The Cooling Effect Of Urban Green Infrastructure At Multiple Scales

Urban Heat Island(UHI)is a typical urban climate feature.As one of the most serious ecological consequences of urbanization,UHI has many negative impacts on urban ecosystems and humans,thus affecting the sustainable development of cities and human health.Urban Green Infrastructure(UGI)can effectively mitigate UHI with shading and evapotranspiration,and provide a variety of ecosystem services.Urban ecology and landscape ecology both are place-based and question-oriented sciences.How to exert ecosystem services function of UGI is both theoretical and practical challenge.Due to the high heterogeneity of urban landscape and the limited urban space,how to build and exert the optimal cooling effect of green infrastructure in complex and limited urban space has been a problem in academic circles and production practice,namely,How large and how many are the green infrastructure in urban areas?Where and what kind of greenspaces should be developed?This paper explored the mitigation effects of UGI on UHI at patch,landscape and city scales respectively.It aims to address the following research questions:(1)How does park size and its landscape composition influence cooling effect?(2)Does the landscape around the park affect its mitigation on UHI?(3)Does urban development density influence the cooling effect of UGI?To answer these questions,80 parks and green spaces within the outer ring in Shanghai were randomly selected as the research object.Two Landsat ETM+images acquired in March and July,2001 were used to retrieve biophysical parameters and land surface temperature.High spatial resolution image(2.5m)was utilized to derived land cover land use types which were further used to calculate landscape metrics of parks and its surrounding landscape.The maximum local cool island intensity(MLCII),the maximum cooling distance(MCD),the maximum cooling area(MCA),the maximum cooling efficiency(MCE),GIS spatial analysis,landscape pattern analysis and statistical methods were employed to quantify the cooling effects of UGI and its influencing factors at patch,landscape and city scales in Shanghai,China.The main findings are as following:(1)The size of UGI influences its cooling effect.The larger the park size is,the lower the mean LST.The fitting curve tends to stabilization when it decreases to a certain value of area.More than 80%of the parks were cool islands.There was no significant quantitative relationship between the maximum cool island intensity(MLCII)and park size in spring,but a significant positive logarithmic relationship existed in summer.The maximum cooling distance(MCD)and the maximum cooling area(MCA)both exhibited a significant positive linear relationship with park size.There was no significant quantitative relationship between the maximum cooling efficiency(MCE)and park size.Compared with the average cooling efficiency of parks in different groups,the small park group had the highest cooling efficiency.(2)The trees and shrubs had the best cooling effect.While the construction land in the park positively contributed to LST,but the shape complexity of the land could reduce the warming effects.The landscape pattern of the surroundings influences park's cooling effect both at patch and landscape levels,but with seasonal variations.The landscape pattern of the buffer zones mainly affects MCA and MCD.(3)Urban development density did influence the mitigation effect of green infrastructure on surface urban heat island.There were significant negative relationships between mean LST and vegetation fraction(F_v)at different levels of urban development density(with 10%interval of impervious surface percentage).Strong negative linear relationships existed between mean LST and F_vwithin the range of impervious surface area(ISA)from 0 to 90%.While exhibited negative logarithmic relationship when ISA is 90%-100%.The mitigation effect of green infrastructure existed when the ISA within the range of 0 to 90%.The optimal threshold value of urban development density is 70%-80%and 20%-30%of vegetation fraction where the UGI can achieve the best cooling effect.According to my results,I gave some suggestions that composition and spatial configuration of urban green infrastructure,especially urban parks,and its surrounding landscape should be considered.The optimal threshold values of urban development density and vegetation fraction should be identified to exert the best cooling effect of UGI in terms of urban geographical location and landscape structure during the planning and construction of UGI in the future.