Temporal and Spatial Effect Distribution on Soil Erosion from Nationwide Forest Restoration Policies in China Revealed by Causal Machine Learning

The Bonn Challenge and the UN Decade on Ecosystem Restoration promote global forest restoration, while the implementation mechanisms and their ecological effects remain insufficiently understood. This study focuses on China’s Natural Forest Protection Program (NFPP) as a case study to address this gap. The study uses a causal machine learning approach, i.e., the forest doubly robust learner, to investigate the individual treatment effect by dividing the NFPP implementation into three phases (1–5, 6–14, and 15–20 years) to capture short-term and long-term policy impacts. Provincial-level panel data (1998–2020), incorporating indicators of the natural environment, socioeconomic factors, and ecological governance are used. The results show that the NFPP significantly reduced soil erosion after 15 years of implementation. The policy’s effectiveness differed regionally, contingent on nonlinear thresholds that delineate specific ″efficiency traps″ and ″safe operating spaces″. Crucially, driving mechanisms underwent a structural transition, shifting from early anthropogenic disturbance dominance to mature natural background regulation. Mitigation outcomes were constrained by stressors such as extreme rainfall and population density. Notably, excessive afforestation in specific regions failed to yield benefits, exemplifying the adverse trade-offs of violating ecological thresholds. These findings underscore the critical need for long-term commitment and precision governance to ensure sustainable ecological resilience.波恩挑战倡议（Bonn Challenge）与联合国生态系统恢复十年（UN Decade on Ecosystem Restoration）共同推动全球森林恢复行动，但二者的实施机制与生态效应仍未得到充分认知。本研究以中国天然林保护工程（Natural Forest Protection Program, NFPP）为案例，旨在填补这一研究空白。本研究采用因果机器学习方法——森林双重稳健学习器（forest doubly robust learner），将天然林保护工程的实施周期划分为三个阶段（1-5年、6-14年、15-20年），以捕捉政策的短期与长期影响，并测算个体处理效应（individual treatment effect）。本研究使用1998-2020年的省级面板数据，涵盖自然环境、社会经济因素与生态治理等多维度指标。研究结果表明，天然林保护工程在实施15年后可显著降低土壤侵蚀程度。政策实施效果存在显著区域异质性，其作用效果受制于刻画特定“效率陷阱”与“安全运行空间”的非线性阈值。尤为关键的是，政策驱动机制发生了结构性转变：早期以人为干扰为主导，后续逐步转向成熟的自然本底调控。生态修复成效会受到极端降雨、人口密度等胁迫因素的制约。值得注意的是，部分地区过度造林并未产生预期效益，这印证了违背生态阈值将带来负面权衡效应。本研究结果凸显了长期坚持与精准治理的必要性，以保障生态系统韧性的可持续性。