Effect of the roadside tree canopy structure and the surrounding on the daytime urban air temperature in summer

As an important component of urban forests, roadside trees provide cooling and humidification effects, thereby improving the microclimate and human thermal comfort during hot summer weather by blocking, reflecting and absorbing solar radiation. However, the complex three-dimensional (3D) structure of roadside trees, diverse planting types and complex landscape heterogeneity complicate efforts to quantify the effects of these factors on above-road air temperatures. In this study, a terrestrial laser scanner (TLS) is used to accurately estimate roadside tree morphological parameters, such as the tree height (TH) and the diameter at breast height (DBH), and canopy structural parameters, such as the canopy thickness (CT), the canopy cover above the road (CCAD), canopy volume over the road (CVAD) and the leaf area index (LAI), to evaluate different 3D canopy geometries (pyramidal, spherical, pileate and cylindrical). A moving meteorological station was used to continuously obtain the horizontal air temperature (AT) distribution to investigate the effects of the canopy structural parameters, the planting types and the land cover types on the above-road AT. The value of ΔAT was positively correlated with TH, CVAD, LAI and CCAD. These structural variables were most significantly correlated with ΔAT under the pileate canopy shape. However, there were no significant correlations between these factors under the cylindrical canopy shape. The physiological equivalent temperature (ΔPET) reached 7.4, 15.8, 4.5 and 17 °C under the cylindrical, spherical, pyramidal and pileate shapes, respectively. The ΔAT of a two-sided planting was approximately 1.7 times greater than that of a one-sided planting; the ΔRH showed a similar trend. Within 230 m of a lake, the ΔAT exhibited a logarithmic relationship with the distance from a given point to the lake. Our study quantifies the influence of the roadside tree structure on the thermal environment, which can serve as a meaningful reference for urban tree species selection and arrangement.