Sometimes even the best efforts to return a forest to a less fire-prone state go up in flames.
This proved to be the case when the Rim fire created an inferno in California's Yosemite National Park last year. A recently published analysis of Yosemite land burned during the record-breaking wildfire demonstrates that when blazes become extreme -- so large and hot that they create their own independent weather patterns -- they can easily destroy forests that have been restored, or thinned of underbrush, through recent controlled burns.
The U.S. Forest Service has long argued that management techniques like the removal of "fuel," such as brush and deadwood, by using controlled burns can significantly diminish extreme fire activity and protect forest habitats.
For example, fire crews noted that a recent blaze in Arizona's Apache-Sitgreaves National Forests was made significantly easier to control thanks to a tree-thinning project there. Not only that, but larger trees within the treated areas proved resilient to the flames and many appeared to have survived the incident (ClimateWire, July 2).
But by examining forest plots where Yosemite had allowed smaller fires to burn and consume fuels within the last 14 years -- a fire regime similar to what the forest experienced before the early 1900s -- researchers behind the new study show that when the Rim fire was at its most severe, the vast majority of trees in these restored plots were still killed. The research appears in the journal Forest Ecology and Management.
The researchers stressed that the Rim fire did not start within Yosemite's restored forests but on adjacent national forest property, and that its rapid spread was in large part due to decades of fire suppression and fuel buildup outside the national park.
"We basically have artificially constrained fires to burn under very, very extreme conditions, and under those extreme conditions, fires generally will be able to overwhelm restoration efforts," said study co-author Brandon Collins, a researcher with the USDA Forest Service's Pacific Southwest Research Station in Davis, Calif.
"To me what it spells out is that we need to be doing fuel reduction efforts at much larger scales than we currently are," Collins added. "We shouldn't be relying on parks to continue their burning programs -- it means we have a bigger problem on the landscape."
A 'legacy of fire exclusion'
The 2013 Rim fire was the third-largest in California's recorded history, consuming over 257,000 acres in the Sierra Nevada. Before it was stopped, it burned through 75,000 acres in Yosemite National Park.
Even before the release of the new study, Yosemite fire ecologists blamed the fire's rapid, destructive growth on both long-term drought and the decades of fire suppression and fuel buildup that have taken place in California's forests (Greenwire, Sept. 13, 2013).
But because the Rim fire gained momentum in untreated, overgrown forests, as well as in forests where logging activity has only left behind a dense population of young, small trees, Yosemite's treated lands were still vulnerable to the intense heat and high winds that resulted.
The authors analyzed before-and-after satellite data on a series of 20-by-50-meter plots in Yosemite's forests. These plots had experienced at least two fires since 1953 before the Rim fire arrived. The researchers' goal was to discover which among the many factors that influence fire behavior, such as brush cover or years since the last fire, was the most important driver of the fire's severity.
Severity, as they defined it, is a measure of how much vegetation was destroyed by the blaze -- a low-severity fire would result in some green remaining, while a high-severity fire would leave behind an entirely charred landscape.
When the Rim fire was less intense, the time since the last fire and the amount of brush cover in the forest did have an important influence on the blaze's severity. But researchers also found that on days that the Rim fire was "plume dominated," meaning that the fire was creating its own weather patterns, the severity was very high. Collins estimates that in plots burned on these days, 90 to 95 percent of the plots' large trees were killed.
"Despite having a recently restored fire regime -- and I mean recent within the last 30 years or so -- these forests are still impacted by the legacy of fire exclusion in the 80 years that preceded it," Collins said.
Low snowpack, high fuels lead to worsening fire situation
According to Max Moritz, a wildfire specialist at the University of California, Berkeley, Cooperative Extension who was not involved in the study, the Rim fire allowed the researchers an unusual opportunity to examine the impact of controlled burns on forest health during a later wildfire. Yosemite, Moritz said, "has some of the few landscapes anywhere, really, where we have anything close to a natural fire regime operating."
"That's one of the reasons this is an interesting study with possibly important findings," Moritz added.
The study includes a common warning about climate change: "Given that spring snowpack in the Sierra Nevada has been decreasing, a trend that is forecast to continue due to temperature rise, along with the growing frequency of extreme fire weather, fires of this size and severity may not be rare events in the future." This means that even more forests could be affected by extreme blazes in the future.
But the researchers said climate change is no reason to give up on efforts to reduce fire risk.
"The theory goes that wildfires are a combination of weather, topography and fuels, and we can't really do anything about weather and topography," said lead author Jamie Lydersen, also of the Forest Service's Pacific Southwest Research Station. "Fuels are the thing you have some control over to some extent."
New fire burns near Yosemite
Wildfires have returned to Yosemite's neighborhood this year, but at this point, the blazes have remained much smaller in size than the Rim fire.
The El Portal fire burned 4,689 acres in Yosemite in late July. And on Monday afternoon, the Junction fire erupted in Madera County, where part of Yosemite is located, burning through 1,200 acres by yesterday. However, the burn area remained 15 to 20 miles away from Yosemite's entrance and therefore was not considered a threat to the national park, said Gary Wuchner, a fire information and education specialist for Yosemite.
A post on the Madera County Sheriff's Office's Facebook page yesterday stated that eight structures in the county had been damaged so far, while more than 500 homes were threatened and more than 13,000 evacuation orders were issued through an emergency notification system. Two shelters were made available for evacuees.
As of yesterday, crews had not yet been able to establish any containment lines around the fire area. According to the California Department of Forestry and Fire Protection, extreme fire conditions enhanced by the current record-breaking drought have made firefighters' task especially difficult.
More than 80 percent of California -- and 100 percent of Madera County -- is currently in extreme drought, according to the most recent U.S. Drought Monitor. So far this year, over 273,000 acres in California has burned. Last year at this time, the total was 211,000 acres.
However, these numbers don't yet approach California's 2008 fire season, which saw about 1.4 million acres go up in flames. That fiscal year, the Department of Forestry and Fire Protection spent $524 million from its emergency fund, compared with $147 million so far this year.
Nationally, the fire season has remained below average. Compared with the 10-year average of nearly 5.4 million acres, more than 2.6 million acres has burned this year, according to the National Interagency Fire Center.