Through three-dimensional observation and numerical analysis of evapotranspiration characteristics and cool island response of individual urban trees, the cause of vegetation cold island effect was analyzed quantitatively. The results show that 1) the absolute value of cool island intensity in various parts of Ficus microcarpa was basically above 3.0°C, while at surface front (SF), the temperature difference could reach the maximum of ?5.19°C and surface top (ST) can reach to ?3.57°C. 2) The evapotranspiration rate of leaves in the east, SF and SR (surface right), was start to rise firstly, and the leaves in the west, SL (surface left) and SBa (surface back), have the highest evapotranspiration rate at noon while the leaves in the west (SL, SBa) reached the peak later. The transpiration of all parts at night was very weak, less 0.05 mm/h. 3) The overall cool island effect intensity were positively correlated with evapotranspiration rate (pearson correlation coefficient was 0.70, with a significance lower than 0.01). For every 1 mm/h increase in evapotranspiration, the temperature of the whole tree decreased by 3.56°C. Evapotranspiration of surface top contribute obviously to the cool island effect of surface top itself with a correlation coefficient ?0.61. Evapotranspiration of surface top, surface front and surface right have the largest contribution to cool island effect of the whole tree (correlation coefficient absolute value is greater than 0.60). Contribution order of different parts’ evapotranspiration to overall cool island effect is: SF>ST>SR>SL> SBa>SBo (surface bottom), that means east and top side>west side>bottom.

通过对城市单株乔木蒸散发特征及其冷岛响应的三维观测和数值分析, 定量解析植被冷岛效应的成因。研究结果表明: 1) 小叶榕各部位冷岛强度绝对值基本上在3.0°C以上, 其中前部最强能达到?5.19°C, 顶部能达到?3.57°C; 2) 早上, 东侧(前部和右部)的叶片蒸散发率先升高, 正午顶部蒸散发达最强, 西边(左部, 后部)的叶片蒸散发较晚达到峰值, 夜间各部位的蒸腾量非常微弱, 不超过0.05 mm/h; 3) 城市乔木整体的冷岛效应强度与其蒸散发的相关系数为0.70, 显著性低于0.01, 蒸散发每升高1 mm/h, 整树可降低 3.56°C, 顶部的蒸散发对顶部的冷岛效应贡献最明显(Spearman 相关系数为?0.61), 顶部、前部和右部的蒸散发对整树的冷岛效应贡献最大(相关系数绝对值均大于0.60), 各部位蒸散发对整树冷岛效应贡献顺序为前部>顶部>右部>左部>后部>底部, 即东侧和顶部>西侧>底部。