Influence of 7 street tree species on thermal comfort of pedestrians in summer in Beijing

Objective The impact of street tree species and structural characteristics on the thermal comfort of pedestrians and the thermal environment of road spaces during summer was investigated to provide a basis for the species selection, structural design, and subsequent maintenance management of street trees in Beijing and other northern regions of China.

Method Taking the Fraxinus chinensis, Platanus orientalis, Ailanthus altissima, Koelreuteria paniculata, Salix matsudana, Ginkgo biloba and Styphnolobium japonicum along the roads in Xicheng District of Beijing as the research objects, this study measured the air temperature, relative humidity, solar radiation, and wind speed within their shaded spaces in summer. The universal thermal climate index (UTCI) was employed as the thermal comfort evaluation metric and the thermal sensation vote (TSV) of pedestrians was analyzed.

Result (1) All the 7 street tree species could significantly improve the thermal environment of road space, reducing the air temperature from 1.0 to 2.1 ℃, reducing solar radiation from 481.27 to 789.18 W/m², and increasing relative humidity from 0.65% to 6.17%. Wind speed reduction range was 0.02−0.38 m/s. (2) All the 7 street tree species could significantly adjust the thermal comfort of pedestrians, and the average UTCI decreased in the order of Salix matsudana > Styphnolobium japonicum > Platanus orientalis > Fraxinus chinensis > Ginkgo biloba > Koelreuteria poniculata > Ailanthus altissima. In summer, the neutral UTCI of pedestrians under road where the 7 street tree species were located was 24.7 ℃. (3) Air temperature and solar radiation were the decisive factors affecting TSV of pedestrians in summer. Leaf area index had a significantly negative correlation with air temperature and solar radiation, and a significantly positive correlation with relative humidity. Leaf area index was the most significant tree structure affecting UTCI, followed by crown width, while average leaf area had less effect. (4) Based on UTCI, TSV and neutral temperature evaluation, it was considered that the performance of 7 street tree species was as follows: Salix matsudana > Styphnolobium japonicum > Platanus orientalis > Fraxinus chinensis > Ginkgo biloba > Ailanthus altissima > Koelreuteria poniculata.

Conclusion In the future, suitable street tree species should be selected according to the thermal environment of road space in different regions, and it is proposed that street tree species with larger leaf density and crown width can provide a more comfortable road environment for pedestrians in summer.