The influence of vertical canopy structure on the cooling and humidifying urban microclimate during hot summer days

Urban forests can mitigate urban heat effectively during hot summer days. Their cooling effects are largely determined by the plant canopy structure, which can be classified into three features: canopy density and horizontal and vertical canopy structures. This study aimed to identify the contribution of these canopy structural features to cooling effect of urban forests and how their contribution varies during the day. Near-surface air temperature and relative humidity were measured repeatedly between 07:00 – 10:00, 11:00 – 14:00, 16:00 – 19:00 and 21:00 – 24:00 during 16 summer days of 2021 using a mobile monitoring system at five urban parks and representative reference areas in Shanghai. To measure canopy structural features around each measurement point with a scale of 5 m, 10 m and 20 m radius buffers, each park was scanned by an unmanned aerial vehicle (UAV) laser scanner and an UAV multiple-spectrum image system. We found vertical canopy structure had great importance in explaining cooling effects and even exceeded the magnitude of air temperature reductions caused by leaf area index and canopy coverage during noon and nighttime. With increasing foliage height diversity (FHD), the cooling effect first decreased and then increased, when the turning point of FHD was approximately 0.5. Generally, sites with a high canopy coverage and an unevenly distributed canopy (FHD < 0.3) in high layers are recommended to provide better cooling effects in summer. Along with canopy density and horizontal structure, vertical canopy structure must be considered to optimize the cooling potential of urban forests in future planning.