Influence of canopy structural characteristics on cooling and humidifying effects of Populus tomentosa community on calm sunny summer days

The measurements of air temperature, relative humidity, and canopy structural characteristics were performed on calm sunny summer days in the Populus tomentosa community and the sun in the Beijing Olympic Forest Park, China. The aim of this study was to investigate in detail the correlations between the detailed cooling and humidifying effects and the canopy structural characteristics of the community. Hourly values of air temperature and relative humidity, canopy density, leaf area index and mean leaf angle were analysed statistically. The cooling effect reached its maximum by 13:00 h, whereas the humidifying effect reached its peak at 09:00 h. Compared with the control open space site, the community seemed to lower the air temperature by 1.0 °C-5.0 °C and to increase the relative humidity by 4%-15% during the daytime. Furthermore, the canopy density had a greater impact in terms of cooling and humidifying effects from 08:00 h to 13:00 h. In addition, these effects were more controlled by the canopy density and by the leaf area index than by the mean leaf angle from 15:00 h to 18:00 h. The results also suggest that the nature of the correlations will vary at different times of the day. Overall, these findings provide new insights into the relationships between canopy structural characteristics and diurnal variations in microclimatic parameters in urban forests, which could be studied in greater detail in future studies.