Monitoring urban greenness dynamics using multiple endmember spectral mixture analysis.

Muye Gan,Ke Wang,Xinyu Zheng,Yang Hong,Jinsong Deng,Clinton N Jenkins

doi:10.1371/journal.pone.0112202

Abstract

Urban greenness is increasingly recognized as an essential constituent of the urban environment and can provide a range of services and enhance residents’ quality of life. Understanding the pattern of urban greenness and exploring its spatiotemporal dynamics would contribute valuable information for urban planning. In this paper, we investigated the pattern of urban greenness in Hangzhou, China, over the past two decades using time series Landsat-5 TM data obtained in 1990, 2002, and 2010. Multiple endmember spectral mixture analysis was used to derive vegetation cover fractions at the subpixel level. An RGB-vegetation fraction model, change intensity analysis and the concentric technique were integrated to reveal the detailed, spatial characteristics and the overall pattern of change in the vegetation cover fraction. Our results demonstrated the ability of multiple endmember spectral mixture analysis to accurately model the vegetation cover fraction in pixels despite the complex spectral confusion of different land cover types. The integration of multiple techniques revealed various changing patterns in urban greenness in this region. The overall vegetation cover has exhibited a drastic decrease over the past two decades, while no significant change occurred in the scenic spots that were studied. Meanwhile, a remarkable recovery of greenness was observed in the existing urban area. The increasing coverage of small green patches has played a vital role in the recovery of urban greenness. These changing patterns were more obvious during the period from 2002 to 2010 than from 1990 to 2002, and they revealed the combined effects of rapid urbanization and greening policies. This work demonstrates the usefulness of time series of vegetation cover fractions for conducting accurate and in-depth studies of the long-term trajectories of urban greenness to obtain meaningful information for sustainable urban development.

Highlights

The rapid urbanization in China in recent decades, accompanied by the continuous increase in urban population and the unprecedented growth of cities, has put significant pressure on urban environments
The R2 values were relatively high, ranging from 0.929 to 0.881, indicating a good fit between the ‘true’ fraction and the fraction derived from Multiple endmember spectral mixture analysis (MESMA)
Spatiotemporal analysis of urban greenness change To construct vegetation cover images for corresponding years, the amount of vegetation in each pixel was represented by the assigned gray value

Summary

Introduction

The rapid urbanization in China in recent decades, accompanied by the continuous increase in urban population and the unprecedented growth of cities, has put significant pressure on urban environments. This pressure has led to severe environmental issues, such as urban heat islands, air pollution, and urban flooding [1]. Unlike the declining trend reported by the above studies, green space coverage may increase with urbanization intensity. Yang et al confirmed the greening trend in larger cities, and they note that rapid urbanization caused a dramatic turnover in vegetation cover [13]. Characterizing and understanding the trends in urban vegetation cover change provide insights into the relationship between urban greenness and urbanization and can help guide sustainable urban development [14]

Methods

Results

Discussion

Conclusion