7. Managing Carbon

In the United States, net carbon (C) has increased in forests and harvested wood product stocks since the 1950s. Annual C storage currently accounts for 13 % of U.S. fossil fuel C emissions, and increased C storage is attributed to reforestation, regrowth of harvested forests, and use of forest products. In a warmer climate, U.S. forests could become a net C emitter of tens to hundreds of Tg C year⁻¹ within a few decades. Carbon mitigation through forest management focuses on (1) increasing afforestation, avoiding deforestation, or both, (2) C management in existing forests, and (3) use of wood as biomass energy or in wood products. The mitigation potential of these strategies differs in timing and magnitude. Longer harvest intervals extend the time that C is stored in biomass, whereas increased growth rates accelerate uptake of C per unit time. Carbon can be stored in wood products for a variable length of time, oxidized to produce heat or electrical energy, or converted to liquid transportation fuels and chemicals that would otherwise come from fossil fuels. Wood products can also be used to substitute for other products that emit more greenhouse gases in manufacturing (e.g., concrete and steel). Life cycle assessment is used to evaluate C management strategies by focusing on the change in C storage or emissions over time. Public lands contain 37 % of the land area of the United States (76 % of which are managed by federal agencies), representing an important component of long-term C storage, although managing these lands for C benefits involves multiple jurisdictions, social objectives, and political factors. Management of C on private lands, and to some extent public lands, is affected by markets, regulations, taxes, and incentives that have only recently evolved to address climate change and C.