| Urban park green space has already been proven to relieve the urban heat island effect (UHI) and perfect the human comfort in summer. The multi-scale research to understand the driving mechanisms of cooling and humidifying effects of urban park green space is a premise for the quantitative evaluation of these two effects and for the appropriate guidance of urban park construction. In this study, Beijing Olympic Forest Park was selected as the research object which was classified as three scales of patches, namely park patch, vegetation patch in the park, plant community in the vegetation patch. Based on the field measurements, the cooling and humidifying effects of the park green space and their driving mechanisms were investigated on calm sunny summer days, and then these two effects of vegetation patch were modeled. Finally, the distribution maps of human comfort in the park during the different periods of the summer day were constructed. The conclusions aim to reveal the driving mechanisms of cooling and humidifying effects in urban park with multi-scale analyses, which contribute to providing the scientific data for the efficient management and future transform of the park, as well as providing the reference for other urban park construction. The main experimental results are as follows:1. The scale of plant community patch(1) Cooling and humidifying effects of common species of plant communities. The diurnal variations in the cooling and humidifying effects of the eight monodominant communities were detected by the field measurement. The results were showed as:there were some differences of cooling and humidifying effects in various plant communities on calm sunny summer days. During the 08:00-18:00, the air temperature was decreased by 3.0-5.8℃, the soil temperature was decreased by 1.8-3.5℃, and the relative humidity was increased by 8.4-20.3%. Meanwhile, during the high temperature periods (10:00-15:00), the communities decreased the air temperature ranged from 2.9℃ to 6.8℃, decreased the soil temperature ranged from 2.0℃ to 4.1℃, and increased the relative humidity ranged from 10.2% to 20.9%.(2) Correlations between the canopy structures and cooling and humidifying effects. The influence of canopy structures on cooling and humidifying effects of the eight monodominant communities were studied by the field measurement. The results were showed as:both canopy density (CD) and leaf area index (LAI) were more important for determining the effects. The correlations tended to vary accordingly at different daytime periods. During the morning time, CD and LAI were major canopy structures to influence the two effects in Koelreuteria paniculata and Ginkgo biloba communities. CD and LAI were significantly related to cooling effect, while only CD was significantly related to humidifying effect in Ulmus pumila and Salix matsudana communities. CD was more important for determining the cooling effect in Populus tomentosa community. During the afternoon time, CD and LAI were major canopy structures to impact the two effects in Populus tomentosa, Ulmus pumila and Salix matsudana communities. CD was significantly related to cooling effect in Fraxinus pennsylvanica community. CD and LAI were more important for determining the humidifying effect in Robinia pseudoacacia community.(3) Correlations between the light intensity and cooling and humidifying effects. The influence of light intensity on cooling and humidifying effects was studied by the field measurement. The results were showed as:The light intensity within the community was a major environmental factor to influence the two effects, which implied that the greater the light intensity was, the weaker the cooling and humidifying effects were. However, the correlations tended to vary accordingly at different daytime periods. During the 10:00-13:00, the correlations were proven to be more significant.2. The scale of vegetation patch(1) Correlations between the proportion of surrounding underlying surface and the structures of patch and the cooling and humidifying effects. The influence of the two respects of factors on cooling and humidifying effects were investigated by the field measurement and artificial digitization. The results were showed as:the proportion of surrounding underlying surface and the patch structures were major factors to influence the two effects. The influence ranges of various types of underlying surface were greatly different. The water cover and tree cover were significantly and positively correlated with cooling effect, whose influence range were 10 m and 50 m, respectively. The impervious surface cover and lawn cover were significantly and negatively correlated with the effect, whose influence range were 20 m and 50 m, respectively. Only lawn cover was significantly and negatively correlated with humidifying effect, whose influence range were 50 m. Besides, the area (A) and tree cover was positively correlated with these two effects. However, the perimeter (P), perimeter area ratio (PA), landscape shape index (LSI) and fractal dimension (Fd) were negatively related to the effects, among which tree cover was more contributory for the effects.(2) The model to forecast the cooling and humidifying effects of vegetation patch. Based on the above analyses, the variables to significantly influence the cooling effect included CD, A. PA, Fd, tree cover, water cover in 10 m buffer zone, impervious surface cover in 20 m buffer zone, tree cover and lawn cover in 50 m buffer zone. Meanwhile, the variables to significantly influence the humidifying effect included CD, A, PA, tree cover, and lawn cover in 50 m buffer zone. Then the cooling and humidifying effects of vegetation patch were modeled with these variables, respectively (See 5.2.4).(3) Relationships between the distances from patch edge to the internal and cooling and humidifying effects. The edge effect of these two effects in vegetation patch was revealed by the field measurement. The results were showed as:during the three periods of the day (08:00-10:00, 10:00-15:00, and 15:00-18:00), the edge effect of air temperature and soil temperature were 5 m while the relative humidity had an irregular change trend.3. The scale of park patchThe spatial and temporal distribution of cooling and humidifying effects and human comfort in the park were analyzed by the field measurement. The results were showed as:there were significant cooling and humidifying effects in the park on calm sunny summer days, which leaded to the reduction of discomfort index. The cooling effect of the park was ranged from 2.7℃ to 6.2℃, and the humidifying effect was from 12.6% to 21.2%. During 08:00-10:00, the discomfort index in the park was lowered by 1.0-5.2. The lowering were 0.6-4.6 and 0.2-3.0 during 10:00-15:00 and 15:00-18:00, respectively. |
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