The relationship of urban green space to urban expansion based on gravity methods

Background/Question/Methods: Urban green space comprising parks, fields, woodlands and other natural ecosystems is a fundamental component of urban ecosystems. Determining the relationship between urban green space and urban sprawl is necessary for understanding urbanization and the provision of urban ecosystem services. We hypothesized that the urban (i.e., population and economic) mean center will migrate over time towards urban green spaces in fast-growing cities. To test our hypothesis, urban and green space expansion was examined in 5 cities in China and 5 cities in the United States that were experiencing high rates of urban expansion. We used Landsat images of those cities from 2000 to 2018 to quantify the extent and migration of the green space combined with annual population and economic data. These data were analyzed using Grether and Mathys’ Center of Gravity Method and circular statistics to determine the relationship between green space and urban expansion. Combining demographic and economic data of the 10 cities in China and US over the 2000-2018 time period, we quantified deviations and differences between movement of the center of gravity of the population, economic and urban green spaces.

Results/Conclusions: Nine out of ten cities followed a similar trend showing a divergent pattern, i.e., the population and economic centers moved in a similar direction but different to that of the green space. The movement of the mean center of the green spaces in the US cities was more consistent than that of the Chinese cities. Only the US city of Phoenix, AZ had a green space center of gravity that moved in a similar direction to its population and economic center of gravity. We conclude that centrifugal forces of urban green spaces that lead to movement of population and economic centers away from green spaces play a larger role in urban change compared with centripetal forces that would pull these centers together with urban green spaces. Over the 18 year time period, the urban green space and urban space expansion in 10 cities show a synchronous growth trend, however, the proportion of green space in the built-up area was decreasing. The urban space expansion rate exceeded the population growth rate, leading to problems of unreasonable urban sprawl that is likely to deplete the provision of ecosystem services.