Scientists have long believed that preventing or dousing forest fires helps combat global warming by saving trees and thus allowing forests to take up more carbon dioxide from the atmosphere. But surprising new data on hundreds of California forest sites suggest the opposite. The work could help quantify the role of forests in the global carbon cycle and shape U.S. federal fire policy.
Lightning-caused fires serve a natural mechanism within forests. They destroy small trees and underbrush while often allowing large trees to remain standing and flourish. But since roughly 1910, U.S. forest managers have sought to fight as many small forest fires as possible. That policy has allowed more shrubs and small trees to grow than in the past. The increasing quantity of vegetation, scientists calculated recently using tree measurements and other data, sucks 50 million metric tons of carbon dioxide out of the atmosphere each year--roughly 14% of the total amount of carbon pulled in by U.S. forests. However, historical data on tree sizes weren't available to allow scientists to confirm that the forests had absorbed that much carbon over the past century.
To do that, ecologist Michael Goulden of the University of California, Irvine, and a grad student used previously overlooked forest inventory measurements taken in the 1930s on 269 California forest wilderness plots. They then compared these data with measurements taken in the 1990s on 260 plots in the same general vicinity. The number of trees per hectare across all plots rose by 4% in 60 years, an increase the scientists attributed to the federal policy on suppressing fires. Yet the total amount of carbon held by trees declined by 34% over the same period, the researchers report this week in Geophysical Research Letters.
The scientists conclude that the large trees in the plots had to compete with the growing population of small trees, making the big trees more susceptible to drought, wind, and insect attack than they would have been without the crowding. Because the large trees died, they didn't absorb as much carbon dioxide. "It's counterintuitive," says Goulden.
"I was really tickled by this finding," says ecologist Richard Houghton of Woods Hole Research Center in Falmouth, Massachusetts. "Nobody thought these little trees would have an effect on the larger ones." But forest scientist Eric Kasischke of the University of Maryland, College Park, argues that some of the older trees might be dying simply "due to the aging process.” Goulden responds that the issue would be important only if the trees in the forest plots were generally the same age, which they weren't.
The results could affect U.S. federal firefighting policy, says Department of the Interior biogeographer Peter Teensma. It's not the practice now, but one way that federal land managers can give large trees space to grow in the wilderness is by thinning out brush and undergrowth with small preemptive fires. The new paper "may make the case we should be doing thinning in some ways in the wilderness lands," he says. U.S. agencies will consider the issue, say federal officials, as they prepare a new forest fires policy for publication next year.