The policy of family-contracted natural forest conservation has recently made remarkable progress in protecting the habitats of giant pandas in Wolong Nature Reserve. However, the optimal outcome could not be derived from rational motivations, as the fairness norm potentially catalyzed the cooperation in the program. To verify this prediction, we undertook field experiments based on one-shot anonymous Ultimatum Games with the local people in the reserve. Subjects behaviors in the games were recorded and compared with rational expectations. The results lend support to the hypothesis with a total of fair or hyper-fair offers from 96.9% of the proposers. The responders reluctance to reject low offers, in addition to the information they provided in the post-game interviews indicate that the evolution of the local fairness norm was based on reputation rather than costly punishment. The result unveils a key factor to the success of the forest conservation in Wolong, and demonstrates the effectiveness of facilitating the participation and cooperation of the local community in the natural resource conservation by crowding in the fairness norm. The study also draws attention to the evolution of social capital for sustainable development of social-ecological systems.