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.