USA — The danger of catastrophic fire was already clear to Scott Stephens when flames erupted almost on cue and chased his team of UC Berkeley researchers out of the Stanislaus National Forest.
The enormous Rim Fire, which started on Aug. 17 and has now blackened 343 square miles of forest in and around Yosemite National Park, was almost licking at his heels.
“I was thinking before the fire that if we ever get a fire in here, most of the old trees will be killed,” said Stephens, the university’s chief fire science expert. “I think that has happened.”
The Rim Fire, now the largest fire in recorded Sierra Nevada history, is a treasure trove of information for scientists studying the effects of forest management techniques. That’s because it burned through a variety of different landscapes, including chaparral-covered canyons, newly planted tree plots, previously burned areas and dense forests.
The key question is what happened on Aug. 22 and 23, when a 200-foot wall of flames burned almost 90,000 acres.
“Almost half of this very, very large fire happened in just two days,” said Max Moritz, a fire scientist at the UC Berkeley Cooperative Extension. “If you are a scientist, that is very interesting.”
Big increase in tree density
A large part of the answer may be found in Stephens’ interrupted expedition. His four-person research team was in the process of measuring tree diameters and densities on 15,000 acres that had been studied by the U.S. Forest Service in 1911.
The group found as many as 400 trees per acre on the land. That’s compared with between 60 and 90 trees per acre in 1911. There was also between 30 and 40 tons of woody debris per acre on the forest floor, compared with 6 to 8 tons 92 years ago, Stephens said. Besides the dramatic increase in tree density, the researchers found more undergrowth species and, although there were still many old growth trees, the average size of the trees was smaller than in 1911, he said.
“We know the last fire in that area was in about 1905. That’s 100 years without fire,” Stephens said. “If you don’t clear trees and brush and do some prescribed burning, you are eventually going to get a very closed forest that is very dense.”
Fires have historically been common in California, where burning actually prompts many native plants and trees to release more seeds. American Indians used to purposely set fires in an effort to clear out excess brush and prompt new growth, but the large trees normally survived. Experts say many areas of California, where fires used to burn every 10 or 15 years, are now more vulnerable to catastrophic fire because the forests are overgrown.
Intense fires kill big trees
High-intensity fires generally kill the big trees, which Stephens said is why forest thinning, prescribed burns, chipping of excess wood and other forest management techniques are necessary.
“All those things would reduce tree mortality in wildfires,” he said. “It is something that could probably reduce mortality in fires by 50 percent.”
The problem is that thinning is difficult in places where there are towns, utility corridors, highways and timber interests blocking the way, and tree thinning and brush-clearing operations are expensive.
“We end up in court a lot on these projects because, lets face it, people don’t like chainsaws,” said Hugh Safford, a forest service vegetation and fire ecologist.
As it is, Stephens fears the 350- to 500-year-old Douglas fir, sugar and ponderosa pine trees he documented in the research area were incinerated.
Logging, fire suppression and a lack of management over the years all played a role in the Rim Fire, experts say, but there are several other factors that contributed to the disaster, including strong winds and extremely dry conditions.