Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures.

Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1 – 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.

人们通常建议增加城市树木的覆盖率来缓解城市高温，因为人们希望树木通过蒸散和遮荫为城市降温。但是，树木也会改变风流和城市空气动力学粗糙度，从而可能限制散热。现有研究表明，在不同的气候和一天中的不同时间，城市树木的降温潜力各不相同。迄今为止，尚未对这些差异进行系统地解释，因为仅通过观察就很难对单个树效果进行分区，而且是不可能的。在这里，我们使用机械化的城市生态水文模型进行数值实验，以消除和增加由城市树木引起的辐射，蒸散和空气动力学粗糙度的影响。针对不同气候的四个城市（凤凰城，新加坡，墨尔本，

结果表明，在夏季的四个气候条件下，仅灌溉良好的树木的蒸散量最多可以使局部2 m气温最高降低3.1 – 5.8°C。由于高蒸气压缺陷限制了蒸腾作用，在峰值温度下气孔关闭阻止了进一步的冷却。虽然遮荫降低了表面温度，但非蒸腾树与辐射的相互作用可以在一天中的某些小时局部升高2 m的空气温度，最高可达1.6 – 2.1°C，从而部分抵消了蒸散冷却作用。此外，在不考虑树木风阻作用的已分析场景中，树木导致城市粗糙度降低，从而抑制了湍流的能量交换并在白天增加了气温。在晚上，由于城市峡谷内大气稳定性的差异，单棵树的影响可能会有所不同。这些结果解释了城市树木的降温效果在昼夜，季节和气候方面的差异，并可以指导未来的野外活动，规划策略和物种选择，目的是利用城市绿化改善局部小气候。