Has Fire Suppression Increased the Amount of Carbon Stored in Western US Forests?

Active 20th century fire suppression in western US forests, and a resultingincrease in stem density, is thought to account for a significant fraction of the NorthAmerican carbon sink. We compared California forest inventories from the 1930s withinventories from the 1990s to quantify changes in aboveground biomass. Stem density inmid-montane conifer forests increased by 34%, while live aboveground carbon stocksdecreased by 26%. Increased stem density reflected an increase in the number of smalltrees and a net loss of large trees. Large trees contain a disproportionate amount ofcarbon, and the loss of large trees accounts for the decline in biomass between surveys.20th century fire suppression and increasing stand density may have decreased, ratherthan increased, the amount of aboveground carbon in western US forests.