Report on Research | Summer 2004 | Fire on the Mountain

By Paul Muhlrad

For humans, forest fires, such as the one that scorched 85,000 acres atop Mount Lemmon last summer, are tragic. But they are natural and necessary events for healthy forests, tree-ring studies show. If land managers had known this a hundred years ago, they could have prevented catastrophic wildfires that now threaten forests throughout the Southwest.

Instead, in summer 2003, Tucson's residents despaired while they watched helplessly as dirty orange smoke plumes hung over the Santa Catalinas. Their beloved weekend playground was up in smoke.

When the ash settled and the smoke cleared over Mount Lemmon, alongside the cabin ruins in the mountaintop village of Summerhaven remained a burnt forest and many questions. How could it have happened? Could we have prevented this catastrophic fire? Will other natural jewels, such as Pinaleno Range, home of the Mount Graham International Observatory, suffer a similar fate?

The 1996 Clark Peak Fire on Mount Graham damaged thousands of acres, reaching into the high fragile spruce-fir zone for the first time in more than 300 years, and threatening the observatory.

For Tom Swetnam, professor and director of the UA Laboratory of Tree-Ring Research, the answers are written in the trees themselves.

Swetnam and his students have hiked forests throughout the western United States, reading the rich history of forest fires recorded in the trees' growth rings. Dendrochronology, the science of dating trees from their growth rings, was born at the University of Arizona, where the Laboratory of Tree-Ring Research has been inconspicuously housed since 1937 under the west bleachers of Arizona Stadium.

Naturally, trees grow most bountifully during years with favorable climate conditions, putting out thick growth rings. Trees record more austere climate conditions with narrow growth rings. Neighboring trees in a forest experience essentially the same growth conditions, and therefore develop similar patterns of thick and thin growth rings, which dendrochronologists can decipher like bar codes, assigning each ring to an exact year, and even to the precise season. Tree ring scientists can date rings even from trees that are long dead, by aligning the patterns of their outer, youngest rings with the patterns of the oldest rings from living trees.

Along with the almanac of growth conditions, tree rings preserve evidence of other historical events, including fire. Most large trees in western forests have experienced fire many times during their long lives, Swetnam says. A typical fire in a healthy forest rolls along the ground level, fueled by the layers of dead leaves and twigs, burning the outer bark layers of trees enough to leave a permanent black scar, but not penetrating sufficiently deep to cause significant harm. Occasionally, a fire will burn parts of the tree completely through the bark layer, but even this severe insult is survivable. The damage scabs over, and the bark eventually grows back to fill over the breach.

By analyzing growth rings and fire scars from live and dead trees on Mount Graham, Swetnam's team has pieced together a fire history dating back almost 600 years. The fire history on Mount Graham is really a tale of two forests. At the top of the mountain, above 9,000 feet, resides the southernmost stand of old-growth corkbark fir and Engelmann spruce in the United States. This is the forest made famous in recent years as home to the Mount Graham red squirrels, sacred ground to the Apaches, and perch for the MGIO cluster of astronomical telescopes, an initiative led by the UA Steward Observatory.

The spruce-fir zone is particularly intolerant to fire, says Swetnam. A spruce tree has such thin bark that if a surface fire reaches the base of one, it will almost certainly die. But trees in the spruce-fir zone show absolutely no fire scars. Remarkably, from 1685, when a ravaging crown fire apparently eliminated every tree on the top of the mountain, to the 1996 Clark Peak Fire, which burned up into the spruce-fir zone and nearly reached the telescopes the spruce-fir forest atop Mount Graham had not experienced a significant fire.

Below 9,000 feet, encircling the spruce-fir zone, lays a mixed-conifer forest dominated by stands of ponderosa and white pine, species well suited to resist fire with their thick bark. For most of these trees fire has been a familiar occurrence, Swetnam explains. Old logs cut from the mixed conifer zone bear witness to centuries of forest fires on Mount Graham at about five- to 10-year intervals.

With constant fires erupting just a stone's throw away, how have the spruce and fir trees escaped certain annihilation for so long? Swetnam says that, although the pine forests pulsed with fire once or twice every decade, once the flames reached the spruce-fir zone, they ran out of fuel. While the mixed-conifer forest floor is littered with loosely packed pine needles and grass cover good fuel for spreading fire ground cover in the spruce-fir zone is almost non-existent. There, the tree canopy is closed, allowing inadequate sunlight to penetrate to the ground for supporting grasses. The tiny spruce and fir needles form a tightly packed mat on the forest floor, devoid of the air pockets necessary to breath life into fire. When ground fires climb up the mountainside from the mixed-conifer forest, they creep along the ground among the spruce and fir trees and simply burn out.

Until around the turn of the 20th century, this was the perennial cycle of fire on Mount Graham. Fires continually burned the understory of the mixed-conifer forests, clearing away tinder before it could accumulate to dangerous levels. Fires burned at a low intensity and stayed on the ground, rarely erupting into the forest canopy, where they could threaten the neighboring firs and spruce. In truth, Swetnam maintains, the continual mid-elevation fires on Mount Graham are what have protected the fragile spruce-fir zone above from being incinerated.

After about 1880, the fires in the mixed-conifer zone abruptly ceased, as Swetnam's research team realized when they saw a complete lack of fire scars in recent growth rings in virtually every tree they examined. Two major human influences probably put an end to the perennial fires on Mount Graham, and indeed throughout much of the west. The first was ranching. Cattle that grazed in the lower mountain slopes devoured much of the grass cover, which in the past had fueled the ground fires. The second was active suppression of all forest fires. The U.S. Forest Service instituted a zero tolerance policy for forest fires, and throughout most of the 20th century the prime goal of forest management was to snuff out all fires as quickly as possible. Bambi and Smokey Bear taught Americans that fire in the forests is a force to be eliminated. As a result, forests in the west no longer exhibit the open park-like character they had in the past. Their understories are overgrown thickets of small trees, shrubs, and leaf litter the fuel that propels stand-replacing canopy fires, as Tucsonans witnessed so dramatically last summer. The trees themselves tell us that fire can be a deadly enemy, or a vital guardian.

As we began to put the finishing touches on this issue of Report on Research, Mount Graham began to burn. UA scientists had feared that a forest fire could destroy the Mount Graham International Observatory and devastate the habitat of the already endangered red squirrel.

However, reports from John Hill, director of MGIO, and John L. Koprowski, director of the Mount Graham squirrel monitoring program, indicate that while serious, the fire neither destroyed all the squirrel habitat nor damaged the telescope complex.

"The telescopes are undamaged." says Hill. "The wind was blowing in a good direction, so there was minimal ash." The complex is surrounded by a 200-foot buffer of cleared land, and astronomers had battened down air intakes and turned off blowers so no ash would be drawn into the buildings.

Hill said the only loss was about two weeks of scheduled telescope time for astronomers.

Koprowski said he and his squirrel-monitoring team will not know the impact on the squirrels for some time, but that the fire did not burn all the small animal's habitat.

"Clearly a lot of the habitat was lost, but not all," he said. He described the fire as having burnt in a "mosaic" pattern, completely scorching and sterilizing some areas while leaving some areas untouched. Ash Canyon, which serves as a corridor up and down the mountain for the squirrels and other animals was mostly spared. Results are "positive and some reason to be optimistic." He added that because there are so few squirrels, any blow to their habitat is serious, "but I think things have turned out to be less severe."

"Most of our study areas were not impacted by the fire," he said. In addition, he said that because of the success in tagging and fitting the squirrels with radio collars the team will be able to resume monitoring soon.

Koprowski says he will know then if the squirrels outraced the flames or "did they hole up and wait out?" "We are actually going to be able to answer that question," he said.

by Lori Stiles
The same trees that have given scientists a striking 600-year history of forest fire in the Pinaleno Mountains have given archaeologists a valuable new tool for dating prehistoric desert dwelling sites.

During the past decade, UA tree-ring scientists made major strides in dating Arizona's prehistoric wood samples, thanks to better local tree-ring chronologies and better dating techniques.

UA dendrochronology Professor Jeffrey S. Dean and others at the UA Laboratory of Tree-Ring Research were the first to date prehistoric Sonoran desert sites to known calendar years using tree-rings. Their breakthrough came in the mid 1990s, when they extracted 21 dates from 1,664 previously collected wood samples and dated three Hohokam sites near Tucson. The three sites were simultaneously occupied and then abandoned by A.D. 1250.

The Hohokam (ho-ho-kam) ancestors of today's Pima and Tohono O'odham people built villages in the Tucson Basin beginning around A.D. 300.

In the late 1990s, senior research specialist Christopher H. Baisan strengthened the Tree-Ring Lab's Pinaleno Mountains chronology, extending the record back to A.D. 882.

This chronology was key in dating charcoal from University Indian Ruin, a Hohokam site near Sabino Canyon and Tanque Verde Roads, seven miles from downtown Tucson.

University Indian Ruin is a 656-by-427-foot area containing at least three separate room blocks and a platform mound. UA archaeologists partially excavated the site between 1930 and 1940. Wood samples collected at the site were numbered and archived at the Tree-Ring Lab during the 1960s.

Tree-Ring Lab archaeological research specialist David J. Street analyzed some of the material collected more than 60 years ago.

"It's extremely difficult stuff to work with because desert trees often don't grow a single ring each year, as do southwestern trees living at higher elevations," he said. Southern Arizona's trees often grow one or more false rings annually, or sometimes miss rings altogether, as they cope with an arid climate.

Street succeeded in dating enough of the material to show that Hohokam lived at University Indian Ruin during or shortly after the winter of A.D. 1371 and until after A.D. 1375. His research on University Indian Ruin only the fourth prehistoric desert site now dated by tree rings suggests that Hohokam people may have occupied the Tucson basin into the 15th century.

Some archaeologists argue that Hohokam culture vanished before Europeans came to southern Arizona. But others think Hohokam were still in the Tucson Basin. New excavations at University Indian Ruin may solve the mystery. Arizona State Museum archaeologists Paul Fish and Suzanne Fish will excavate University Indian Ruin in 2005 as they conduct an anthropology department archaeological field school.