Are big, severe wildfires normal?

Are big, severe wildfires normal?

27 July 2012

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USA– The conventional wildfire wisdom goes something like this: Western forests are out of whack due to past fire suppression and logging practices. Forests that used to be open and free of undergrowth have turned into dense “dog hair” thickets of young trees that burn like kindling. Combine that with millions of acres of trees killed in a climate-change-fueled beetle outbreak, and we’ve ushered in an era of mega fires that are bigger, hotter and more intense than ever before.

We’ve heard this narrative over and over and over, and with good reason: there are a lot of scientific studies supporting it.

But a team of researchers from the University of Wyoming complicate that view with their latest study, which suggests that today’s wildfires aren’t any worse than those that burned in the late 19th century.

Bill Baker, a professor of ecology and geography, and Mark Williams, a recent graduate student, set out to test assumptions that dry, Western forests were historically more open and park like, and severe fires less frequent.

They set their sights on eastern Oregon, along Colorado’s Front Range, and on Arizona’s Black Mesa and Mogollon Plateau. First, they had to figure out what forests in these places looked like before European American settlement. They decided to combine tree-ring data, which has been widely used to reconstruct historic forests, with General Land Office surveys from the late 1800s. They recruited teams of students to browse through 13,000 hand-written records created by surveyors who were charting undeveloped federal land so it could be divided up and sold. The surveyors took meticulous notes on the landscape they walked through, putting bearing marks on trees every mile and describing when they entered a clearing, a burned forest or a stand of old-growth trees.

What the researchers found surprised them. Forests in the four regions were historically much denser than expected, especially in the Front Range. Open park-like forests made up less than half of all the forested areas they examined.

Baker and Williams also found that low-severity fires, which kill less than 20 percent of trees, weren’t as widespread as predicted. Instead, high-severity fires, which destroy more than 70 percent of trees in a burn area, were common in all four forests. Just as common, in fact, as they are today. The Front Range in particular, scorched by two destructive fires already this season, has a long history of big, severe fires.

The team from Wyoming rounded out their studies by criticizing common forest management practices. Except where necessary to protect homes, they believe widespread culling of small trees and understory shrubs to thin forests, the doctrine behind projects like Arizona’s ambitious 4 Forest Restoration Initiative, is a bad idea. This strategy, they say, will “move dry forests outside their historical range of variability, rather than restore them, probably with negative consequences for biological diversity.”

Baker and Williams’ studies were met with strong opposition from some of the preeminent fire ecologists in the field. Thomas Swetnam, who directs the tree-ring research lab at the University of Arizona, said the team’s findings were inconsistent with a large body of published research, and called the papers “deeply flawed in multiple ways.” In an email he specifically critiqued the finding that high-severity fires aren’t any larger today than in the past.

“There is no reliable, convincing evidence that high severity crown fires creating canopy holes of this size occurred anytime in the past 500 to 1,000 years, probably much, much longer. These are extraordinary events, with major unsustainable, irreversible impacts,” he wrote, that can be explained by the conventional wisdom: unnaturally dense forests, drought and high temperatures.

Peter Brown, a fire ecologist and director of Rocky Mountain Tree-Ring Research in Ft. Collins, Colo., who studied under Swetnam, said he equated Baker with “creationists trying to tear down evolution.” Ponderosa pine forest ecology, Brown said, is a theory, but one supported by studies from multiple disciplines. “You can’t just tear down that whole body of understanding and the complete set of evidence for that theory with one little bit of data that has its own tremendous flaws in it in the first place.”

The flaws, Brown says, stem from the Wyoming team’s use of GLO surveys, which he believes do not document a large enough area to provide an accurate depiction of the forest. But Baker says and his students walked hundreds of miles along the paths the surveyors traveled, looking for anything that would suggest their notes were wrong. He found that the trees the surveyors marked were 95 to 98 percent accurate.

Baker doesn’t disagree with Swetnam’s argument that current climate conditions are likely to make modern fires worse. He and Williams just hope to place the current rash of wildfire in its historic context. Big, severe wildfires are “a natural part of our forest ecosystems; we maybe didn’t know that until recently. But now that we do we just have to figure out how to live with that.”

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