UNIVERSITY PARK, Pa. -- Forest composition, ground cover and topography are the best predictors of forest fire severity in the Western U.S., according to Penn State physical geographers who also see that the long history of fire exclusion on federal lands leads to uncharacteristically severe burns and potentially changes the dynamics of forests and their recovery.
A hunter's illegal campfire in Stanislaus National Forest adjacent to Yosemite National Park started what would become the Rim fire, the third largest fire in California history, that burned from August through October 2013. The fire burned about 400 square miles inside and outside Yosemite, with 78 square miles burned on the worst day.
"We would never be able to do an experiment on this, never be able to burn the forest in this way, so this natural experiment is a perfect opportunity to see what happens," said Alan H. Taylor, professor of geography.
Taylor and Lucas Harris, graduate student in geography, studied the forest's recovery in the aftermath of the Rim fire. They report their results in the current issue of Ecosystems.
"This area burned at uncharacteristically high severity and did so even though fire weather was not particularly extreme," said Taylor. "The fire does not appear to have restored the forest to before fire suppression, but altered it."
Forest fires occur naturally, usually initiated by lightning strikes. Native Americans also started fires accidently or deliberately to improve plant growth and hunting.
"If a forest burns every ten years, ponderosa pine is pretty fire resistant," said Harris. "But after 100 years of fire suppression, there are a lot of pine needles on the forest floor and they are highly flammable. We found areas of ponderosa pine burned more severely than areas with other trees."