Impacts of strengthened warming by urban heat island on carbon sequestration of urban ecosystems in a subtropical city of China

Urban heat island could strengthen the warming trend of air temperature and cause the change of carbon cycle of urban ecosystems. The primary goal of this study was to explore the above phenomena in a subtropical city of Shenzhen. We selected four sites along a thermal gradient of the city and ran an ecological model to simulate hourly gross primary productivity and net ecosystem change of carbon for the purpose of analyzing the difference in the effects of urban heat island on vegetation growth and carbon sequestration. The work was also intended to help us understand future warming effects on carbon cycle of terrestrial ecosystem in subtropical and tropical areas. The results of the modeling experiments showed that strengthened warming temperature by urban heat island could constrain vegetation carbon sequestration in hot seasons and promote vegetation growth in cool seasons. The seasonal sequence of carbon sequestration for the subtropical city was: spring > summer > autumn > winter. For the study period, the maximum amount of monthly carbon sequestration by urban ecosystems happened in May. In summer and autumn, vegetation gross primary productivity and carbon sequestration first increased with the increase in temperature, and then decreased after hourly temperature reached a threshold value. In addition, the number of days with high temperature constraints on vegetation carbon sequestration were significantly higher in the two urban sites than those in the two suburban sites. The above phenomena was driven by the fact that strengthened warming by urban heat island constrained vegetation photosynthesis and transpiration and caused the increase of ecosystem respirations. The above results indicated that the strengthened warming by urban heat island could create additional constraints on carbon sequestration of urban ecosystems in the subtropical regions. The results implied that future global warming may cause the weakening of vegetation carbon sequestration in the subtropical and tropical ecosystems.