Changes in urban vegetation cover and analysis of the influencing factors: a case study of Harbin, Heilongjiang Province, China

Urban vegetation has an important role in ensuring the ecological security of cities. Rapid urbanization in China has radically changed the urban vegetation cover. Isolation of the contributions of human activities from observed vegetation can aid in understanding the effects of human activities on urban ecosystems. In this study, the Google Earth engine platform was employed, and the 1986-2017 archived Landsat data were used to derive the annual normalized difference vegetation index (NDVI), i.e., the observed NDVI. This was combined with annual mean temperature and annual precipitation data; then, Theil-Sen median and Mann-Kendall trend analysis was used to study the spatiotemporal variation characteristics of NDVI. Residual analysis and relative effect analysis were used to analyze the contributions of human activities to vegetation cover changes in Harbin. The results of the study showed the following: (1) In 1986-2017, the NDVI of Harbin ranged from 0.36 to 0.67, and NDVI fluctuations are large. From a temporal scale, there were no clear trends in the observed annual mean NDVI values, but NDVI showed an increasing trend before 2000 and after 2010. (2) From a spatial perspective, the mean NDVI value of 32 years showed spatial variation, which was low in the center and high in the periphery, and the area of the vegetation improvement regions (38.04%) was greater than the area of the vegetation degradation regions (21.33%). (3) Regions in which human activities played a dominant role in vegetation degradation (relative effect of more than 50%) accounted for 67.14% of the entire degeneration regions, while regions in which human activities played a dominant role in vegetation improvement (relative effect of more than 50%) accounted for 84.64% of the entire improvement regions. Our study shows that over 32 years, an overall greening trend was observed in Harbin, but urban vegetation cover changes showed spatial variability, and human activities had a dominant effect on vegetation changes. Vegetation degradation was mainly caused by urban expansion. Greening projects in recent years such as tree planting, greening of the old city area, building of urban parks, and greening of city roads in Harbin can improve urban vegetation cover and are important measures for improving the urban ecological environment.