Response and multiscenario simulation of trade-offs/synergies among ecosystem services to the Grain to Green Program: a case study of the Chengdu-Chongqing urban agglomeration, China

As a policy tool for advancing ecological progress, the Grain to Green Program (GTGP) has been operating for 20 years and has played an important role in improving ecosystem service value. However, there are few studies on the trade-off/synergy changes in ecosystem services during the implementation of the GTGP and how to select the optimal scheme for regional ecological security based on these trade-offs. Thus, we took the Chengdu-Chongqing urban agglomeration (CCUA) in southwestern China as the study area and estimated the regional food production, carbon sequestration, water yield, soil conservation, and habitat quality services. Then, the trade-off/synergy relationships among ecosystem services were clarified, and the influential mechanism of the GTGP on trade-off changes was evaluated. Finally, different risk scenarios were generated and the regional ecological security zone was simulated under the principle of the best protection efficiency and the highest trade-off degree. The obtained results indicated that the trade-offs/synergies of regional ecosystem services changed significantly during the implementation of the GTGP. Among them, food production, water yield, and soil conservation have always had trade-offs, while carbon sequestration, soil conservation, and habitat quality have all had synergistic relationships. The relationships between carbon sequestration and water yield and food production changed from noncorrelated to trade-off/synergistic, and the relationships between habitat quality and food production and water yield were not obvious. The GTGP has been an important factor affecting the change of trade-off intensities. On the one hand, it has strengthened the synergistic relationships among carbon sequestration, soil conservation, and habitat quality; on the other hand, it increased the constraints of water resources on soil conservation and vegetation restoration. The scenarios with a decision risk coefficient of alpha = 1.6 were the most suitable, and the ecological security zone corresponding to this scenario was located in the northwestern region of the study area, with high carbon sequestration and habitat quality services. In the future, the intensity or direction of the GTGP should be adjusted to solve the conflicts among ecosystem services in other areas except the northwest region.