California's biggest wildfire was just a 40-acre canyon blaze west of Yosemite National Park on Aug. 17. Within just a few days, the Rim fire had grown into a monster, menacing foothill communities and incinerating the landscape with 200-foot flames burning at 1,200 degrees.
The still-growing Rim fire footprint is now more than two-thirds the size of Los Angeles and burning into state history one of the five biggest fires on record.
What happened to that 40-acre fire at the Rim of the World, a peaceful overlook at the Tuolumne River canyon in the Stanislaus National Forest? Federal scientists say the fire hit overdrive when it moved from the scar of an old fire into overgrown forest that had not burned in a century. The thick forest made the Rim fire historic.
By nightfall Monday, it had expanded more than 235,000 acres. Nearly 5,000 people still are involved in fighting the fire, which was 70 percent contained as of Monday, at a cost of $66 million and rising. It will be weeks before it is contained.
Experts already are saying the ecosystem damage is huge. In a terrifying whoosh, the Rim fire changed the forest for at least a generation, and maybe a lot longer.
As the climate changes and more fires take place, there is a chance that big, fire-resistant trees might not recover. Fields of flammable shrubs might be all that are left in some places, fire ecologists say. Further, they say much of the Sierra's 25 million acres is overgrown and primed for these kinds of large, hot wildfires.
Someday, many parts of the Sierra may resemble Southern California mountains, where evergreen shrublands dominate the landscape, said Hugh Safford, regional ecologist for the Forest Service's Pacific Southwest Region and research associate at the University of California at Davis. "If you've seen those mountains, you know how different they are from the Sierra," he said. "That's the kind of change I'm talking about."
The key is the increase of big fires, which has been documented as the climate has warmed in the past two decades, ecologists said. These blazes quickly torch acres of vegetation from the top of centuries-old pine and fir trees to the grasses, forbs and sedges below.
"A fire like this will kill a third to half of the forest it burns through," said Malcolm North, research scientist for the U.S. Forest Service and forest ecology professor at UC Davis. "It is devastating."
One reason Sierra forests became overgrown was a flawed federal policy. Many decades ago, before fire's role was understood, federal authorities snuffed out all fires. The practice interrupted the frequent, low-intensity fires that kept the forest healthy in the past.
Fire suppression must continue to protect communities such as Groveland in the Rim fire, but there are some hard facts to face, North said. Federal land managers are trying to thin the forest with controlled fires, as well as cutting and clearing heavily forested areas. But there's simply too much forest that needs thinning.
"We've gotten ourselves in a really big hole," North said. "We're not going to dig our way out the way we're doing it now."
The situation makes fire authorities nervous every summer, particularly now, after two dry winters. The Rim fire probably will burn until storms hit in autumn. Safford said scientists won't know the extent of the damage until they investigate after the smoke clears, but there is little doubt about what they'll find: "The Tuolumne River canyon will be fundamentally different over the next 50 years."
Will the trees come back at some point? They should eventually return, said Yosemite-area scientist Jan van Wagtendonk, research forester emeritus with the U.S. Geological Survey's Western Ecological Center. "But if another fire burns through, it goes right back to chaparral," he said. "That has happened already in some places."