Past and Prospective Carbon Stocks of United States Forests: Implications for Research Priorities and Mitigation Policies

Abstract:

United States forests and wood products have been a significant and persistent carbon sink of 100-200 million tons annually since 1950, currently offsetting about 12% of U.S. emissions of CO₂. This carbon sink is caused by recovery of forest C stocks following timber harvest and abandonment of agricultural land over the last 150 years, and more recently the growth-enhancing effects of N deposition, increasing atmospheric CO₂, and climate variability. The forest carbon sink would have been significantly larger if not for continued losses of forest to other land uses such as urban development, and increasing impacts from natural disturbances such as fire and insect outbreaks. Projections of the future U.S. C sink have raised concerns that it may disappear in a few decades because of slower growth, continued losses of forest area, and increasing demand for timber products especially bioenergy. However, continuing atmospheric and climate changes may delay this projected decline in the sink strength for another 50 years or longer. Research is urgently needed to improve projections of land-use changes and demand for timber, quantify the large-scale effects of atmospheric change and climate variability, and develop modeling approaches that can effectively integrate these multiple factors. Policy decisions to meet emissions reduction targets are partially dependent on assumptions about the magnitude of the future forest carbon sink; therefore, it is important to have convincing projections about how these various driving factors will affect forests in the future.