| Urbanization,as a rapid process of land use and cover change dominated by human,inevitably occupies ecological resources,thus triggering a series of environmental problems and affecting regional ecological security.Nanjing,as one of the most important central cities in the east,plays a leading role in the level of social and economic development.And the urbanization rate has reached the level of developed countries so that the contradiction between urban expansion and ecological security is significant.Therefore,this study takes Nanjing metropolitan area as the study area,and based on the gradient division of urban area,describes the characteristics of urban expansion and the spatial differentiation of ecological security level in 1995,2005 and 2015 respectively,and explores the relationship between urban expansion and ecological security.The results are as follows:(1)During 1995-2015,the urban built-up land tended to expand acceleratively.The compactness gradually declined,and the expansion degree was relatively high.As time goes on,the degree of urban sprawl was higher,and spatially there were many new patches with higher expansion degree in Binjiang area of Jiangning District,Pukou District,Liuhe District,Qixia District and other regions.Based on the analysis of urban concentricity index,during the study period,the growth of urban built-up land mainly benefited from outward expansion of urban area,but seldom from the potential built-up land in the main urban area.However,during 2005-2015,although the expansion extent became much wider and the intensity was higher,the potential built-up land in the urban core area was well and fully utilized,because of relevant policies issued by the government to strengthen the regulation and control of urban construction land.Based on the analysis of urbanization intensive index,urban expansion in the former period mainly occurred in circle 4-13,while in the latter period,mainly in circle 5-18,with a wider scope and greater intensity than in the previous period.Generally speaking,the urban expansion in the two periods mainly took place within the main urban area and the suburban area near the main city.Based on the analysis of the landscape pattern of urban built-up land,the fragmentation degree of the urban built-up land increased with the increase of distance from the city center,and decreased over time.There was spatial differentiation for the change of patch density:the patch density of urban built-up land in the main urban area tended to decrease over time,which reflects that space in the main urban area has been well utilized,while the patch density of urban built-up land in the periphery of the main urban area increased,mainly due to the disordered spread of newly-added urban construction land.(2)For the ecological security assessment,the fragmentation of green spaces including cultivated land and forest land increased,and there was spatial differentiation for the change of landscape pattern in landscape level.The ecological quality of Nanjing urban area has improved during the research period,but there were still some areas whose ecological conditions were deteriorating.Specifically:On the class level,for cultivated land,its fragmentation degree increased with time and decreased as distance from the city center increased.There was spatial differentiation for the variation of its patch density:In the main urban area,the scattered cultivated land was occupied by new built-up land,so the patch density of the cultivated land declined with time.However,outside the main urban area,due to the occupation of urban construction land,the area of single farmland patch decreased and the number of patches increased,so the patch density raised.For the forest land,its fragmentation degree increased and connectivity between patches was weakened.However,compared with arable land,variation of indicators for forest land was not significant,reflecting that the expansion of urban construction land had a weaker impact on the area and fragmentation of forest land.On landscape level,in the main urban area and its vicinity,because of more and more disturbance from human activities,the landscape shape was becoming more regular,and its fragmentation degree was decreasing.In the contrary,in the outer suburbs,the fragmentation degree of patches was increasing,the landscape types were becoming richer and landscape boundaries were more complex because of less disturbance from human activities.From the perspective of ecological quality,generally speaking,the eco-environment quality of Nanjing metropolitan area has improved during1995-2015,especially in the main urban area and most part of Liuhe District in the north of the Yangtze River.However,there were still some areas where the ecological condition was deteriorating,such as both sides of the Yangtze River and along the southern part of the Qinhuai River.In terms of urban circle analysis,from 1995 to 2005,the eco-environment quality of most of the circle layers showed a downwardtrend,especially in circle 5-14 which was the main area for urban expansion,indicating that large-scale urban expansion have negative impact on habitat quality.During 2005-2015,the Nanjing government carried out large-scale programs for urban renewal and landscape greening so that the ecological index of most circles,especially in the main urban area,has been improved.(3)Urban expansion is significantly relevant to regional ecological security,as described in the following:1)The scale of urban expansion is closely related to the fragmentation degree of cultivated land and forestland,as well as the fragmentation degree,landscape dominance degree,diversity and landscape shape in landscape level.And at the same time,urban expansion is also significantly correlated with the decline of eco-environment quality.2)the fragmentation degree of construction land is positively correlated with eco-environment quality.3)There is a significant correlation between urban expansion mode and the fragmentation degree of cultivated land.The smaller the landscape expansion index is,that is,the looser the urban expansion structure is,the higher the fragmentation degree of cultivated land is.And also,urban expansion mode is significantly correlated with landscape diversity index and largest patch index.We can see that overspreading urban expansion can increase landscape diversity and reduce patch dominance.Meanwhile,urban expansion mode is also closely related to ecological quality,and compact urban expansion mode is more advantageous for ecology protection.(4)It is suggested that the pattern of urban spatial expansion should be optimized in Nanjing metropolitan area and the expansion mode should change from scale expansion to intensive expansion.And at the same time,efforts should be made to protect those land use types with high ecological quality such as cultivated land,forest land and grassland,and to further strengthen the greening construction in urban built-up areas,so as to make connection between fragmented habitat patches. |
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