Retrieval of three-dimensional tree canopy and shade using terrestrial laser scanning (TLS) data to analyze the cooling effect of vegetation

Urban warming has become a serious problem due to global warming and rapid urbanization. One important phenomenon is the increasing urban heat island (UHI) effect, which has a serious negative impact on energy consumption, environmental pollution, and human well-being. Trees lower land surface and air temperatures by providing shade and through the process of evapotranspiration and therefore are useful in effectively mitigating the UHI effect. The cooling effects of trees vary depending on the tree crown size and density and the optical properties of their leaves. Selection of the best species to plant is important in achieving effective mitigation of the UHI effect. In this research, we examined four woodlands. Three of these woodlands are dominated by species (Cinnamomum camphora, Metasequoia glyptostroboides, Magnolia grandiflora) that are frequently planted in Nanjing, China and one is a mixed woodland. Terrestrial laser scanning (TLS) was employed to detail the vegetation canopy structure and capture the volume of three-dimensional point clouds of the leaves (L_V3DPC), as well as the shade at each case study site. Meteorological parameters were measured at each site. Statistical analysis was used to assess the cooling effects of the different woodlands and their impacts. This research revealed that trees can influence the microclimate beneath their canopies and that the degree of the impact is different for different tree species. Statistical analyses showed that the woodlands studied exhibit obvious temperature reductions during the daytime (05:00–19:30) and weaker temperature reductions during the nighttime (19:30–05:00). The temperature reduction was greatest for M. glyptostroboides, followed by C. camphora, M. grandiflora, and the trees in the mixed broad-leaved woodland. These results indicate that small-leaved species tend to be more effective at cooling than large-leaved species. Comparisons of the Leaf Area Index (LAI) and Sky View Factor (SVF) with L_V3DPC and shade, respectively, show that L_V3DPC and shade better reflect the impact of the vegetation canopy on the cooling effect. Multiple linear regression analyses showed that shading by trees is of prime importance in cooling the thermal environment. The high significance of L_V3DPC and shade indicate that the tree canopy is a major component of the contribution of trees to microclimatic environments, particularly the cooling effect under the tree canopy. This paper presents an innovative technique for determining tree canopy shade using TLS data for the purpose of analyzing the cooling effect of trees. The findings can be used as a guide to aid in the selection of the best species for urban greenspace planning and design to cool the thermal environment and enhance energy savings in urban environments.