Abstract: [Objective] Previous researches on the cooling and humidifying effects of urban green space mainly base on the large-scale and obscure the influence of plant community structure and tree species. However, this study based on the fine-scale investigated the daily dynamics in air temperature and relative humidity and the accurate cooling and humidifying effects of the five landscape plant communities during a calm sunny summer day, aiming to provide a reference for their landscape application as well as provide the theoretical basis for planning and construction of the urban green space system. [Method] In the Beijing Olympic Forest Park, five landscape plant communities with different canopy structure characteristics, including Populus tomentosa community, Ulmus pumila community, Salix matsudana community, Koelreuteria paniculata community, and Ginkgo biloba community, were selected as the research object. The open turfgrass lawn served as control open space site. The air temperature, relative humidity, wind speed, and canopy structure characteristics were measured in the field on calm sunny summer days. The collected data were processed and quantitatively analysed in every minute through the EXCEL2010 and SPSS19.0 software, to detect the detailed diurnal variation in the air temperature and relative humidity of the every studied community and to compare the cooling and humidifying effects among the five communities. [Result] The results showed that diurnal variation in the air temperature and relative humidity of the five plant communities displayed a single peak in the 24 h of a day and the periods in which a state of high temperature and low humidity was maintained were different among the communities. The period of high temperature and low humidity appeared at 11:00 to 16:00 in P. tomentosa community, 12:00-17:00 in G. biloba community, and 11:00 to 17:00 in S. matsudana community which was relatively longer than other communities. However, there was no obvious high temperature and low humidity state in U. pumila and K. paniculata communities. Additionally, all the five plant communities had the significant cooling and humidifying effects during 08:00 to 18:00 when they lowered the air temperature by 3.0 to 4.3℃ with an average of 3.7℃. Meanwhile, compared with the control open space site, they increased the relative humidity by an average of 11.3% and ranged from 10.2% to 12.8%. However, these two effects in the studied communities were not significantly different from the control during 18:00 to 08:00 except in the U. pumila community. [Conclusion] On the basis of measured data per minute, there was a unimodal diurnal variation in these two microclimate indices in the five frequently-used communities and the diurnal variation in air temperature was more stable than that in relative humidity. Moreover, these plant communities had different influence on the air temperature and relative humidity within the internal environment likely because of the difference in canopy structure characteristics. These cooling and humidifying effects tended to be significant during the daytime while they were not significant during the night. Therefore, this study might form a theoretical reference for people to pleasantly employ the urban green space with the five tree species. Furthermore, other frequently-used tree species would be investigated on this issue, to provide a reference for the scientific planting design to build a urban green space with the pleasing temperature and humidity, as well as to promote the comfort degree for outdoor activities.

Key words: plant community, air temperature, relative humidity, heat island effect, Beijing Olympic Forest Park