Projecting the CO2 and Climatic Change Effects on the Net Primary Productivity of the Urban Ecosystems in Phoenix, AZ in the 21st Century under Multiple RCP (Representative Concentration Pathway) Scenarios

Urban vegetation provides ecological services that promote both the ecosystem integrity and human well-being of urban areas, and thus is critical to urban sustainability. As a key indicator of ecological health, net primary productivity (NPP) provides valuable information about the performance of urban ecosystem in response to the changes in urban climate and atmosphere in the 21st century. In this study, a process-based urban ecosystem model, HPM-UEM (Hierarchical Patch Mosaic-Urban Ecosystem Model), was used to investigate spatiotemporal dynamics of urban ecosystem NPP in the Phoenix city, AZ under three representative concentration pathway (RCP2.6, RCP4.5 and RCP8.5) during the 21st century. The results indicated that, by the end of the 21st century, the urban ecosystem’s NPP would increase by 14% (in RCP2.6), 51% (in RCP4.5) and 99% (in RCP8.5) relative to that in the late 2000s, respectively. Factorial analysis indicated that CO2 fertilization effect would be the major driver of NPP change, accounting for 56–61% of the NPP increase under the scenarios. Under the RCP2.6 scenario, the strongest NPP increase would be found in the agricultural lands located in the west and southeast of the city. Under the RCP4.5 and RCP8.5 scenarios, the strongest NPP increase would be found in the mesic residential areas that mainly located to the eastern, southern, and southwestern of the Phoenix Mountains Preserve. Although higher ecosystem NPP in the future implies improved ecosystem services that may help to alleviate the heat stress (by providing more shading) and air pollution in the city, this will be at the cost of higher irrigation water usage, probably leading to water shortage in the natural ecosystems in this arid region. Furthermore, this study indicated the rich (such as in mesic residential area) would enjoy more benefits from the improved urban ecosystem services than the poor (such as in xeric residential area).