Scene from a 2009 fire north of Los Angeles.
A new study by scientists at U.S. Forest Service and University of Washington shows prescribed burning – in addition to forest thinning – can dramatically reduce wildfire severity in the dry, overstocked forests of the Pacific Northwest.
How dramatically? This study shows it can save three times as many trees as thinning alone. It can prevent 75 percent of the bigger trees (larger than 8 inches in diameter) from dying in a wildfire.
For three years, researchers studied the 175,000-acre Tripod Fire in Washington’s Okanogan-Wenatchee National Forest. They compared the damage in tree stands that had been thinned and fire-treated with stands that had only been thinned.
Section of Okanogan-Wenatchee National Forest that had only been thinned before the wildfire.
The result? The fire killed 80 percent of the trees that hadn’t been treated with fire. In stands that had been treated, 60 percent of the trees survived. Even better, in fire-treated stands, the wildfire only killed 25 percent of the bigger trees that have higher timber, habitat and carbon storage value.
“The objective of fuel treatments is not to eliminate wildfires, but to reduce their intensity in areas where we want to protect resources,” said research biologist Dave Peterson with the U.S. Forest Service Pacific Northwest Research Station. “If we implement treatments across large areas and place them strategically, we can manage these low-elevation forests sustainably, even in a warmer climate.”
Section of the forest that had been thinned and treated with a prescribed burn before the wildfire.
I called Peterson to learn more about the implications of this study. Here is what we discussed:Q: What’s new about these research findings? A: “We have a lot of intuition that prescribed burning will reduce wildfire severity, but we have little empirical data to verify that. In the case of this 175,000-acre fire, they happened to have very good records of what they did and when prior to the fire. So, we had an opportunistic situation there where we could go into areas that have been treated and those that haven’t, and we could quantify it. Having that over such a large geographic area gives us some valuable data. It’s a validation.”
Dave Peterson, research biologist with the Pacific Northwest Research Station
Q: How often is prescribed burning used along with thinning to safeguard against wildfire in the Pacific Northwest?A: ” There are two different things typically used. The first step in really dense forests is to reduce stand densities to around 200 trees per acre or less. The accumulation of those fuels over several decades can lead to more severe fires if you haven’t had a fire in a long time. But if you do a thinning operation, it can create some fuels itself, and you have to follow it up with surface fuel treatment. That can be done with prescribed burning, or it can be done with mechanical equipment, where someone goes in and collects the material off forest floor. Or sometimes it’s done manually.”
Q: What are some of the reasons prescribed burning wouldn’t be used?A: “In some places, the use of fire may be considered more safe than in others. A big factor now is air quality. There are restrictions on the number of days per year you can burn so it doesn’t affect nearby communities. Manual removal and prescribed burning can be fairly equivalent in their effectiveness – but prescribed burning is a lot cheaper. You can run a fire through an area quickly, and it takes more human power it to do mechanical activities.”
U.S. Fish and Wildlife Service
The Chewuch River in Okanogan-Wenatchee National Forest.
Q: Have air quality concerns reduced the amount of prescribed burning in recent years?A: “We have seen a steady increase in the air quality factor as more and more people are moving to these wild-urban interface areas. If you were to go there 40 to 50 years ago, there weren’t all that many people living in these remote places. Now you have people living all over the place. It’s not the fires or the smoke that’s the problem. It’s the people living in those places that are the problem. Regulatory agencies are responsible for that and have to take that into consideration. They want to avoid impacts on people taking in heavy intakes of smoke.”
Q. Which forests would be comparable to the forest where you did your research?A: “The majority of east side Cascade forests in Oregon at low elevation would probably be comparable to this site. Oregon has certainly had its fair share of big fires. A lot of these occurred in dry forests with mixed conifer and Ponderosa pine that have not experienced fire as often as they did prior to 1900. So, I’d say it’s quite applicable across the east side of the Pacific Northwest.”Q: Sometimes it’s hard to get permission to do thinning on federal forests. Would wildfire still be reduced without thinning?A: “Thinning is a real challenge, and an expensive part of the operation. The challenge is if you have these really dense stands, if you were going to try to introduce fire in them, there’s a much greater chance of having a crown fire. Try to envision small trees acting as ladders into the crown and conducting fire from the surface into the upper canopy. If you burn the whole canopy, the tree’s going to die. There are fuels on the ground, and also fuels in the air. “If you can continue to keep it at reasonable densities, run subsequent fires in there, the flame won’t be high enough to get up to the crown. The bigger trees can tolerate high heat because they have thick bark. But you want to keep it out of the crown of the trees. “People have tried to do that in California, in the Sierra Nevada, where the forest is more open, they have used prescribed fire without thinning and have had some success. But here it’s a much riskier proposition. Most of us are unwilling to take the risk of having a big crown fire.”Q: How do you think your findings can be used?A: “What we’d like to do is have this science inform guidelines so resource mangers can use it to make forests more resilient to fire and more resilient to climate change. One of my jobs is to provide folks in national parks, forests and (Bureau of Land Management) with the scientific information they need to make good decisions. “The climate change connection is very important. Managing for reduction of wildfire severity is perfectly compatible with management for climate change. One of the best things we can do to create resilient forests is reduce very large, severe disturbances. We can tolerate less severe forest fires – which forests did for hundreds of years before 1900. But having stopped for a century or so and then having very severe fires is not necessarily something these forests are adapted to. So, this management approach will help with climate change, as well.”
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