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.
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.
Then something happened.