Objective Street trees are effective way to improve pedestrian comfort and walking environment, especially in summer. Therefore, it is of great significance to explore the influence of street trees on the thermal comfort of pedestrians and the thermal environment of road space

Method The air temperature, relative humidity, solar radiation and wind speed were measured on 7 roads in Xicheng district of Beijing. The Universal Thermal Climate Index (UTCI) was used as the evaluation index for thermal comfort, and the Thermal Sensation Vote (TSV) was analyzed. The effects of street tree types and structural characteristics on the thermal comfort of pedestrians in summer were discussed.

Result (1) All the 7 street trees could significantly improve the thermal environment of the road space, reducing the air temperature from 1.0 to 2.1 ℃, reducing the solar radiation from 481.27 to 789.18 W/m², and increasing the relative humidity from 0.65% to 6.17%. Wind speed reduction range was 0.02 ~ 0.38 m/s; (2) All the 7 street trees could significantly adjust the thermal comfort of pedestrians, and the average UTCI was decreased in the order of Salix matsudana > Styphnolobium japonicum > Platanus orientalis > Fraxinus chinensis > Ginkgo biloba > Koelreuteria poniculata > Ailanthus altissima. Salix matsudana and Styphnolobium japonicum had stronger effects. In summer, the average neutral UTCI of pedestrians under the road where the 7 street trees 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 significant negative correlation with air temperature and solar radiation, and a significant 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 little effect. (4) Based on UTCI, TSV and neutral temperature evaluation, it was considered that the performance of the 7 street trees was as follows: Salix matsudana > Styphnolobium japonicum > Platanus orientalis > Fraxinus chinensis > Ginkgo biloba > Ailanthus altissima > Koelreuteria poniculata.

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