SACRAMENTO, Calif. — Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the explosive conflagration chewing up the rugged landscape on the edge of Yosemite National Park, forestry experts say.
The fire had ravaged 282 square miles by Tuesday, the biggest in the Sierra’s recorded history and one of the largest on record in California.
Containment increased to 20 percent, but the number of destroyed structures rose to 101, and some 4,500 structures remained threatened. The types of lost buildings were not specified. Firefighters were making stands at Tuolumne City and other mountain communities.
The blaze was just 40 acres when it was discovered near a road in Stanislaus National Forest on Aug. 17, but firefighters had no chance of stopping it in the early days.
Fueled by thick forest floor vegetation in steep river canyons, it exploded to 10,000 acres 36 hours later, then to 54,000 acres and 105,620 acres within the next two days. On its 11th day, it had surpassed 179,400 acres, becoming the seventh-largest California wildfire in records dating to 1932.
Federal forest ecologists say that historic policies of fire suppression to protect Sierra timber interests left a century’s worth of fuel in the fire’s path.
“That’s called making the woodpile bigger,” said Hugh Safford, an ecologist with the U.S. Forest Service in California.
Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years, forest ecologists have warned that Western wildfires will only get worse.
“Every year the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious,” Safford said. “People can deny it all they want, but it’s happening. Every year the fuels are a little bit drier.”
The Rim Fire’s exponential growth slowed only after hitting areas that had burned in the past two decades, and Safford said that shows the utility of prescribed and natural burns that clear brush and allow wildfires to move rapidly without killing trees.
“If you look at the Sierra Nevada as a whole, by far the largest portion hasn’t seen a fire since the 1910s and 1920s, which is very unnatural,” said Safford, who has authored several papers on the increasing wildlife severity across California’s mountain ranges. “This one isn’t stopping for a while.”
Since a 1988 fire affected nearly one third of Yellowstone National Park, forestry officials have begun rethinking suppression policies. Yosemite has adopted an aggressive plan of prescribed burns while allowing backcountry fires caused by lightning strikes to burn unimpeded as long as they don’t threaten park facilities.
“Yosemite is one of the biggest experimental landscapes for prescribed fire, and it’s going to pay off,” Safford said. “The Rim Fire is starting to hit all those old fire scars.”