Does National Forest City Construction promote the synergistic development of environment and welfare?

China's National Forest City Construction (NFCC) policy represents a novel approach to urban sustainability that integrates forest ecosystems into urban landscapes to simultaneously enhance ecological protection and residents' welfare. This study investigates whether and through which mechanisms the NFCC policy promotes the synergistic development between urban ecological environment and residents' welfare. Employing a quasi-natural experimental design with a multi-period Difference-in-Differences (DID) methodology, the study analyzes data from 285 Chinese cities from 2000 to 2022. The baseline regression results demonstrate that the NFCC policy significantly improves the coupling coordination degree (CCD) between urban ecological environment and residents' welfare. The mechanism analysis reveals that green technology innovation, government green development attention, and public environmental protection concern are key pathways through which the policy facilitates synergy. Heterogeneity analysis shows that the positive effects of the NFCC policy on CCD are statistically significant primarily in western regions, environmentally-focused cities, and provincial capitals. These findings provide valuable insights for optimizing the design and implementation of urban sustainability policies.