Missoula, Mont. — Thinning forests without also burning accumulated brush and deadwood may increase forest fire damage rather than reduce it, researchers at the Forest Service reported in two recentstudies.
Studies of fires in 2002 in Colorado, top, and Oregon raise questions about the effectiveness of thinning forests without burning debris.
The findings cast doubt on how effective some of the thinning done under President Bushs Healthy Forests Initiative will be at preventing fires if the forests are not also burned.
The studies show that in forests that have been thinned but not treated with prescribed burning, tree mortality is much greater than in forests that have had thinning and burning and those that have been left alone. Another study, on Blacks Mountain Experimental Forest in Northern California, had similar findings.
The studies, combined with other recent research showing that climate change is reducing snowpack and making the fire season longer and more intense, have prompted researchers to urge the Forest Service to use prescribed fire more.
We need fire on the ground, said Dr. Ronald H. Wakimoto, a professor of forestry at the University of Montana who studies fire. The only thing that stops fires is previous fire or prescribed fire.
A study of a 500,000-acre wildfire in Oregon in 2002, called the Biscuit fire, showed that the mortality rate of trees in forests that had neither thinning nor prescribed burns was a little more than half. The study, published in late 2005 in The Canadian Journal of Forest Research, found that 80 to 100 percent of the trees in forests that had only been thinned died in the blaze, while 5 percent of the trees died in forests that had been thinned and burned.
A 2003 study of another large blaze, the Hayman fire in Colorado in 2002, published as a case study by the Forest Service, showed that fires killed 50 percent of the trees in a natural, unthinned forest but killed 90 percent in a thinned forest, because the fire on the ground was hotter.
When thick stands of small trees are cleared and space is created between larger trees, it causes a mat of litter several inches thick, including pine needles, slash from thinning and other debris, to dry out. New growth of shrubs and grasses, stimulated by thinning, also adds to the fuel load. When a fire gets started, it burns hotter and moves faster than when it was slowed by thicker growth.
The forest floor is hotter, drier and windier, Dr. Wakimoto said. When we thin, were not getting the shade, and the vegetation doesnt slow the winds.
He said thinning near urban areas was particularly worrisome. If they dont treat the fuels on the ground, the fire will get to the homes faster, he said. Thinning may create room to fight fires, he said, but it creates a false sense of security because serious fires can still happen.
Thinning is often done to prevent crown fires, which move through treetops, but unless crowns are 20 feet apart, which is usually not the case, surface fires can still create crown fires.
The most efficient way to decrease these fire risks is with prescribed fire, which can be difficult in areas with houses.
So-called activity fuels, the debris left by thinning, were a big part of the problem in the Biscuit fire. Thinning needs to be done completely, said Crystal L. Raymond, a researcher at the College of Forest Resources at the University of Washington, including activity fuels, which can be done with prescribed burning.
Forests around the West have adapted to frequent fires, which were set by lightning and for centuries by American Indians. But regular, minor fires have been thwarted because of increasing numbers of homes in grasslands and forests. When a fire does start, it feeds on accumulated fuel and is more damaging than smaller, recurring fires would have been.
Federal officials say they understand the role of prescribed burns but have not used them widely. Its an issue of sequence, said Mark E. Rey, under secretary for natural resources and the environment at the Agriculture Department. Well follow the thinning with prescribed burns.
Thinning has been controversial, even when accompanied by prescribed burns, since the Healthy Forests Restoration Act was signed into law in 2003.
A 1996 federal study, the Sierra Nevada Ecosystem Project, was conducted by a team led by Dr. Don C. Erman, emeritus professor of ecology at the University of California, Davis. Dr. Erman said thinning could be effective only if it was repeated as often as every two years, which would be prohibitively expensive. Its a treadmill you have to be on all the time, he said. Prescribed fire may extend the period between thinnings, he said, but not by much.
Prescribed fire can also be controversial, though for a different reason. A 2000 blaze in New Mexico called the Cerro Grande fire that started as a prescribed fire burned more than 40,000 acres and 400 houses near Los Alamos.
A report by the Office of the Inspector General at the Agriculture Department issued in March was critical of the Forest Services management of the Healthy Forests Initiative, saying the program had no consistent analytical process to determine which areas were most at risk and did not set priorities for projects.
Mr. Rey said those things had been addressed. They looked at early treatments, he said. Many of the things the inspector general found are things we are aware of and that weve moved on to correct.