Dynamics of Hierarchical Urban Green Space Patches and Implications for Management Policy

Zhoulu Yu, Yaohui Wang, Jinsong Deng, Zhangquan Shen, Ke Wang, Jinxia Zhu, Muye Gan
2017 Sensors  
Accurately quantifying the variation of urban green space is the prerequisite for fully understanding its ecosystem services. However, knowledge about the spatiotemporal dynamics of urban green space is still insufficient due to multiple challenges that remain in mapping green spaces within heterogeneous urban environments. This paper uses the city of Hangzhou to demonstrate an analysis methodology that integrates sub-pixel mapping technology and landscape analysis to fully investigate the
more » ... otemporal pattern and variation of hierarchical urban green space patches. Firstly, multiple endmember spectral mixture analysis was applied to time series Landsat data to derive green space coverage at the sub-pixel level. Landscape metric analysis was then employed to characterize the variation pattern of urban green space patches. Results indicate that Hangzhou has experienced a significant loss of urban greenness, producing a more fragmented and isolated vegetation landscape. Additionally, a remarkable amelioration of urban greenness occurred in the city core from 2002 to 2013, characterized by the significant increase of small-sized green space patches. The green space network has been formed as a consequence of new urban greening strategies in Hangzhou. These strategies have greatly fragmented the built-up areas and enriched the diversity of the urban landscape. Gradient analysis further revealed a distinct pattern of urban green space landscape variation in the process of urbanization. By integrating both sub-pixel mapping technology and landscape analysis, our approach revealed the subtle variation of urban green space patches which are otherwise easy to overlook. Findings from this study will help us to refine our understanding of the evolution of heterogeneous urban environments. the presence of UGS is believed to provide numerous psychological benefits, such as relieving stress and anxiety [10] and promoting social cohesion [11] . Studies have also emphasized the importance of aesthetic views and restorative opportunities provided by UGS for urban residents [12, 13] . Urbanization exerts a complicated influence on UGS [14] [15] [16] [17] . Unplanned urban expansion decreases UGS area and degrades the quality of natural vegetation. The expansion of built-up areas generally destroys a great amount of the original vegetation cover and later replaces it with new artificial green patches, resulting in a high turnover rate for vegetation cover [18] . Vegetation in less developed regions is often more isolated and fragmented because economic growth and urbanization is regarded as a higher priority than maintaining urban vegetation [19] . However, in arid or semiarid cities, urbanization may result in an increase of vegetation by replacing primeval landscape with vegetated human settlements [20] . Since the mid-1980s, China has experienced arguably the most widespread and intensive urbanization in history, especially in the Yangtze River Delta, characterized not only by its enormous scale but also by unparalleled land use change [21] . More importantly, this process will not be halted in the short term. The latest National New-type Urbanization Plan of China projected that the proportion of permanent urban residents of China's total population will rise from 35.7% in 2013 to reach 60% by 2020 [22] . The remaining green space within cities is increasingly recognized as being vital to the amelioration of urban living conditions [14, 16] . Having realized the importance of urban green space, a number of global cities have set long-term strategies and policies to incorporate urban green space to help achieve a high-quality built environment. London has constructed a zonal greenbelt to shape patterns of urban development and create a green network by connecting green patches through a green wedge, green galleries, and channels to provide valuable habitat for flora and fauna [23] . Singapore has put forward the construction concept of being the "Garden City", and set goals constantly in different development periods [24] . The Japanese government has practiced a controlled green land system which limits development area with laws and regulations. In addition, they have also adopted auxiliary systems such as loss compensation, land expropriation, and tax cuts to boost the enthusiasm of citizens to protect the green space [25] . Similarly, various green space planning strategies have been put in place by local governments to promote sustainable development [26, 27] . There is great debate regarding the reliability and use of data approaches to quantify and track the changes, trends, and patterns of UGS over long periods. Owning to the increasing availability of image data from multiple sources, the quantification of spatiotemporal patterns for green space frequently relies on remote sensing [28] [29] [30] . However, data such as Lidar and high-resolution images are still not easily accessible for many regions or users due to the high costs of data acquisition. Moreover, it is usually impractical to provide full coverage of extensive metropolitan areas, with limited data available over long periods. With the advantages of global availability, repetitive data acquisition, and long-term consistency, Landsat series satellites have become the best compromise to overcome these limitations [14, 18, 26, 31] . Green spaces within cities are generally highly fragmented and heterogeneous, characterized by abundant dispersed patches [31, 32] . The commonly-used 30 m resolution Landsat data may fail to capture most of these small UGS patches (for example, lawns, street trees, residential yards), as well as their changes [30, 33] . In recent years, multiple endmember spectral mixture analysis (MESMA) has provided a great advantage to overcoming this problem associated with using Landsat data to map urban components in highly heterogeneous urban environments [34] [35] [36] [37] . Understanding the patterns of UGS is a complex issue, and utilizing landscape metrics analysis can be extremely helpful because of its ability to reveal the spatial features and changing processes in urban structures [38, 39] . The size, accessibility, and connectivity of UGS are deemed to have a significant impact on the well-being of urban residents and the maintenance of biodiversity. Coupling remote sensing and landscape metrics analysis can provide valuable information on the spatial characteristics of UGS distribution and variation under urban development. This study presents a new approach that combines sub-pixel mapping technology and landscape analysis with long-term time series Landsat images to describe the detailed pattern of UGS variation in a heterogeneous urban environment. The method involves two steps. First, MESMA was applied to time series Landsat images to estimate vegetation cover fraction (VCFs) at the sub-pixel level.
doi:10.3390/s17061304 pmid:28587309 pmcid:PMC5492285 fatcat:et6fnpcrxzf6rn43iy5ryzwmg4