Wildfire Corner: Thinning Trees to Change Fire Behavior

By Guy Gifford, Landowner Assistance Forester and Fire Prevention and Firewise Coordinator, Washington State Department of Natural Resources, Northeast Region, guy.gifford@dnr.wa.gov

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Figure 1: This untreated area experienced a crown fire and significant mortality. (Photo by Guy Gifford, Washington State Department of Natural Resources)

I’ve written multiple articles in Wildfire Corner about how proactive work has helped save homes and change the intensity of wildfire. So far, I have not had a personal stake in the examples I’ve described but, this year, the Pine Meadows fire in Spokane County burned an area that I’m very familiar with. It burned 140 acres, including forest stands that I’ve helped landowners to thin and reduce the risk of crown fires.   

The area had experienced fire in the past, with several structures being lost nearby to a wildfire in 1991.  Some time later, I helped secure funding from the American Jobs Act to do some much-needed thinning work in the ponderosa-pine-dominated forests. The work focused on removing small trees, less than 8 inches in diameter and taller than 4.5 feet. Reducing stand density this way increased space between tree crowns to about 5 feet, creating what some foresters call “roads in the skies.”  Doing this minimizes the risk of a full crown fire if one tree torches, similar to how a road on the ground can stop fire from spreading.  

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Figure 2: As the fire moved back into a thinned area, the flames moved back to the ground. (Photo by Guy Gifford, Washington State Department of Natural Resources)

Ladder fuels, which can be small bushes and branches that carry fire to the crowns of trees, were also removed. Trees were pruned 10 feet from the ground unless that would mean removing more than 50 percent of the crown. Sometimes, clumps of trees would be left to meet other landowner objectives such as aesthetics and wildlife habitat, but they were given more space from surrounding tree crowns. More than half the area was treated, with the focus being around structures.

Cut to another decade later, the wildfire started in the afternoon on August 16 as the result of a nearby structure fire. It burned the entire 140 acres in only 90 minutes. Winds coming from the southwest caused the fire to spread rapidly, and one structure was lost early in the fire’s development.

The fire spread into an area that had been thinned and pruned, and it stayed on the ground.  As the fire was pushed by the wind, it reached an area that had not been treated, and the fire quickly climbed into the canopy and started spreading to other crowns, which significantly increased the fire intensity (Figure 1). When fire gets into the crowns of the trees, fire resources have to wait until the fire comes back to the ground before they can attack the fire.

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Figure 3: The thinned area around the home. (Photo by Guy Gifford, Washington State Department of Natural Resources)

As the fire continued to spread to the northeast, it came into another area that was thinned and pruned, and it again came back to the ground. This work was done around a home to increase the chance of the home surviving a wildfire. Looking at Figure 3, we can see that it worked. You can see the house in the background and how the tree needles are only brown, not scorched, which means the fire stayed on the ground and the fire intensity was reduced. 

As I look back at this project and the work we did, there are a few notable lessons to take away:

  • Thinning and pruning trees in the forest made a huge difference in reducing the severity of the fire and keeping it on the ground.
  • A fire doesn’t have to be big to destroy a home. The size of this fire was quite small but did serious damage.
  • In the early stages of a fire and when it is moving quickly, homeowners may not always receive evacuation notices.

If you own forestland or a home in a forested area in Washington, wildfire is an inevitability, especially in eastern Washington where fires are more frequent. However, you can control how that fire behaves with proactive forest management and application of Firewise principles. The fall season is a great time to take a look at your property and determine if you need to do some thinning and pruning in your forest to reduce the fire intensity of the next wildfire.