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How The Smokey Bear Effect Led To Raging Wildfires05:30

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Adams (left) talks with Swetnam in their laboratory, nestled under the football stadium. (NPR)closemore
Adams (left) talks with Swetnam in their laboratory, nestled under the football stadium. (NPR)

First of a five-part series

The history of fire in the American Southwest is buried in a catacomb of rooms under the bleachers of the football stadium at the University of Arizona.

Rex Adams, a senior research specialist at the Laboratory of Tree-Ring Research, steps away from a band saw that he uses to slice tree ring samples. (NPR)
Rex Adams, a senior research specialist at the Laboratory of Tree-Ring Research, steps away from a band saw that he uses to slice tree ring samples. (NPR)

Here rules professor Thomas Swetnam, tree ring expert. You want to read a tree ring? You go to Tom. He's a big, burly guy with a beard and a true love for trees.

Tree sections are stacked floor to ceiling. They're like rounds chopped from a carrot, the carrot being a tree trunk. They're the size of dinner plates. When the football team scores, they rattle on their shelves.

Growth rings tell how old the sectioned tree was. But when Swetnam holds up one, he points to something else: fire scars. They're black marks, about the size of a fingernail clipping, left by fires.

"The first time here, back in the 1600s, it looks like, and it created a wound there. Basically the fire was hot enough to burn through the bark," he says. But the fire wasn't hot enough to kill the tree. So the next few rings show normal growth.

"Until the next fire occurs, and it creates another scar," he says. "And another, and another, and another, and another, and another."

Swetnam leaves his laboratory for the evening. Fires in the Southwest have been getting bigger and bigger over the past two decades. The Wallow fire in Arizona, Swetnam says, was "a tornado of fire." "It burned more than 40,000 acres in the first eight hours," he says. (NPR)
Swetnam leaves his laboratory for the evening. Fires in the Southwest have been getting bigger and bigger over the past two decades. The Wallow fire in Arizona, Swetnam says, was "a tornado of fire." "It burned more than 40,000 acres in the first eight hours," he says. (NPR)

Scars from thousands of sections show how often fires burned in the Southwest. It was every five or 10 years, mostly — small fires that consumed grass and shrubs and small seedlings, but left the big Ponderosa pine and Douglas fir just fine. This was the norm.

Then something happened.

"Around 1890 or 1900, it stops," Swetnam says. "We call it the Smokey Bear effect."

Settlers brought livestock that ate the grass, so fires had little fuel. Then when the U.S. Forest Service was formed, its marching orders were "no fires."

And it was the experts who approved the all-out ban on fires in the Southwest. They got it wrong.

By cross-dating different tree-ring samples from a given area, researchers can get a clearer picture of how a forest burns. (NPR)
By cross-dating different tree-ring samples from a given area, researchers can get a clearer picture of how a forest burns. (NPR)

That's the view of fire historian Stephen Pyne.

"The irony here is that the argument for setting these areas aside as national forests and parks was, to a large extent, to protect them from fire," Pyne says. "Instead, over time they became the major habitat for free-burning fire."

So instead of a few dozen trees per acre, the Southwestern mountains of New Mexico, Arizona, Colorado and Utah are now choked with trees of all sizes, and grass and shrubs. Essentially, it's fuel.

And now fires are burning bigger and hotter. They're not just damaging forests — they're wiping them out. Last year, more than 74,000 wildfires burned over 8.7 million acres in the U.S.

That included the huge Wallow Fire in Arizona.

Swetnam stands in the catacombs of storage boxes that house thousands of tree-ring samples. His laboratory is buried under the bleachers of the University of Arizona football stadium. (NPR)
Swetnam stands in the catacombs of storage boxes that house thousands of tree-ring samples. His laboratory is buried under the bleachers of the University of Arizona football stadium. (NPR)

"It burned more than 40,000 acres in the first eight hours," says Swetnam, the tree ring expert. "A tornado of fire."

Fires in the Southwest have been getting bigger and bigger over the past two decades.

"Now the fire behaviors are just off the charts," Swetnam says. "I mean, they are extraordinary. Actually, I think in some cases, they're fire behavior that probably these forests haven't seen in millennia or maybe even tens of thousands of years."

Over the past several years, even as fewer fires have struck the Southwest, they've burned more land. The U.S. Forest Service now spends about half its budget on firefighting.

Many fire experts embrace controlled, or "prescribed," fires — purposely set fires that do the cleanup job that small natural fires once did. It takes the tinder out of the tinder box.

A perfect tree-ring slice is a window in time, a slice of a forest's history. (NPR)
A perfect tree-ring slice is a window in time, a slice of a forest's history. (NPR)

But people have built homes and towns close to forests; they don't like the smoke, and prescribed burns sometimes get out of control. The Cerro Grande Fire in New Mexico in 2000 was a controlled fire — until it jumped fire lines and destroyed hundreds of homes.

I talked to veteran fire manager William Armstrong of the U.S. Forest Service about that, sitting on a ridge near Santa Fe, where he has done prescribed burns himself. Armstrong says people must accept fire in their lives.

"Large blocks of forest — if they want those — then what they must understand is that fire is inevitable," he says.

He says to save forests from total annihilation — and the wildlife and water supplies they protect — you have to set some fires and let some natural fires burn.

"The choice is not whether or not these forests burn," Armstrong says. "The choice is how they burn. What kind of intensity are we going to see those burn at?"

A tree's rings are marked on graph paper. Experts use this technique, called skeleton plotting, to help them cross-date tree-ring samples. This sample evinces no fire scars in the latest years of the tree's life. (NPR)
A tree's rings are marked on graph paper. Experts use this technique, called skeleton plotting, to help them cross-date tree-ring samples. This sample evinces no fire scars in the latest years of the tree's life. (NPR)

But as fire experts like Craig Allen, an ecologist with the U.S. Geological Survey in New Mexico, are now discovering, fire is increasingly out of their control.

"Basically, the mountains in the Southwest — you can almost think of them as caskets of fuel," Allen says. "Gunpowder has been building up in these things for a century, and now it's dangerous to try to defuse."

Copyright NPR 2016.

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