Spatial Structure And Its Ecological Performance In Chinese Prefecture Cities

Along with urbanization,cities have become the most prominent places for environmental contradictions,and spatial structure adjustment has become one of the main means of urban ecological governance.Excessive population concentration will lead to congestion,over-exploitation of resources,ecological space reduction,pollution and heat emissions,thus leading to the decline of urban ecological quality.However,the dispersal of population may also lead to energy waste,inadequate environmental regulations and other problems.Therefore,the monocentric urban structure that emphasizes high density and the polycentric structure that emphasizes the combinations of spatial decentralization and re-clustering,which one has higher ecological performance?This is the first focus of this research.The spatial structure does not directly affect the ecology,but it can affect the urban ecology by affecting the spatial layout of industrial enterprises.Industrial enterprises occupy a large amount of impervious surface and are the main pollution source and heat source in the city.The mediating effects of industrial enterprises spatial distribution is the second focus of this research.The spatial structure is sensitive to spatial scale.For example,a monocenyricity at the scale of a municipal district may become a polycentricity at the scale of a city-region,which will directly affect the impacts on the urban ecological performance.Therefore,the third concern of this research is to explore whether the ecological performance of spatial structure varies with spatial scales.In view of the fact that a unified urban ecological analysis framework has not been formed in previous studies,we construct a framework of urban ecological connotations of“pollution reduction,”“green increase”,and“disaster prevention”,explore the impacts of population spatial structure on urban air quality,green space and heat island effect from the municipal district and city-region,and examines the mediating effects of the spatial distribution of industrial enterprises.The main findings are as follows:Regarding air pollution,a monocentric population structure at the municipal district is conducive to reducing PM_2.5 emissions,and a polycentricity at the city region is conducive to reducing PM_2.5 emissions.Mechanism analysis found that in municipal districts,monocentricity centralized industrial enterprises in the city center;while in city region,polycentricity decentralized industrial enterprises to sub-centers.As a result,industrial enterprises centralized in the center of municipal districts can improve their technological level through knowledge spillovers,and their proximity can share environmental facilities.In addition,they are subject to high environmental regulations to reduce PM_2.5 emissions;while industries scattered in the city region use the saved land rent and labor costs for environmental protection,and the industrial enterprises clustered in the sub-center will also be affected by the agglomeration economy,thus reducing the pollution to the peripheral areas.For urban green spaces,whether in municipal districts or city region,the polycentric population distribution not only helps increase the area of green spaces,but also improves the accessibility.From the mechanism empirical analysis,the polycentric structure brought about the decentralization of industrial enterprises in the municipal districts and city region.It may result in two outcomes.On the one hand,the impervious surface of the city center?primary center?will be transformed into a green space.The suburbs?sub-centers?have more funds to build green spaces due to the agglomeration of enterprises.On the other hand,as the polycentricity radius becomes smaller,and the decrease in density will increase the traffic efficiency,the accessibility of the green space is also improved.Regarding the heat island effect,whether at the municipal district or city-region,the polycentric population distribution is conducive to decrease the intensity of the heat island.Because the polycentric population structure draws the main heat source?industrial enterprises?from the city center to the suburbs,thus reducing the temperature difference between the city center and the periphery in the municipal district and the city-region.Therefore,from the perspective of urban ecological integrity and spatial multi-scale,there is no single spatial structure with absolutely high ecological performance.In the practice of urban planning,specific problems should be analyzed in detail,and appropriate planning strategies should be selected according to local conditions.The contributions of this research is that in the research framework,we constructed a complete urban ecological connotation framework and provided an overall understanding of urban ecological performance research.In the research results,it is found that there is no perfect spatial structure that can uniformly improve the performance of all urban ecological elements.Therefore,it is necessary to conduct specific analysis of specific issues in guiding urban planning.On the spatial scale,we explored comparative studies on two spatial scales:municipal districts and city regions,which not only complemented the lack of multi-scale comparison of the ecological performance of the spatial structure,but also improved the previous studies on the lack of attention to the scale of administrative divisions in Chinese cities.The findings have important practical significance for urban management and urban planning.Chinese cities are promoting the constructions of new cities and industrial parks,which can indeed improve the green space performance and reduce the intensity of heat islands to a large extent.However,for municipal district,the monocentric population structure is more conducive to decreasing PM_2.5 emissions.Therefore,“one size fits all”implementation of polycentric planning has ecological risks.In the process of urban ecological planning,appropriate spatial structure schemes should be selected according to different ecological problems and different spatial scales,so as to achieve the purpose of improving the overall ecological performance of cities.