Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
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Local thinning projects yield cords of wood, healthier forests

By Bridger Skaarer   

mound of wood
Photo by Bridger Skaarer
The end result after thinning has taken place near Bigelow trailhead on Tucson’s Mount Lemmon.
Stacks of wood interspersed with small boughs and scaling up to large logs line a portion of the main highway on Mount Graham outside Safford, Arizona. Most stacks look like miniature huts, with some almost resembling teepees, while others sloppily form mounds of wood. These piles of firewood are the result of forest thinning, an attempt to reduce wildfire potential.

Thinning operations are taking place on Mount Graham in the Pinaleños, as well as many other mountain ranges in southern Arizona’s Coronado National Forest system, including the Catalina Mountains north of Tucson. By removing small trees and lower limbs of larger ones and putting them into piles, foresters are working to protect forests, homes and other structures from the threat of wildfires. As a side benefit, this provides a cheap source of fuel available to the general public.

“Thinning has become very common after the fires in Montana in 2000,” said Shane Lyman, the fire management officer of the Nogales Ranger District, which includes the Santa Rita and Tumacacori mountains.

Almost 1 million acres burned in Montana in 2000, a year in which over 7 million acres burned nationwide. Soon after, the infamous Rodeo-Chediski fire in 2002 burned about 468,000 acres and charred approximately 400 homes in northern Arizona, while the Biscuit fire burned another half a million acres that year in Oregon.

The spate of uncharacteristically large wildfires prompted new legislation to help prevent large fires. Congress passed the Healthy Forest Restoration Act in 2003. The main objective of the new law was to promote:

Since the Healthy Forest Restoration Act was passed, thinning has become more common in the western United States, said Lyman. unthinned area

Photo by Bridger Skaarer
An overgrown stand of ponderosa pine a few miles above Bigelow trailhead that has not been thinned.

Ironically, forest policy of the past helped set the scene for modern-day large fires. Ponderosa pine forests have historically had a high-frequency, low-intensity fire pattern. This kept ponderosa pine forests with open canopies that discouraged fire from reaching the crown of trees.

“Fire suppression for the last 100 years has allowed forests to build up large fuel loads,” explained Peter Ffolliot, a professor at University of Arizona for more than 40 years, with a long background in forest systems and management. This has really made it tough to fight wildfires and is one of the causes of large fires such as the Rodeo-Chediski and Wallow fires.

The Wallow fire near Alpine burned 30 structures and consumed over a half million acres, surpassing the Rodeo-Chediski fire as the largest wildfire in Arizona history.  

Dating back to the early 1900s, the U.S. Forest Service began suppressing wildfires.
Modern society has generally viewed fire as a negative disturbance. An advertising campaign by the Forest Service also changed the public’s view on forest fires. In 1944 the Forest Service distributed posters with a picture of a black bear named Smokey, with the slogan “Only YOU can prevent forest fires. The warning became well known and gave the public the view that forest fires were bad.  

Most fires from 1900 to 2000 were put out and not allowed to burn. For that matter, firefighters tried to suppress many of the recent big fires, but found them uncontrollable. Between fire suppression efforts and a general tendency to let forests grow unmanaged since the mid-1990s, the region has been facing large and destructive fires, explained Fflolliot.

Many foresters agree that thinning helps to prevent wildfires in western dry forests by reducing the risk that fires will reach the forest canopy where suppression becomes much more difficult.
Before people began suppressing fires in western forests, fires would start on the surface, burning herbaceous material such as grasses and weeds. Once enough heat builds up, it will start to burn shrubs, young trees and low-lying limbs of larger trees.

Surface fires historically burned the small, young trees. Now, many of them overpopulate the forest floor. This vegetation, referred to as ladder fuels, allows fire to build up and reach the canopy of trees. Wildfires often get large and uncontrollable once a fire moves from the surface. Thinning can help keep wildfires from becoming so severe by removing shrubs, shorter trees, and low-lying limbs of large trees.

thinned forestPhoto by Bridger Skaarer
Sunlight filters through an area of thinned forest near several homes along Mount Lemmon Highway.
Fflolliot explained how thinning has been shown to help lower the severity of fire. In the cases of the Rodeo-Chediski and Wallow fires, there are many minimally burned areas within the perimeter of those two fires. Many of those unburned areas were thinned in the past, he noted.

Once an area is thinned, typically, the slash and debris is then stacked into piles. This is what many people see when driving up to the top of Mount Graham or Mount Lemmon. This wood is available to the general public; all it takes is the purchase of a fuel wood permit. The cost varies by district, but runs in the range of $20 per cord of wood or less. A cord is 4 feet by 4 feet by 8 feet.

“Allowing the public to use this wood and take it home helps out the Forest Service in a few ways. It costs money to dispose of the slash. It takes time and work put in by firefighters to burn these piles,” said Lyman.

Collecting the wood instead of leaving it to burn helps the forest also.

“Burning piles of wood can alter the soil structure,” explained Don Falk, an associate professor in the UA School of Natural Resources and the Environment with an extensive background in fire science. “Highly concentrated heat output alters the soil properties to make it impermeable to water, forming a hydrophobic layer.”  

With the Forest Service thinning the forest and these new fuel wood permits available, everyone can help reduce the threat of wildfires, and also get some cheap firewood.

Bridger Skaarer is a senior majoring in rangeland management. He was raised on a cattle ranch south of Willcox, Arizona. As a summer job, he is a wildland firefighter for the federal Bureau of Land Management.


Coronado National Forest

National Interagency Fire Center list of historic fires

Healthy Forests Initiative

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