Heat Mitigation Benefits of Street Tree Species during Transition Seasons in Hot and Humid Areas: A Case Study in Guangzhou

The potential microclimatic effects of street trees are influenced by their ecological characteristics, planting patterns, and street orientations, especially in subtropical hot and humid areas. To investigate these effects, four typical street tree species in Guangzhou were selected for study during the transition seasons: Khaya senegalensis, Terminalia neotaliala, Ficus microcarpa, and Mangifera indica. Air temperature (AT), relative humidity (RH), solar radiation (SR), surface temperature (ST), wind speed (WS), and the leaf area index (LAI) were monitored. The cooling effects of these four species and the resulting improvements in human thermal comfort (HTC) were assessed. The influences of tree planting patterns and street orientations on cooling benefits were systematically analyzed. The results indicate that, during transition seasons, the four street trees, on average, can block 96.68% of SR, reduce AT by 1.45 degrees C and ST by 10.25 degrees C, increase RH by 5.26%, and lower the physiologically equivalent temperature (PET) by 8.34 degrees C. Terminalia neotaliala, reducing AT and PET by 1.76 degrees C and 12.4 degrees C, respectively, offers the greatest potential for microclimate improvement. Among the four tree species, the variations in ST (Delta ST) and PET (Delta PET) were minimal, at only 0.76 degrees C and 0.25 degrees C, respectively. The average differences in AT and PET between inter-tree and under-tree environments were 0.06 degrees C and 0.98 degrees C, respectively. The AT reduction rate was 1.7 times higher in the double-row planting pattern compared to the single-row planting pattern. Street trees planted in the northwest-southeast (NW-SE) orientation exhibited a 16.96% lower WS reduction than those in other orientations. The northeast-southwest (NE-SW) orientation showed the least potential to enhance human thermal comfort. Compared to NE-SW, the northwest-southeast (NW-SE) orientation achieved twice the rate of AT reduction, while the north-south (N-S) orientation improved it by 1.3 times. This data analysis aids in assessing the impact of green infrastructure on urban climates and demonstrates the year-round microclimatic benefits of street trees.