Yosemite Forest Fire Reveals Severe Wildfires May be on the Rise

How do we know how severe a forest fire is going to be? Scientists have discovered that forest composition, ground cover and topography are the best predictors of forest fire severity in the Western U.S.

The third largest fire in California history was the Rim fire, a fire that started when a hunter's illegal campfire in Stanislaus National Forest went out of control.

In this latest study, the researchers examined the forest's recovery in the aftermath of the Rim fire. This revealed a bit more about what might have led to the fire itself.

"This area burned at uncharacteristically high severity and did so even though fire weather was not particularly extreme," said Alan H. Taylor, one of the researchers, in a news release. "The fire does not appear to have restored the forest to before fire suppression, but altered it."

If a forest burns about every ten years, ponderosa pine is relatively fire resistant. However, suppressing fires means that fuel can build up over time. After 100 years of fire suppression, there are a lot of highly flammable pine needles on the forest floor.

"Fuels and terrain were the major factors contributing to the severity of the forest fire," said Lucas Harris, one of the researchers. "The only one of these that can be controlled is the fuel."

In the early 1900s, the U.S. Forest Service instituted a policy of total fire suppression or fire exclusion in the forests that they managed. The researchers examined fire severity from 1899 in Yosemite, the last year a fire burned in the study area, for a baseline of forest composition and fire's effects on the forest.

In some areas that have not burned for 100 years, workers go in and remove underbrush to reduce fuel load. However, this is particularly labor intensive. Controlled burning is also sometimes used to reduce the fuel load on the ground.

"In pine forests where fires naturally occur every five to 10 years, 100 years of fire exclusion creates an understory with abundant surface fuel and small trees that allow fires to move into the tree canopies," said Taylor. "Normally, with frequent fires, only the understory burns with some burn scarring of tree trunks, but the trees survive. However, in Big Oak Flat during the Rime fire, there was an unusually high proportion of moderate and high severity fires compared to 1899."

These findings reveal exactly why the Rim Fire occurred. This, in turn, suggests what sort of practices that officials should institute in order to help manage forests.

The findings are published in the journal Ecosystems.

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