Fewer Wildfires, Less Carbon Stored?

May 7, 2008 -- Fire suppression in the western United States may be decreasing the amount of carbon stored in the forests there, finds new research in a study that contradicts what scientists have long believed.

This could have implications for managing forests, if carbon uptake becomes a priority.

"Everyone has just assumed that, with fire suppression, open forests would accumulate biomass in lots of little, young trees," said Richard Houghton of Woods Hole Research Center in Falmouth, Mass., who was not involved in the work. "This is a nice study in that it raises a question about something we assumed."

Atmospheric measurements suggest that there is a large carbon sink in North America, and researchers have tried to piece together what it might be. One component of that sink was believed to be an increase in carbon in forests where fires have been suppressed over the last century.

"That makes so much sense, it almost couldn't not be the case," said Michael Goulden of the University of California, Irvine, who authored the study with graduate student Aaron Fellows.

But their new work shows otherwise.

The team looked at measurements of tree size and number taken as part of an extensive survey of California in the 1920s and 1930s and compared them with U.S. Forest Service measurements from the 1990s. They used the diameter of the trees and species type to estimate the amount of carbon in each tree using relationships derived by the Forest Service. The results were published online in Geophysical Research Letters.

The pair found that at middle elevations, the number of trees per area increased by 19 percent over the time period, while the amount of stored carbon dropped by 39 percent. The researchers blame the loss of large trees for the change.