After surpassing 400,000 acres on Thursday, the Bootleg Fire in southern Oregon remained not only the largest active fire in the nation, but it also became the state’s third largest megafire officially recorded since 1900, according to an inventory kept by the Oregon Department of Forestry.

The growth of the fire was spectacular. It began July 6 and quickly quadrupled in size, then doubled on back-to-back days, chewing through 143,000 acres by July 11.

It increased nearly threefold after that, etching its name higher in the record books.

“A slow day for this fire was 10,000 acres,” said Marcus Kauffman, a spokesman for the Oregon Department of Forestry.

But as big as this raging fire is – and it is a leviathan – fire research suggests there were much larger wildfires before the historical record was established in Oregon, such as the Silverton Fire in 1865 that reportedly hit up to 1 million acres.

The Oregon Department of Forestry, for its part, acknowledges it does not keep records before 1900.

Indeed, some of that historical fire research indicates that megafires were frequent occurrences in the ponderosa pine and mixed conifer stands where the Bootleg Fire started in the Fremont-Winema National Forest.

The difference this time, and a cause for significant concern as climate change increases the frequency of drought and extreme weather events like last month’s heat wave, is the severity of the fire and the ecological disaster it portends.

Keala Hagmann, an ecologist associated with the University of Washington, and Andrew Merschel, a doctoral candidate at Oregon State University’s College of Forestry, were able to reconstruct a precise historical record of fires in a 150,000-acre study area of the Fremont-Winema region back to 1650, and in some areas date fires back to the 1300s.

How did they do that?

Just over a decade ago, Hagmann found 100-year-old timber inventory records covering almost a million acres of the landscape at the National Archives and Records Administration in Seattle. Recognizing that they had rare post-fire tree survival data from earlier fires, the duo set out to determine how fire behaved before the U.S. Forest Service, the State of Oregon and local residents adopted the aggressive fire suppression tactics that have characterized the last century.

By taking cross sections of stumps and logs over a 150,000-acre portion of the Fremont-Winema, they were able to identify fire scars and drought signatures and date them precisely. Replicating the same process over the broad landscape, they were also able to estimate the size of those individual fires within the study area, though many of the conflagrations extended beyond the study boundaries.

The upshot: Prior to the last 100 years, fires of comparable size to the Bootleg have occurred every 15 to 25 years in that ecosystem, typically coinciding with periods of drought, Merschel said.

One of the more recent historical examples of a large blaze in that area came in 1918, a year of severe drought in Oregon, when fire burned some 200,000 acres of the forest, according to the researchers.

That fire is not on the state’s list but appears in an annual report from the Department of the Interior for what was then the Klamath Indian Reservation.

Jim Gersbach, a spokesman for the forestry department, expressed surprise that a large fire post-1900 would be missing from the state’s list but noted the difficulty in tracking the size of some historic fires with full accuracy.

That fire was almost 20 years before the Forest Service adopted its 10 a.m. rule, which decreed that every fire should be suppressed by 10 a.m. the day following its initial report. Forest ecologists now believe that policy, coupled with logging and grazing practices over the subsequent century, interrupted the natural and healthy cycle of fire in forests throughout the west, leaving a tinderbox of fuels kindling the severe fires being seen today.

There were also big fires on the Fremont-Winema, according to the researchers, in 1741, 1773, 1783, 1795, 1822, 1829 ... and on.

Based on the evidence of those fires, combined with the inventory of standing timber in the early 20th century, they determined that those fires were generally low intensity, and left behind an open forest structure with large, older and fire-resistant trees spread ubiquitously across the landscape.

Flash forward to the present.

Early severity maps from the Bootleg Fire suggest it burned with high intensity, torching vast swaths of trees as it moved across the landscape. Recovery of those ponderosa pine forests could take centuries, Merschel said, if it’s even possible under today’s conditions.

“Seeing this fire today, we have an iron-clad case that the severity of fires is much different than what it was 100 years ago,” said Merschel, who is pursuing his doctorate in forest ecology.

“It’s been really alarming that all of this burned up in the course of two weeks,” he added. “What is shows is that we’re really running out of time to restore these dry forests. We’re running out of time to get this right.”

“Restoring” the forests, however, remains a controversial topic. Lawmakers in Oregon just passed a bill that makes a tiny down payment on the vast backlog of thinning and prescribed fire projects they suggest are necessary to reduce risks in the most fire-prone forests in the state. The need is vast, they suggest, at as much as 5.6 million acres and a cost of $4 billion over 20 years. The federal government, meanwhile, which owns some 60 percent of Oregon’s forests, has done little to address the problem.

Conservation advocates, meanwhile, maintain that such projects are generally ineffective, as the incidence of wildfires encountering areas that have been treated is low; those areas still burn with high intensity when fire intrudes, and the carbon loss associated with commercial thinning and post-fire salvage logging is counterproductive.

Either way, severe fires are hardly unprecedented, though the underpinnings vary according to available fuels, topography and weather.

Take, for example, the first of the succession of fires known as the Tillamook Burn, in 1933. Driven by heavy east winds, that fire reportedly burned through more than 200,000 acres in one 20-hour period. Similarly extraordinary wind conditions during last year’s Labor Day fires consumed almost a million acres in a few days in areas on the west side of the Cascades, where fire is infrequent, leading to the most devastating season on record.

Those were wind driven fires, experts say. The Bootleg, while it saw some big winds, was primarily fuel-driven.

“There’s only one way to get a 40,000-foot pyrocumulus cloud,” said John Bailey, a professor in OSU’s College of Forestry, referring to the massive columns of smoke that rose over the Bootleg Fire, then collapsed, stoking the fire with massive downdrafts.

“It requires fuel, and fuel that is ready to burn,” he said. “It’s like going from a campfire to a bonfire.”

The biggest signal the Bootleg Fire is sending, Bailey agrees, is how land management patterns and historical fire suppression efforts have changed forest conditions and fire dynamics, and how much risk that poses during an era of climate change.

“I was a fire suppressor back in the '80s,” he said. “We thought it was the right thing to be doing to save the forest, and we were wrong. And we’re paying the price now.”