理具有重要理论和现实意义

Protective or shelter forests play an important role in coping with natural disasters and ecological problems in ecological fragile areas. However, the artificial protective forests have emerged decline phenomenon, especially for the protective forests in semiarid sandy region. Previous studies have indicated that water imbalance was the major cause inducing the decline of protective forests, but the native tree species did not show obviously decline in the same precipitation condition. Therefore, determination of mechanisms in water use for the introduced and native tree species have become an important scientific issues, which further reveal mechanisms in the protective forests decline. In this study, we select the introduced tree species (Pinus sylvestris var. mongolica and Populus spp.) and native tree species (Pinus tabuliformis and Populus simonii) in the southern edge of Horqin Sandy Land as the objects of this study. Water condition, the most important factor influencing the survival and growth of the protective forests, is considered as the main topic in this project. Based on high pressure flow meter and heat dissipation approaches and so on, dynamic characteristics of hydraulic architecture, sap flow and its correlation with influencing factors and comparision of soil water and groundwater contribution to tree transpiration are studied for the introduced and native tree species with different ages. The specific objectives of this project are to 1) determine the trade-off between hydraulic safety and efficiency, tree transpiriaton patterns and sources of water for transpiraiton in the introduced and native tree speices, 2) to reveal the water use mechnisims for the introduced and native tree speices and their interspecific differences in hydraulics and ecophysiology. The results obtained in this project would have important theoretical and practical significances in tree species selection, stand structure configuration and water management for protective forests in semiarid sandy region.