The Carbon Balance of the Southeastern US Forest Sector as Driven by Recent Disturbance Trends

This study documents annual carbon stocks and fluxes from 1986 to 2010 at 30-m resolution across southeastern U.S. forests, analyzing trends and regional greenhouse gas exchange. We used forest inventory data to guide a carbon cycle model representing postdisturbance carbon dynamics for diverse forest and site conditions. We mapped carbon stocks and fluxes with stand age inferred from spaceborne disturbance monitoring and biomass. We assessed the fate of harvested biomass with a wood products model. We found that pine forests experienced the largest biomass removals, with paper products leading all end-uses. Averaging across all SE forestlands, mean annual net ecosystem productivity decreased from 116 gC center dot m(-2) center dot year(-1) in 1986 to 71 gC center dot m(-2) center dot year(-1) in 2007, and 85 gC center dot m(-2) center dot year(-1) in 2010, equating to a range of 25 to 41 Tg C/year (mean of 34 Tg C/year). Interannual variability in forest-atmosphere carbon exchange is dominated by the extent of harvesting, with removals ranging from 23 to 56 Tg C/year. Region-wide live biomass stocks varied little over time, averaging 5.0 kg C/m(2) for aboveground biomass or 1,780 TgC, with average annual biomass growth balanced by harvest removals. However, net biome productivity exhibited large interannual variability, spanning a sink of 16 Tg C/year in 1986 to a source of -30 Tg C/year in the year of peak harvest. Two thirds of harvest removals are emitted within 50 years, 8% as methane, causing the forest sector to act as a large CO2-equivalent source. Uncertainties are estimated at +/- 25%.