Evaluation Of Tree Shade Effectiveness And Its Renewal Strategy In Harbin Typical Historic Districts

As an essential part of the city,the quality of the thermal environment in urban historic districts has a profound impact on the daily life style of citizens.Optimizing the amount of tree shade in the street canyon without changing the building form is an important way to improve the quality of the thermal environment of the district.In this study we evaluate the shading effectiveness of trees in typical historical districts of Harbin and propose strategies to renew trees from numerical and spatial dimensions,using a fast and large-scale calculation of tree shade in urban street canyons based on big data and machine learning.This paper provides a reference for the tree shade evaluation and update of other northeastern cities under the influence of Middle Eastern railway culture and the whole city.In this study,firstly,Sky View Factor(SVF)is used as the shading index of the street canyon,and Seg Net(Convolutional Neural Network)is used for image semantic segmentation of Baidu Street View to calculate the real-time SVFp of the street canyon environment in Harbin historic district;secondly,the SVFs of the street canyon with only the shading of the building are simulated using the DEM data of the building.The shading amount(SVFd)of trees in the Harbin historic district was obtained by calculating the difference between the two SVFs.Various statistical methods were used to analyze the correlation mechanism between the morphological indicators of the street canyon and the shading amount of trees,and to evaluate the shading performance of trees in the Harbin historic district.The results showed that:(1)the thermal environment of Harbin historical district was significantly influenced by the trees in the street canyon,and the average contribution of tree shade was 56.3%;(2)due to the special 45°angle direction of the street canyon morphology in Harbin,the contribution of tree shade in the three street canyons facing northeast-southwest,northwest-southeast,and 30°east-northwest converged(55.8%,56.1%,56.4%);the amount of tree shade varied with the street canyon.The increase in the height to width ratio was characterized by a stepwise linear decrease,with an average decrease of 31.6%;the amount of tree shade was most negatively correlated with the average block building height(ABH)within the 40 m buffer zone(r =-0.417).At the same time,this paper aims to propose tree renewal strategies for Harbin’s historic districts.Firstly,based on numerical analysis,this paper generates a tree shading data visualization and distribution map to visualize the shading status of trees.Secondly,this paper introduces the Geo Da autocorrelation analysis method for the first time at the neighborhood scale,which expands the important dimension of the quantitative analysis of tree shading in the street canyon.The results show that the shade amount of trees is positively and spatially correlated in the global geospatial autocorrelation analysis of the historical district,with Moran’I = 0.487.Based on the local spatial autocorrelation,we can obtain the "high value aggregation" of the shade amount of trees in the Harbin historical district and the "high value aggregation" of the shade amount of trees in the Harbin historical district.This study is based on the idea of "low value agglomeration" and further research on the distribution of agglomeration directions.This led to the proposal of a "top-down" macro-level tree renewal strategy for the street canyons in Harbin’s historic districts.Finally,based on the results of spatial autocorrelation and numerical analysis of tree shading,all streets in a typical neighborhood are divided into six spatial levels,so that a "bottom-up" micro-scale tree shading renewal strategy can be obtained.