Fire suppression versus global warming

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This Science article reports on a counterintuitive result: extensive fire suppression reduces, rather than increases the amount of carbon captured by trees.
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've heard arguments before that fire suppression is bad policy because it blocks the thinning effect of occasional fires, with the result that wildfires become more serious. [*] [Note that I'm not saying that's the cause of this year's severe wildfires. According to the fireman I talked to on Wednesday, the vegetation is especially dry this year—as dry now as it usually is at the end of the summer.] It's interesting to ask whether there's some optimal, nonzero, amount of fire suppression, or whether it would be best to just let fires burn except where they actually threaten human activity. Unfortunately, this is a topic I know basically nothing about.

3 Comments

There's a third option: regularly setting controlled fires under safe(r) conditions, to prevent buildup of brush, rather than just hoping nature does the job. Is that being done anywhere?

@Dan

They regularly do controlled burns in NM at least. Sometimes they mess up though, as they caused the very bad wildfires near Los Alamos in 2000.

It should be obvious that in the long run a fixed area of forest should be carbon neutral -- otherwise its carbon volume would move toward zero or infinity.

So the question is whether fire suppression changes the long term equilibrium of carbon content of a forest, which would be true if the average tree size was being reduced by fire suppression (tree size reduction by logging doesn't count). Shouldn't that be easy to measure?

Wired Magazine had an article a few months ago suggesting that a carbon reduction measure would be to log old growth forests and plant young trees in their place. Young trees capture far more carbon than old trees, and by turning old trees into lumber we prevent them from re-releasing all that carbon by rotting on the forest floor.

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