Spatiotemporal trends of terrestrial vegetation activity along the urban development intensity gradient in China's 32 major cities

Terrestrial vegetation plays many pivotal roles in urban systems. However, the impacts of urbanization on vegetation are poorly understood. Here we examined the spatiotemporal trends of the vegetation activity measured by MODIS Enhanced Vegetation Index (EVI) along Urban Development Intensity (UDI) gradient in 32 major Chinese cities from 2000 to 2012. We also proposed to use a new set of concepts (i.e., actual, theoretical, and positive urbanization effects) to better understand and quantify the impacts of urbanization on vegetation activities. Results showed that the EVI decreased significantly along a rising UDI for 28 of 32 cities (p<0.05) in linear, convex or concave form, signifying the urbanization impacts on vegetation varied across cities and UDI zones within a city. Further, the actual urbanization effects were much weaker than the theoretical estimates because of the offsetting positive effects generated by multiple urban environmental and anthropogenic factors. Examining the relative changes of EVI in various UDI zones against that in the rural area (ΔEVI), which effectively removed the effects of climate variability, demonstrated that ΔEVI decreased markedly from 2000 to 2012 for about three-quarters of the cities in the exurban (0.05<UDI≤0.25) and suburban (0.25<UDI≤0.5), and only half of the cities in the urban (0.5<UDI≤0.75) and urban core (0.75<UDI≤1). The stable or even increasing tendencies of ΔEVI in the urban and urban core of many cities could primarily be attributed to the importance of positive effects derived from the urban environment and the improvement of management and maintenance of urban green space. More work is needed to quantify mechanistically the detailed negative and positive effects of urban environmental factors and management practices on vegetation activities.