Fewer Wildfires, Less Carbon Stored?

Discovery News

May 7, 2008

By Jessica Marshall

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.

"Not all trees are equal. A big tree has much more carbon in it than a small tree," Goulden said. "For every large tree that is lost, you would need 50 or more small trees to offset that amount of carbon. As a result there's actually a net loss of carbon from those forests."

Large trees can't compete well against the small trees in periods of drought, Goulden explained, which are inevitable in the west.

"It's the fires that thin out the small trees, and it's having the small trees thinned out that allows the big trees to make it through dry periods," he explained.

The findings may be specific to western forests of certain types where drought is common. For instance, David Wardle at the Swedish University of Agricultural Sciences said that his studies have found the opposite for Swedish boreal forests.

But the findings could have implications for forest management.

"If someone wanted to manage the forest to maximize the amount of carbon they could obtain, what you want to do most is maintain those big trees," Goulden said.

The work is also important for researchers trying to understand the carbon budget. While this finding is significant, it won't change the entire budget, Houghton said.

"I don't think anyone supposed this would be a big proportion of the sink, but it was a piece," he said.

Goulden is proud to be able to have used the impressive data set collected long ago.

"I just imagine these guys riding horses around and recording these data," Goulden said. "I'm sure the guys never thought it would be useful for this."