When a surface fire did manage to crown, it would kill trees and make a hole in the forest, after which, more often than not, the fire would drop back down and resume its slow crawl beneath the pines. Stoked by thick ladder fuels, crown fires of recent years have spread farther and faster, leaving much bigger gaps. Case in point: the Rattlesnake fire, which started in the pines of the Chiricahuas during a hot spell in 1994. Helicopters attacked it with water and flame retardant, but the fire raced uphill into a dense mixed-conifer forest and burned uncontrollably for three weeks. When the rains finally came, the erosion of the denuded slopes loosed so much soil that a lake below the forest was completely filled in.
The hiking trail that Hodges chose skirted the fire's damage, which was still plain a decade later. Aspens had sprung up in the open patches, as well as opportunistic wildflowers like meadow arnica, with its floppy yellow blossoms, and scarlet-bearded penstemon, jauntily erect. Fifty yards farther along, the hulking Douglas fir and ponderosa seemed not to have been touched. There were bluebirds, hairy woodpeckers, and yellow-eyed juncos. A family of goshawks—dark-winged, acrobatic raptors—swooped low over the trees.
The higher you go in these mountains, the more you feel yourself being funneled into the sky. Unexpectedly the earth will fall away. Views burst outward, uncovering the khaki-colored desert far below, with blackened snags fringing the picture. Earth, air, and fire are close at hand, while the fourth element, water, is organizing into thunderheads. Great, grumbling clouds advance upon the Chiricahuas from the north, but the rain they release doesn't reach the ground. Instead, waves of fine droplets are parted by the mountain, as by a ship's prow, and the storm sails south.
The Laboratory of Tree-Ring Research is in the football stadium of the University of Arizona in downtown Tucson. Hunting for information on the deep history of fire, you worry that you're in the wrong place. But the faculty offices and the lab archives, full of old wood, are situated in the wedge of space between the west seats of the football field and the stadium's outer wall.
Tom Swetnam, the scientific director, and Don Falk, an ecologist, unlock a door in a corridor near the 40-yard line. Out flows a warm, ligneous aroma. The shelves inside are stacked to the ceiling with platter-sized sections of trees. Some of the wood is dusty and loose, and some is kept in sleeves in labeled drawers.
The scientists make their way downfield past wood from South Africa, past 5,000-year-old bristlecone pine, pale with spidery whorls, past giant sequoia, its slabs like the paws of defensive tackles.
At about the 10-yard line, Swetnam bends and pulls out some wood from the Animas Mountains. The Animas are a sky island system in New Mexico. "Southwestern white pine?" he hazards, handing the piece off to Falk.
Sniffing it, Falk says, "I think so."
Dendrochronology, they explain, involves much more than dating a tree by counting its rings. A record of climate change, insect attacks, and wildfires, among other contingencies, is wrapped up in the rings. The birth date of this white pine, 1695, is marked in pen at the pith, the tree's central stem. Subsequent dates indicate the fire scars. At 1851, for example, there is a dark fleck of discoloration and a particularly narrow ring, a sign of poor growth during that year. Historical accounts speak of a very large fire in the region in 1851. For a contrast they show a section of younger wood from another sky island, the Huachucas, totally devoid of scars in the 20th century.
It is interesting to see scarring from the internal vantage of the tree. A small part of the tree's circumference is killed, so the wood there stops growing. The rest of the cambium layer and bark continue to expand around the injured portion. Even as the scar is renewed by ensuing burns, the new growth pinches from two sides. If the tree escapes fire for a time, the scar may close over and disappear from sight. The tree-ring history of the mountains makes clear that fires used to burn frequently but did not wound each tree every time.
"A rule of thumb," Swetnam says, "is that once or twice a decade, over 20 to 30 acres, a ground fire swept through the sky island pine forest. These surface fires occurred at longer intervals in the heights of the sky islands—in the mixed conifers. Also, crown firing was more common up and out of the ponderosa." Asked why a fire would crown more readily in the mixed conifers, Swetnam says emphatically, "Because it's a freight train coming up from the pines below." The freight train roars where the fuels are thickest.
In thinking about how to restore today's forests to more balanced conditions, the researchers at the tree-ring laboratory favor the standard combination of mechanical thinning and prescribed burning. Also, they say that forest managers and the public should expect a greater number of crown fires, although these will be alarming. Whatever the policy, there can be no ideal forest to shoot for, no perfect mix of species and spacing to restore. As Falk notes, global warming during the last century would have changed the forest even if fires had not been suppressed.
