Urban forests with various structures can bring considerable but divergent biophysical cooling and humidification effects on their local climate. Thus, it is crucial to unravel the 3D thermal environment within urban forests and their relationship with forest structure, which are helpful for the urban forest planning and design. In this study, we continuously observed the air temperature (Ta) at different vertical layers from canopy to land surface as well as the soil surface temperature (Ts) from the forest center to 5 m outside the forest edge across 22 urban forests in Hefei city, China. Indicators of forest structure such as tree height, diameter at breast height (DBH), crown diameter and leaf traits were associated with their 3D thermal environments for exploring the underlying mechanisms. We found that Ts was 1.43 °C lower than the understory air temperature (Ta understory) in forest center but 10.90 °C higher than Ta understory outside the forest. Additionally, tree height largely influenced the buffering distance from forests center to the places with Ts = Ta understory (LTs=Ta understory Lcenter), being 4.41 m, 5.80 m and 7.75 m in short (< 7 m), medium (7–9 m) and tall (>9 m) canopy forests, respectively. The temperature difference between forest center and 5 m outside the forest (ΔTemperature) varied significantly at different vertical layers, with ΔTs greater than 10 °C, ΔTa understory and ΔTa bottom canopy at around 2 °C, and no difference for ΔTa upper canopy. Regression analysis showed different relationships of forest structure and leaf traits with ΔTemperature between vertical layers. Tree height, forest area and DBH showed significant positive relationships with LTs=Ta understory Lcenter. The study, for the first time, demonstrate the 3D thermal environments of urban forests, quantify the role of forest structure and leaf traits in predicting forest cooling.

中文翻译：

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揭示森林中心和边缘的 3D 热环境差异——以合肥市 22 个城市森林为例


具有不同结构的城市森林会给当地气候带来相当大但不同的生物物理冷却和加湿效果。因此，解开城市森林内部的三维热环境及其与森林结构的关系至关重要，这有助于城市森林的规划和设计。本研究连续观测了合肥市 22 个城市森林从冠层到地表不同垂直层的气温 （Ta） 以及从森林中心到森林边缘外 5 m 的土壤表面温度 （Ts）。将树高、胸径 （DBH）、冠径和叶片性状等森林结构指标与其 3D 热环境相关联，以探索其潜在机制。结果表明，Ts 比林中心林下空气温度 （Ta 下层） 低 1.43 °C，但比林外 Ta 林下层温度高 10.90 °C。此外，树高在很大程度上影响了从森林中心到Ts = Ta林下层（LTs=Ta 林下层Lcenter）的缓冲距离，短（< 7 m）、中等（7–9 m）和高（>9 m）树冠林分别为4.41 m、5.80 m和7.75 m。不同垂直层的森林中心与林外 5 m 温差 （ΔTemperature） 变化显著，ΔT 大于 10 °C，ΔTa 林下和 ΔTa 底冠在 2 °C 左右，上层冠层无差异。回归分析显示，垂直层之间森林结构和叶片性状与 Δ 温度的关系不同。树高、林面积和 DBH 与 LTs=Ta 林下 Lcenter 呈显著正相关。 该研究首次展示了城市森林的 3D 热环境，量化了森林结构和叶片性状在预测森林降温中的作用。