Green change in the core build-up areas of China: Information from MODIS data

Urban built-up areas are the core area of global population agglomeration, where the interactions between human beings and the surrounding ecological environment supporting their survival are particularly intense. With the acceleration of urbanization since the 21st century, the level of green vegetation in the core built-up area (CBA) and its response to human activities are of great interest. In this study, an innovative green index (GI) was developed for evaluating green level in CBA from three aspects of quantity-stability-balance. GI can be measured by integrating MODIS EVI data and metrics including green coverage index, nearest neighbor index, and coefficient of variance. Spatiotemporal pattern, regional differences and geographical variances of green level during 2001-2018 in China's CBAs were explored using geographic analysis methods (i.e. mutation test, trend analysis, spatial autocorrelation, Theil index, and scale variance). Results showed that during 2001-2018, GI and its different aspects of green coverage (GC), green stability (GS), and the balance of the greenness distribution (BGD) were generally high in South China and low in North China. Average green level in China's CBAs experienced a V-shaped change. Green level in 54.58% of CBAs indicated mutation phenomenon, and the green level in most of these CBAs improved after the turning point. Regional differences in green level were reduced, and gaps within group were far larger than gaps between groups. Scale variance in green level at the regional, provincial, and city levels fell by 96.91%, 53.27%, and 46%, respectively, and the contribution of the difference at the city level to total difference in China was far higher than that at the provincial and regional levels. More attention should be paid to green in CBA at the city level, and sustainable urban development and land management policies should be implemented in the future.

Automated summary

This study developed a green index (GI) to evaluate green levels in China's CBAs and found that the green levels were higher in South China and lower in North China. From 2001 to 2018, the average green level in China's CBAs experienced a V-shaped change with improvements in most regions post turning point. Regional differences in green levels decreased, and city-level differences contributed significantly more to the total difference in China compared to provincial and regional levels. More attention should be focused on greenery in CBAs at the city level to implement sustainable urban development and land management policies in the future.