Improved forest management (IFM) offset programs reward landowners for the additional carbon sequestered by adjusting forest management practices, but the credibility of the credits issued hinges on how the counterfactual baseline is set. Static baselines fixed at project initiation cannot reflect ex-post changes in regional forest dynamics, making it difficult to distinguish project-driven sequestration from broader trends. This study develops a dynamic ex-post baseline framework that updates regional reference conditions over time using a difference-in-differences design and applies it to IFM projects in California’s Compliance Offset Program. The results show that static baselines tend to systematically over-credit projects, whereas the dynamic baseline approach better aligns issued credits with the actual additional sequestration generated, offering a practical pathway to enhance the environmental integrity of forest-based offset programs and informing the design of similar mechanisms in other jurisdictions, including China.