In a stand of ponderosa pine trees high in the Santa Catalina Mountains overlooking Tucson, Arizona, forest-and-fire ecologist Don Falk squatted with me next to a 100-foot-tall tree born a decade or two before American independence. At the base of the trunk, the tree's thick cinnamon-colored bark gave way to a shallow opening a foot wide and two feet high that looked like a series of successively smaller triangles. Falk ran his hand along the charred edges of the opening and explained what we were looking at: a window into the forest's past, and fire's role in shaping it.
Falk studies fire-scarred trees to understand how frequent, severe, and widespread fires have been in an area, and how those patterns have shifted over the centuries—which is also a key to understanding why some fires are bigger, more unpredictable, and more destructive these days, “How do you know anything on Earth has changed?” he asks. “You have to be able to compare it to how things were in the past. This is how we know the history.”
Long before the Mexican-American War, when this land still belonged to Mexico, a fire swept up this mountain slope. Short flames wrapped around the tree and curled like an eddy in a stream, lingering on the back side, where accumulated leaves and pine needles caught fire. The flames stayed long enough to penetrate the bark and killed a portion of the cambium, which produces new cells. The tree slowly healed itself, pushing edges of new growth onto the dead area, year after year. But the scar remained. The next fire that came through left another scar, and the next fire another. If we examined a cross-section of the tree, we could use the rings to figure out the exact year of each fire.
Falk works down in the valley at the University of Arizona's Laboratory of Tree-Ring Research, which occupies a gleaming new four-story glass-and-metal cube and holds 2 million wood specimens from around the world, the largest archive of its kind. The lab's founder, an astronomer named Andrew Ellicott Douglass, created a new discipline called dendrochronology: the analysis of tree rings to interpret and date past events. He used rings to date ancient Aztec and Pueblo ruins in the southwest by studying trees used in their construction, and he found that trees in the region grow more in wet years than in dry years, a first step in our understanding of climate change.
Falk, his face tanned by long days in the field, walked with me through the pines. He stopped at a large ponderosa-pine stump, two feet across, cut smooth by a chainsaw. To understand wildfire today, everything we've done to try to control it, and the problems those efforts have wrought, this was a good place to start. He brushed fallen pine needles from the stump and offered a quick reading of the tree's fire history: Born in the mid-1700s, it shows scarring from fires every decade or two, the rings curled like breaking waves around the wound. But something curious happens after the marks from an early-1900s fire: the scars stop. The tree rings continue out toward the edge, for decades, slowly healing that last fire wound, until the tree died several years ago.
Where did the fires go? Grazing animals consumed some of the fuels that would have carried fire. Then, a century ago, we embarked on a campaign to banish fires from forests, with a goal of extinguishing them soon after they started. But that wasn't such a good thing for the forest. When fires don't come through regularly, fuels accumulate. A couple of centuries ago, forests like this one in the southwest might have had a few dozen trees per acre, widely spaced, with an open, savannah-like floor. Today an acre might be crowded with thousands of mostly smaller trees. When fires do burn, they're more destructive, often killing the big trees along with the small.
“What's being released in a fire is the accumulated capital stored up through years of photosynthesis,” Falk says. “You're not destroying the carbon, hydrogen, or oxygen molecules. They're just being liberated.” And on a tremendous scale: even a relatively small fire of a couple hundred acres can pump out energy equivalent to the atomic bomb dropped on Hiroshima, and can push a mushroom cloud of hot air, ash, and soot miles into the sky.
Falk and I stopped for lunch in a meadow and sat on a scorched and toppled pine tree. Evidence of the 2003 Aspen Fire, which burned 84,000 acres across these mountains, surrounded us: dead trees that stand like giant toothpicks; huge patches of pine forests that have been replaced with shrub. Falk regarded this with a scientist's remove. “There's no good or bad fire,” he said. “That's a human construct. We can say what's normal, or historical, or expected, but how can you say it's bad?”
While Falk studies fires to better understand how they have changed over the years and altered the landscape, wildland firefighters study fire as soldiers might analyze enemy capabilities. They catalog mental snapshots of fire behavior they have encountered: how the flames ripped through a grassy canyon or hopped off the ground and leaped into the treetops; the strange calm before a sudden wind shift; the fire tornados that can spin flame in new directions. Early on, a rookie matches these images with what he's learned in training or heard from the veterans.
“The mind pulls up the most similar slide, and it says, ‘I recognize this situation as similar to this other one that I experienced, so probably a similar approach is a good way to go about it,’” Larry Sutton says. He heads up risk management for the Forest Service at the National Interagency Fire Center, in Boise, Idaho. “The danger is, what if this situation doesn't match any of your slides?”
For nearly a century, scientists have been trying to create a bigger reference library to explain why fires behave the way they do. Starting in the 1920s, Harry Gisborne gathered data on weather, terrain, and fuel conditions to predict the likelihood and potential severity of a fire in a given area. The legacy of that work can be see today at the entrance to many parks and forests—an arrow pointing to a color-coded scale for fire danger. When the Mann Gulch Fire flashed up a grassy Montana hillside and killed 13 firefighters in 1949, Gisborne went there to study the fire's progression, and died from a heart attack while hiking the steep terrain.
An aeronautical engineer named Richard Rothermel continued Gisborne's quest. At the Forest Service's Fire Sciences Laboratory in Missoula, Montana, he used wind tunnels and combustion chambers to test how fast fires ignite and spread in different wind speeds, slopes of terrains, temperatures, and types of vegetation. He translated his observations into mathematical equations that could be used on the fire line by sketching a graph or punching a few numbers into a special calculator.