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.
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.”
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.
“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. 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.