Resilient Cities in a Changing World AMPS | City, University of London Page 55 The Figure 14 illustrates the maximum difference in PET between the three experimental groups and the blank control group, highlighting the maximum improvement in thermal comfort achieved by each experimental group in the street canyon. The data shows that the PET trends for each tree species across different orientations are generally consistent. Notably, in the northwest-southeast and NE-SW orientations, Salix matsudana, Populus alba, and Ulmus pumila achieved maximum temperature reductions of 9.8°C, 4.0°C, 5.1°C and 6.4°C, 6.7°C, and 7.7°C, respectively. Comparisons among tree species reveal that, in the NE-SW orientation, the 15- year-old Ulmus pumila tree provides the most significant improvement in PET values, while in the northwest-southeast orientation, the 30-year-old Salix matsudana demonstrates the most substantial reduction in PET. These results offer valuable insights for urban street tree renewal strategies. Additionally, we analyzed the cooling slope of street trees at different growth stages. The slope of each segment in the function graph represents the rate of PET reduction, indicating the change in PET for each additional year of tree age. Generally, the slope from ages 5 to 15 years is steeper than that from 15 to 30 years. Specifically, during the 5-15 year stage, Ulmus pumila on NE-SW oriented streets has the largest slope (k1 = 0.44), meaning the PET value decreases by 0.44°C for each additional year of tree age. In comparison, the slopes for Populus alba and Salix matsudana are 0.33 and 0.16, respectively. Furthermore, the improvement in PET value at 5 years of age is highest for Ulmus pumila (3.3°C), compared to Populus alba (2.7°C) and Salix matsudana (2.3°C). Therefore, planting Ulmus pumila saplings in NE-SW orientations is recommended for the fastest improvement in street thermal comfort. In the 15-30 year period, the improvement effect of Ulmus pumila and Populus alba on thermal comfort slows, while Salix matsudana's impact on PET continues to increase with tree age. Trend predictions suggest that after 30 years, Salix matsudana will provide greater and more sustained improvement in thermal comfort compared to the other two species, making it the best choice for long-term benefits. In the NW-SE oriented streets, Salix matsudana exhibits the largest slope (k1 = 0.28), meaning the PET value decreases by 0.28°C for each additional year of tree age. In comparison, the slopes for Populus alba and Ulmus pumila are 0.17 and 0.22, respectively. Additionally, at 5 years of age, Salix matsudana provides a PET reduction of 3.1°C, which is higher than that of Populus alba (1.5°C) and Ulmus pumila (2.9°C). During the 15-30 year stage, similar to the NE-SW (NE-SW) orientation, the improvement effect of Ulmus pumila and Populus alba on thermal comfort slows down, while Salix matsudana's effect on PET continues to increase with tree age and remains higher than that of the other two species. Therefore, in the NW-SE orientation, planting Salix matsudana is recommended for achieving a faster and more efficient improvement in PET values. In both NW-SE and NE-SW orientations, the improvement in PET value due to Salix matsudana increases with tree age. This is likely because the leaf area index of mature Salix matsudana is relatively low, resulting in higher crown porosity. Consequently, the dense crown does not significantly impede air circulation within the street canyon. Instead, it provides enhanced shading and more humid air, which contributes to greater pedestrian comfort. Conversely, as the age of Ulmus pumila and Populus alba increases, the reduction in PET value due to decreased air velocity becomes more pronounced compared to the shading effect. This leads to a gradual decrease in overall thermal comfort. CONCLUSION Our study examined the impact of street tree growth on thermal comfort in street canyons. The results indicate that in the NW-SE and NE-SW orientations, Salix matsudana, Populus alba, and Ulmus pumila achieved reductions in PET values of up to 9.8°C, 4.0°C, 5.1°C and 6.4°C, 6.7°C, and 7.7°C, respectively. The PET spatial distribution map demonstrates that, at the peak temperature, the PET value consistently decreases with tree maturity, with blocks featuring 30-year-old trees showing the lowest average PET values. The improvement in thermal comfort is more pronounced for trees aged 5-15 years
