Forests matter in the global and U.S. carbon cycle because they store much carbon in wood, dead wood, and soil and because this storage can be easily changed. Currently, forests and long-lived wood products annually store 313 million metric tons of carbon per year (1 million metric tons = 1012 ), which offsets about 20 percent of U.S. fossil fuel output of 1580 million metric tons of carbon per year. Woodland expansion onto grasslands offsets another 120 million metric tons of carbon per year. The large U.S. carbon sink is likely the result of recovery from past heavy wood use and logging, but increasing CO2 and nitrogen deposition may also be increasing productivity. Forest carbon goes through a predictable cycle after a disturbance, with losses either during or after the disturbance and recovery with regrowth. As long as forests replace themselves and the frequency of disturbance does not increase dramatically, forested landscapes are very resistant to changes in carbon over the long term. However, if tree regeneration is unsuccessful after disturbances such as fire, insects, or storms and the forest changes to meadow or shrubland, then the carbon stored in the former forest is lost to the atmosphere.