How Urban Trees Cool Cities

Cities are often warmer than nearby rural areas because roads, roofs, and concrete absorb solar energy during the day and release it slowly at night. This phenomenon is known as the urban heat-island effect. Trees reduce local temperatures in two ways. Their leaves provide shade, preventing surfaces from heating as quickly, and they release water vapour through a process called transpiration.

城市通常比附近乡村地区更温暖，因为道路、屋顶和混凝土在白天吸收太阳能，并在夜间缓慢释放。这种现象被称为城市热岛效应。树木通过两种方式降低局部温度：树叶提供阴影，避免地表过快升温；同时，树木还通过蒸腾作用释放水蒸气。

However, the benefits of urban trees depend on where and how they are planted. A tree beside a pavement may cool pedestrians immediately, while a distant park may have less effect on a neighbourhood street. Species selection also matters because trees differ in canopy size, water needs, and resistance to disease. Effective programmes therefore combine environmental data with local knowledge rather than simply counting how many trees are planted.

然而，城市树木的益处取决于它们种在哪里以及如何种植。人行道旁的树可以立刻让行人感到凉爽，而较远的公园对社区街道的影响可能较小。树种选择也很重要，因为不同树木在树冠大小、需水量和抗病性方面存在差异。因此，有效的项目会结合环境数据与当地知识，而不只是计算种了多少棵树。