Institute of Biometeorology - CNR, Florence, Italy.
Department of Agriculture, Food, Environment and Forestry - University of Florence, Florence, Italy.
Int J Biometeorol. 2019 Apr;63(4):467-479. doi: 10.1007/s00484-019-01678-1. Epub 2019 Feb 2.
In a context of urban warming, the effects of trees on outdoor thermal stress are important even during the increasingly hot autumn season. This study examines the effects of a deciduous tree species (Tilia x europaea L) on surface temperature over different ground materials and in turn on human thermal comfort, with a particular focus on tree shade variation due to leaf fall. Grass, asphalt, and gravel-covered ground surfaces, both sun-exposed and under the Tilia, were monitored in Florence, Italy, during the summer (2014) and autumn (2017) seasons. The Index of Thermal Stress (ITS) was used to gauge the micrometeorological effects of the changing tree canopy, with tree defoliation quantified by the Plant Area Index. On clear summer days, thermal discomfort was especially pronounced over exposed asphalt, and even more intense above exposed gravel due to its higher reflectivity-while shaded surfaces showed large reductions in thermal stress. Even though incoming solar radiation decreases over the course of the fall season, the direct radiation under the gradually defoliating tree canopy actually increases. Due to this diminished shading effect, the differences in surface temperature between exposed and shaded asphalt shrink dramatically from about 20 to 3 °C. However, since ambient conditions become milder as the season progresses, the Tilia demonstrated a double benefit in terms of ITS: providing thermal comfort under its full canopy at the beginning of autumn and maintaining comfort even as its canopy thins out. At the same time, tree species with earlier defoliation may be unable to replicate such benefits.
在城市变暖的背景下，即使在越来越热的秋季，树木对户外热应激的影响也很重要。本研究考察了落叶树种（欧洲椴）对不同地面材料表面温度的影响，以及对人体热舒适度的影响，特别关注因落叶导致的树荫变化。在意大利佛罗伦萨，监测了夏季（2014 年）和秋季（2017 年）暴露在阳光下和欧洲椴树下的草地、沥青和砾石覆盖的地面。使用热应激指数（ITS）来衡量树冠变化的微气象影响，用叶面积指数来量化树木落叶量。在晴朗的夏日，暴露在阳光下的沥青表面的热不适感尤其明显，而暴露在阳光下的砾石表面则因反射率较高而更强烈——而阴凉表面则大大降低了热应激。尽管随着秋季的到来，太阳辐射会减少，但逐渐落叶的树冠下的直接辐射实际上会增加。由于这种遮阳效果减弱，暴露和遮荫的沥青之间的表面温度差异从约 20°C 缩小到 3°C。然而，由于环境条件随着季节的变化而变得更加温和，欧洲椴在 ITS 方面具有双重优势：在初秋时提供了满树冠的热舒适度，即使树冠变薄，也能保持舒适度。与此同时，落叶较早的树种可能无法复制这种好处。
