Quantitative assessment of human-induced impacts based on net primary productivity in Guangzhou, China

Urban expansion and land cover change driven primarily by human activities have significant influences on the urban eco-environment, and together with climate change jointly alter net primary productivity (NPP). However, at the spatiotemporal scale, there has been limited quantitative analysis of the impacts of human activities independent of climate change on NPP. We chose Guangzhou city as a study area to analyze the impacts of human activities on NPP, as well as the spatiotemporal variations of those impacts within three segments, using a relative impact index (RII) based on potential NPP (NPPp), actual NPP (NPPact), and NPP appropriation due to land use/land cover change (NPPlulc). The spatial patterns and dynamics of NPPact and NPPlulc were evaluated and the impacts of human activities on NPP during the process of urban sprawl were quantitatively analyzed and assessed using the RII. The results showed that NPPact and NPPlulc in the study area had clear spatial heterogeneity, between 2001 and 2013 there was a declining trend in NPPact while an increasing trend occurred in NPPlulc, and those trends were especially significant in the 10–40-km segment. The results also revealed that more than 91.0% of pixels in whole study region had positive RII values, while the lowest average RII values were found in the > 40-km segment (39.03%), indicating that human activities were not the main cause for the change in NPP there; meanwhile, the average RII was greater than 65.0% in the other two, suggesting that they were subjected to severe anthropogenic disturbances. The RII values in all three segments of the study area increased, indicating an increasing human interference. The 10–40-km buffer zone had the largest slope value (0.5665), suggesting that this segment was closely associated with growing human disturbances. Particularly noteworthy is the fact that the > 40-km segment had a large slope value (0.3323) and required more conservation efforts. Based on the above results, we suggest that continuous efforts may be necessary to improve the intensity of protection and management in the urban environment of Guangzhou.