A CARBON BUDGET FOR FORESTS OF THE CONTERMINOUS UNITED-STATES

The potential need for national-level comparisons of greenhouse gas em issions, and the desirability of understanding terrestrial sources and sinks of carbon, has prompted interest in quantifying national forest carbon budgets. In this study, we link a forest inventory database, a set of stand-level carbon budgets, and information on harvest levels in order to estimate the current pools and flux of carbon in forests o f the conterminous United States. The forest inventory specifies the r egion, forest type, age class, productivity class, management intensit y, and ownership of all timberland. The stand-level carbon budgets are based on growth and yield tables, in combination with additional info rmation on carbon in soils: the forest floor, woody debris, and the un derstory. Total carbon in forests of the conterminous U.S. is estimate d at 36.7 Pg, with half of that in the soil compartment. Tree carbon r epresents 33% of the total, followed by woody debris (10%), the forest floor (6%), and the understory (1%). The carbon uptake associated wit h net annual growth is 331 Tg, however, much of that is balanced by ha rvest-related mortality (266 Tg) and decomposition of woody debris. Th e forest land base at the national level is accumulating 79 Tg/yr, wit h the largest carbon gain in the Northeast region. The similarity in t he magnitude of the biologically driven flux and the harvest-related f lux indicates the importance of employing an age-class-based inventory , and of including effects associated with forest harvest and harvest residue, when modeling national carbon budgets in the temperate zone.