1987-2015年深圳市主城区景观演变过程多时相识别

Multi-temporal detection of landscape evolution in western Shenzhen City during 1987-2015

城市扩张具有典型的阶段性特征,城市化初期的不透水面快速蔓延和中后期的生态环境约束往往呈现非线性关系。基于多时相数据对典型城市发展中关键指标的变化拐点进行识别,将有助于认识城市景观演变的时间节点,理解城市化的生态响应过程。本文选用深圳市西部1987-2015年长时序Landsat影像共27期,逐年提取新构建的归一化裸露指数(MNDBI*)与归一化植被指数(NDVI*),从而在时间上寻找城市增长的转折点,在空间上识别不同空间位置的景观演变特征。研究结果表明,1987-2015年深圳市城市增长十分明显,并以2003年为拐点经历了"快速增长"至"平稳约束"的变化;与此对应,植被指数的关键拐点与城市化拐点基本重合,印证了城市建设用地扩张和生态系统响应的时空关联特征。此外,从空间分异来看,深圳市南部各区的城市化约束期出现相对更早,拐点一般在1995-1998年间;而北部各区的快速城市化时期持续更长,一般在2003-2006年后趋于平稳。在空间上,以市图书馆为中心刻画西、北、西北、东北剖面线方向的指标动态,发现采样中心附近的景观类型变化较小,而更远辐射半径经历了更大幅度的城市化;这说明深圳市南部城市发展较早达到饱和,且近30年的城市扩张以由南向北的放射式蔓延为主。面临城市化带来的生态威胁,相关环保措施的有力实施仅能延缓城市化导致的生境退化步伐,城市扩张所带来的生态破坏依然不容小觑,合理而有力的政策颁布、实施与监管在未来的城市发展中极为必要。

The expanding impervious surface along with the process of urbanization has produced great impact on ecological land in most of megalopolises all over the world. A typical urban expansion process usually shows a phase character, and the rapid increase of urban area has a non-linear correlation with vegetation degradation. Therefore, it is essential to recognize the important turning points in the curve of urban growth and the expansion pattern, which would also make a clear understanding of ecosystems＇ responses to urbanization. Landsat TM/ETM ＋ dataset was widely used in the detection of landscape change, however, most of the studies used the visual interpreted data per 5 years or above to characterize a long-term variation, and less attention was paid to the identifications of urban sprawl stages. In this study,with western Shenzhen City as a case study area, two new indexes of MNDBI＊ and NDVI＊were built based on long-term Landsat dataset during 1987-2015, in order to identify the turning points of urban sprawl in temporal dimension and analyze the landscape evolution characteristics at different expansion axes. The results showed that western Shenzhen had experienced a rapid urbanization and the water area as well as vegetation coverage decreased dramatically. Specifically, the urban growth could be divided into two phases, i.e. rapadly growing period and stable period by the year 2003, and the vegetation degradation changed in1999. The similar turning points confirmed the consistent spatiotemporal correlation between urbanization and the ecosystem responses. Furthermore, from the perspective of spatial differentiation, urban sprawl consistently occurred in the northern districts and coastland with sea reclamation until 2003-2006. The southern districts in western Shenzhen experienced the stable urbanization period much earlier than the other ones, with the turning points were identified during 1995-1998. Urban landscape evolution showed various characteristics in different profiles, a large number of sample points were selected through the west, north, east,northwest, northeast directions per 120 m, and the two indexes were calculated in 1987 and2015 in four directions from central Shenzhen. Obvious urbanization as well as vegetation degradation could be found in a further expansion radius. Also, it is indicated that the north and west profiles should be the major urban expansion axes. In a word, Shenzhen＇s urbanization has exerted a complex effect on the eco-environment, and in order to realize vegetation restoration,a series of environmental protection policies were promulgated and implemented in recent 10 years. However, this could only postpone the process of habitat degradation rather than ecological restoration. Therefore, more powerful policies are required in the future development of Shenzhen City, and more attention should be focused on the response of ecosystems to the urbanization process.