Research On The Urban Tree Canopy And Its Influence On Urban Thermal Environment In Capital Functional Core

Under the background of global warming and the rapid development of Chinas urban construction,urban thermal environment problems such as the increase of high temperature days and the advance of high temperature days have seriously affected the urban ecological environment management and urban green development.As an important part of urban greening,urban forest always plays a positive role in adjusting the urban thermal environment.The introduction of the concept of urban forest provides a new perspective for landscape architecture to deal with urban environmental problems.Therefore,it is of great significance to study the relationship between urban forest crown cover and urban thermal environment for scientific planning and construction of urban forests and effective play of their functions in regulating urban thermal environment.Based on the data of 2005,2009,2015 and 2019,the characteristics and evolution of urban forest crown cover and the spatial distribution of urban thermal environment in Beijings capital functional core area were studied.The impact of urban forest crown cover on urban thermal environment was studied from two aspects of forest crown cover patch characteristics and landscape pattern.The main results are as follows:(1)The land cover types in the functional core area of Beijings capital are divided into 5 categories,and the area of impervious patches accounts for about 72% of the whole area with a large proportion;The second is the patch area covered by the crown,accounting for about 24% of the whole area.The area of water body,bare land and grassland patches is relatively small,accounting for about 3% of the whole area.From 2005 to 2019,the overall dynamic transformation process between impermeable surface and canopy coverage is shown.The canopy patches of park green space have diversified patch areas and shapes,and the patch aggregation degree is relatively high,mainly distributed in the north,middle and south of the functional core area of the capital.Street trees and canopy covered patches of protective green space form a green network relying on urban roads and rivers,but the connectivity of canopy covered patches in some areas is poor due to the restriction of construction land.The patches covered by the crown of the affiliated green space are relatively evenly distributed in the whole region,but due to the division of construction land and roads,a series of patches with small area and simple and regular shape are formed,and the degree of patch fragmentation is relatively high.(2)From 2005 to 2019,the phenomenon of urban heat island in the functional core area of Beijings capital is relatively obvious,with medium-temperature patches accounting for the largest proportion,followed by sub-high-temperature patches.In space,the medium temperature patches cover the whole region,the sub-high temperature patches gather,and the ultra-high temperature patches and high temperature patches are distributed in dots.According to the proportion index of urban heat island in the fourth phase,it can be found that the phenomenon of urban heat island in the functional core area of the capital shows a slight easing trend.(3)There is a correlation between land cover types and urban thermal environment in the capital functional core area: Regression analysis of random data shows that the surface temperature in the functional core area of the capital is negatively correlated with Normalized Vegetation Index(NDVI)and Modified Normalized Difference Water Index(MNDWI).When NDVI is in the range of 0.25-0.45,the cooling effect is obvious.It means when the vegetation coverage is in the range of 25%-45%,the cooling effect of vegetation is obvious.While MNDWI is in the range of 0.25-0.50.There is a positive correlation between surface temperature and Normalized Difference Build-up Index(NDBI),and buildings and impermeable face have obvious warming effect on urban surface temperature.(4)The relationship between urban tree canopy patch characteristics,urban forest scale and landscape pattern and urban thermal environment: When the urban forest canopy patch area is 13-25 ha and the patch perimeter is 7-10 km,the efficiency of regulating thermal environment is the greatest.The patch shape index of urban tree canopy has a linear positive correlation with the ability of urban forest to regulate thermal environment.At the landscape pattern level,when the urban tree canopy coverage is more than 33%,increasing the tree canopy coverage can effectively reduce the urban surface temperature.Average land surface temperature was negatively correlated with urban tree coverage,largest patch index(LPI),patch cohesion index(COHESION)and aggregation index(AI),and positively correlated with patch density(PD)and landscape shape index(LSI).The purpose of reducing urban surface temperature can be achieved by adjusting the spatial layout of urban forest,i.e.by increasing the connectivity between urban forest patches and improving the aggregation degree between patches.In conclusion,this paper puts forward the optimization strategy of urban forest based on the regulation of urban thermal environment from(1)increasing the urban tree canopy coverage rate and optimizing the characteristics of urban forest patches;(2)optimizing the spatial layout of urban forests;(3)strengthening the conservation and management of urban trees and shrubs in order to provide some suggestions for the construction of urban forest in Beijings capital functional core district.