桉树的大面积种植和基于经济效益考虑的现行轮作模式过度消耗水分和营养，土壤无法得到有效补充和恢复，人们对林区的水量平衡和环境效应产生担忧。气候变暖导致区域降雨分配不均，桉树持续的非保守性蒸腾与土壤水分供应的不协同越发突出。至今，相关研究偏重桉树蒸腾与环境、种植密度及耕作方式的关系，忽略树龄对蒸腾耗水的影响，对桉树水分利用时间规律的认识出现偏差。我们认为，与树龄紧密相关、决定输水效率的水力结构的调整，间接影响树木水分利用对土壤水分梯度的响应，据此提出“不同土壤水分梯度下桉树的水分利用与生长存在年龄权衡”的观点。本项目将设计自然土壤水分梯度，研究不同龄级桉树的树干液流、水力导度、冠层气孔导度、生长参数和水分利用效率的变化，阐明桉树整树水分利用响应土壤水分变化的年龄效应和水力调节机理，为种植管理措施的设计和预测未来气候变化情景下桉树林区的水文动态提供实验支撑，有明显的学术价值和实践指导意义。

Large-scale cultivation and short rotation of eucalypt trees based on economic considerations have led to an excessive consumption of water and serious depletion of nutrients in soils. Such practice makes the recharge and rehabilitation of soil difficult, which has largely raised concerns on regional water balance and environmental effects in the eucalyptus cultivation zone. On the other hand, the uneven distribution of regional rainfall induced by climate warming has intensified the contradictive relation between soil moisture supply and the strong transpiration of eucalyptus trees. Previous researches have emphasized more on the relationship between transpiration of fast-growing eucalyptus and environment factors, plant density, and cultivation way, whereas ignored the age effect on eucalyptus’ water transpiration, which might lead to misunderstanding of water use of eucalyptus. We argue that the adjustment of hydraulic structure which is closely related to tree age and determines water transportation efficiency, could indirectly affect the response of tree water use to soil moisture. Accordingly, we propose that there is an age tradeoff between transpiration and growth of trees under different soil moisture conditions. To illustrate age effect on the response of whole-tree water use to changes in soil moisture and to reveal the relevant regulatory mechanisms, the sap flow of different age eucalyptus trees, their hydraulic conductance, canopy stomatal conductance, growth indexes, and water use efficiency will be examined under different natural soil moisture gradients. These results would provide experimental support for management of eucalyptus cultivation and for prediction of eucalyptus regional water balance under future climate change, which are of academic and practical importance.