Abstract
Forests play a crucial role in regulating regional climate and mitigating local air pollution, but little is known about their responding to such changes. This study aimed to examine the potential responses of Pinus tabuliformis , the major coniferous tree species in the Miyun Reservoir Basin (MRB), along an air pollution gradient in Beijing. Tree rings were collected along a transect, and ring width (basal area increment, BAI) and chemical characteristics were determined and related to long-term climatic and environmental records. The results showed that Pinus tabuliformis showed an overall increase in intrinsic water-use efficiency ( iWUE ) at all sites, but the relationships between iWUE and BAI differed among the sites. The contribution of atmospheric CO 2 concentration ( c a ) to tree growth was significant at the remote sites (> 90%). The study found that air pollution at these sites might have caused further stomatal closure, as evidenced by the higher δ 13 C levels (0.5 to 1‰ higher) during heavy pollution periods. The analysis of tree ring δ 15 N also revealed the potential of using δ 15 N to fingerprint major nitrogen (N) deposition, as shown in the increasing tree ring δ 15 N, and major nitrogen losses due to denitrification and leaching, as shown in the higher δ 15 N in tree rings during heavy rainfall events. Overall, the gradient analysis indicated the contributions of increasing c a , increasing water deficit and elevated air pollution to tree growth and forest development. The different BAI trajectories suggested that Pinus tabuliformis has the ability to adapt to the harsh environment in the MRB.