高温天气植被蒸腾与遮荫降温效应的变化特征

Characteristics of vegetation shading and transpiration cooling effects during hot summer

开展城市中不同树种植被遮荫与蒸腾降温效应的量化研究是科学优化植被温度调控服务的重要基础。以南京市栖霞区某小型绿地单元为研究区,对高温晴朗天气下不同树种典型植株树干液流进行了观测,采用"单位叶面积上的平均液流速率×叶面积指数"的扩展方法实现了由单株到林分尺度上冠层蒸腾量与蒸腾降温效应的估算,并根据林上、林下太阳辐射值计算了不同树种与整个绿地单元的遮荫降温效应,进而阐明了蒸腾与遮荫降温对总降温效应贡献率的变化特征。研究结果表明:1)3个树种树干液流均呈现昼高夜低的变化趋势,树干液流通常在6:00左右启动,正午前后达到峰值,且存在明显的"午休"现象,而在同一树种内树干液流会随着胸径的增大而显著增大;2)林分尺度上的冠层蒸腾量与蒸腾降温效应均为杨树>雪松>香樟,杨树峰或谷出现的时间(11:00—19:00)均明显晚于雪松(10:00—15:00)和香樟(9:00—16:00);3)3个树种遮荫降温效应总体上与太阳辐射的日变化规律基本一致,但树种间日平均降温效应的差异较小;4)3个树种与整个小型绿地单元的总降温效应在夜间均非常微弱,且全部为蒸腾降温,而在白天遮荫对总降温的贡献率(60%—75%)则明显高于蒸腾降温(25%—40%)。

Quantifying the transpiration cooling effect and cooling effect of the shade of different tree species in urban areas is important for improving the vegetation temperature regulating ecosystem services.A small green space patch located in the Qixia District of Nanjing was selected as the study area.The Granier thermal diffusion probe technique was used to monitor the sap flow of three tree species in extremely hot weather.The method ＂average sap flow of per unit leaf area × leaf area index＂was used to estimate the canopy transpiration and transpiration cooling effect.Differences in solar radiation under and above the vegetation canopy were calculated and the cooling effect of the shade of different tree species and the whole green space patch was captured.Finally,we determined the contribution of the transpiration cooling effect and cooling effect of the shade to the total vegetation cooling effect.The results indicated that 1） Diurnal variation of sap flow was regular and was observed to be high in the daytime and low at night.Sap flow typically began at approximately 06：00,peaked at noon,and then showed an obvious midday depression.In the same tree species,sap flow increased with increasing DBH.2） Thevolume of the stand canopy transpiration and transpiration cooling effect were found to have the following order：Populus tomentosa Carr Cedrus deodara Cinnamomum camphora,and Populus tomentosa Carr（11：00—19：00） clearly peaked later than C.deodara（10：00—15：00） and C.camphora（09：00—16：00）.3） The cooling effect of shade of the three tree species was mostly consistent with the diurnal variation of solar radiation and difference in the average cooling effect between tree species was small.4） The cooling effect of three tree species and small green space patch was very weak at night and nearly equal to the transpiration cooling effect.The contribution of the cooling effect of shade（60% —75%） to the total cooling effect was significantly higher than that of transpiration cooling effect（25% —40%） during the day.