Changes in terrestrial carbon storage in the United States. 2: The role offire and fire management

1 Areas burned annually in the United States between 1700 and 1990 were der ived from published estimates of pre-European burning rates and from wildfi re statistics of the US Forest Service. Changes in live and dead vegetation following fire and fire exclusion were determined for 18 types of biomes a nd added to a book-keeping model to calculate the long-term effect of fire and fire management on carbon storage. 2 Over the 290-year period, burning declined by an estimated 98%, first, be cause wildlands were converted to agricultural lands, essentially eliminati ng fire from 236 x 10(6) ha and, secondly, because wildfires were excluded and suppressed in the remaining forests and non-forests. 3 Adding fire and fire management to an analysis of land-use change (compan ion paper) reduced the emissions of carbon over the period 1700-1990 by 25% (8 PgC). Less carbon was released because fire reduced the average biomass of forests cleared and burned, and because fire exclusion led to an increa se in carbon storage in forests. 4 The wildfire statistics of the USDA were insufficient for addressing two kinds of change: fire exclusion before 1926 and changes in the burning of n on-forest ecosystems. We estimate here that as much as 4 and 12 PgC, respec tively, may have accumulated in vegetation as a result of these changes, bu t the estimates are uncertain and likely to be upper limits. 5 The maximum rate of carbon accumulation attributable to all changes in la nd use, including fire management, was 300-400 TgC/year and occurred around 1980. Less than half of this uptake was in forests. Uptake by forests was constrained by the fact that most forests were already accumulating carbon in response to earlier harvests. Fire exclusion added little to this uptake .