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Wildfires May Improve Forests' Ability to Sequester Carbon

When saving trees means less carbon storage

Wildfires wreaked havoc across southern California last year, resulting in billions of dollars in irreparable damage. Not surprisingly, land managers and agencies this season have mobilized fire crews and equipment to stop the flames before they spread. In the meantime, however, researchers studying the amount of carbon that forests and vegetation harbor have stumbled on a finding that presents an added quandary to fire management: suppressing fires means that less carbon is stored in trees.

The team, led by Michael L. Goulden of the University of California, Irvine, compared the biomass of California’s wild forests in the 1930s with those in the 1990s using data compiled by two forest census takers: the Wieslander Vegetation Type Mapping Project at the University of California, Berkeley, and the U.S. Forest Service’s Forest Inventory and Analysis program. In evaluating the two sets of data, the scientists found that the density in midaltitude conifer forests increased by 34 percent during the 60 years that elapsed. Yet contrary to conventional wisdom—that more trees mean additional carbon storage—they found that the amount of carbon held actually decreased by 26 percent in the same period.

“This is a nice example of a counter­intuitive result,” remarks Richard Houghton of the Woods Hole Research Center, who was not a part of the study but had come up with the idea for it when researching large carbon sinks in North America.

The logic behind the unanticipated finding comes down to the size of the trees that are being saved by fire suppression, says Goulden, whose study is being published this summer by Geophysical Research Letters. Over the past few decades firefighters have stopped the ground blazes common in California that would have otherwise likely wiped out the smaller trees and undergrowth. Instead these forests now have many small and midsize trees, adding to the forest’s density.

In turn, Goulden adds, when forests in the western U.S. inevitably go through periods of extreme drought, the entire forest is put under severe stress. The larger trees, which require more water to survive and experience higher exposure to the drying sun and wind, tend to be the casualties of the drought. Trees measuring 90 centimeters or more in diameter might contain as much carbon as 50 to 75 small trees that measure between 10 and 30 centimeters in diameter. “In terms of trying to nail down the United States net carbon budget,” Houghton says, “we should be more careful of counting forest thickening as a sink.”

Preserving the heftier trees is the easy solution to augmenting carbon storage and allowing them to play their ecological roles—they offer varied habitats and shape the land—but the responsibility of fire management remains complex. Nathan L. Stephenson, a research ecologist at the U.S. Geological Survey’s Western Ecological Research Center, is not so sure that Goulden’s findings will change much of the current efforts in fire management.

As the climate changes and puts stress on plant life, Stephenson says, it is probably better for the forest to get back to the way it used to look: thinner and less crowded. In fact, the national parks of the Sierra Nevada Mountains, with which he closely works, already use prescribed fire to thin forests. Burning or cutting down trees will release some carbon into the atmosphere. But at least, Stephenson notes, “you reduce the chance that you’re going to lose all [the carbon] in a catastrophic wildfire.”

Note: This story was originally published with the title, "The Puzzling Inferno".

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