Evaluating carbon additionality under short-term harvest deferrals in U.S. southern plantation forests

The growing demand for nature-based solutions to mitigate climate change has heightened interest in forest carbon programs, particularly in the southeastern United States, a region with abundant forest resources. Challenges such as additionality, leakage and permanence requirements, and competition between timber and carbon markets are among the complexities of implementing effective forest carbon programs. This study examines the impacts of one-year harvest deferral programs on carbon sequestration and timber market dynamics using an agent-based simulation model. The model incorporates interactions between landowners, a wood-consuming mill, and a carbon credit developers. Through a simulation experiment, we explored varying levels of carbon demand and prices to analyze how forest landowners respond under different scenarios and to examine the outcomes for stand age structure, stumpage and delivered prices for timber, and carbon sequestration over a 100-year period. Results indicate that a one-year harvest deferral program has leakage effects, mainly on timber market dynamics by increasing stumpage prices and extending the rotation age. In addition, our model shows that this type of program can lead to additionality in scenarios with higher carbon demand but also higher program costs. Lastly, we observe permanence at the landscape level through an increasing and higher carbon volume than in scenarios without a carbon program, particularly at moderate carbon prices and higher levels of carbon demand. This study contributes to a growing literature by providing insights into economic and ecological trade-offs of forest carbon offset programs and their implications for sustainable forest management in the Southeastern US.