中文摘要：

应用Granier热消散探针,长期监测华南丘陵地马占相思（Acacia mangium）林14棵样树的树干液流（Sapflow）,由此计算整树和林段的蒸腾速率,结合同步记录的环境因子,求算冠层平均气孔导度（Gc）.Granier探针的灵敏度较高,能精确测定即使是微弱的液流活动.观测结果显示,树木个体之间的液流密度（Js）和整树蒸腾（Et）受树形特征影响较大.马占相思林径级大的树木个体数较少,但占据林段边材总面积和林段蒸腾的比例较大.Js和Et的日变化主要受光合有效辐射（Qo）和空气水蒸气压亏缺（D）的控制,土壤含水量（θ）对较大胸径树木Et的影响大于胸径较小的树木,个体之间Js和Et的差异随θ的下降而缩小.一年中,林段蒸腾（E）在光照和水热条件较好的7月最高,9～12月,由于土壤水分供应的减少致使E值降低,E对D的敏感性下降.Gc与主要环境因子的关系与E相似,如果θ长期偏低,Ge会明显下降,是造成E降低的主要原因.成熟马占相思林在光照充足、水热条件较好的情况下的蒸腾活动旺盛,但对土壤水分胁迫的忍受力较低.

英文摘要：

Background and Aims Canopy stomatal conductance is considered as a sensitive parameter of forest ecosystem in responding to environmental changes and can be used to monitor the impacts of water stress, air pollution, trace gas and other environmental factors on forest function. Sap flow of an Acacia mangium forest was measured to scale up stand transpiration and canopy stomatal conductance. It aimed at understanding water use of A. mangium forest in different environmental water conditions and its responses to soil moisture change at an ecosystem level. Methods The Granier＇s thermal dissipation probes were applied to monitor sap flow of 14 sample trees in an Acacia mangium forest in hilly land of South China. The sap flow data were used to calculated whole-tree and stand transpiration, and to estimate mean canopy stomatal conductance （ Gc ） by combing with synchronous measurement of environmental factors. Key Results Sap flux density （ Js ） and whole-tree transpiration （ Et ） were obviously affected by tree morphological features. Even though there were less individual trees with large diameter at breast height （DBH） in the sample plot, they comprised relatively larger proportions of total sapwood area and transpiration of the forest. Daily variations of Js and Et were mainly controlled by photosynthetically active radiation （Qo） and vapor pressure deficit （D）. Soil moisture （0） had greater effects on Js and Et of trees with larger DBH than on those trees with smaller DBH. The differences of Js and Et across individuals reduced with decreasing θ. The highest stand transpiration （E） occurred in July when the radiation and hydmthermal condition were favorable, whereas the reduced water supply in soil during the time period of September-November resulted in lower E value and its sensitivity to D. Gc responded to major environmental factors in a similar way with E. Long-term lower θ would significantly decreased Gc, which would cause reduction in E. Conclusions Mature A. mang