The influence of trees shade level on human thermal comfort and the development of applied assessment tools

Abstract

With the intensification of global warming and the urban heat island effect, vegetation plays a crucial role in shading and cooling urban spaces. The extensive planting of trees not only contributes to urban cooling but also enhances human thermal comfort by providing shade. Implementing government policies and regulations is an effective strategy for promoting urban greenery. However, Taiwan currently lacks a comprehensive database and assessment methodology for evaluating tree shading capacity, making it challenging to objectively assess the extent of urban green coverage.

This study conducted empirical measurements on 38 common tree species in Taiwan and utilized ENVI-met simulations to model six scenarios of the Leaf Area Index (LAI). Based on shading capacity, a tree shading capacity index was established, and the impact of varying LAI values on human thermal comfort was analyzed. Additionally, two evaluation formulas—Green Plot Ratio (GPR) and Greenery Cooling Capability (GCC)—were developed to assess the amount of greenery at a given site and its cooling effectiveness.

The results indicate that high-LAI trees provide significant shading and cooling benefits in urban environments. As LAI increases, the reductions in ΔTmrt and ΔPET become more pronounced. A strong correlation was observed between greenery coverage and cooling effectiveness. Specifically, when GPR reaches 2.2, the average PET of a site decreases by 1.8 °C, and when GPR increases to 3.2, PET decreases by 4.4 °C. The GPR proposed in this study has already been incorporated into Taiwan’s green coverage regulations.