In August 2021, the Caldor Fire entered the Lake Tahoe Basin, burning over 222,000 acres and forcing about 30,000 residents to evacuate. While more than 1,000 structures were destroyed outside the basin, the communities of Christmas Valley and Meyers were largely spared—a phenomenon many referred to as the “Christmas Valley miracle.”
Federal, state, and local land managers had spent decades conducting fuel-reduction treatments in these areas to protect communities from wildfire. Researchers from the University of California, Davis (UC Davis) sought to determine which specific treatments most effectively protected forests during this event. Their findings were published in Forest Ecology and Management.
The study identified three main results: trees in treated areas were three times more likely to survive fire than those in untreated sites; mechanical- and hand-thinning over multiple years was found to be the most effective treatment method, followed by mastication—where heavy machinery grinds trees into chips or mulch; and hand thinning with subsequent burning of leftover fuels was also successful. However, leaving unburned piles after thinning led to higher fire severity and tree mortality compared to doing nothing.
“Generally, the treatments were very successful at increasing the resistance of forest to wildfire, but the major hiccup at Lake Tahoe is the big backlog of unburned fuel piles,” said lead author Hugh Safford, a UC Davis forest ecologist. “The presence of thousands of fuel piles in the Caldor Fire caused higher fire severity than we expected to see.”
Safford noted that during both the Caldor Fire and earlier Angora Fire in 2007 he observed issues related to unburned fuel piles. According to staff from the Lake Tahoe Basin Management Unit, as many as 250,000 unburned piles remained on U.S. Forest Service lands within the basin during 2021. Thinning alone was not sufficient if these piles were left behind.
“You’ve got to sweep up. If you’re finished cooking, you have to clean the kitchen,” Safford said. He pointed out that limited capacity and funding for cleanup work has long been a challenge for agencies like the Forest Service.
Researchers also expressed surprise that controlled burns were not among the most effective treatments observed during this particular fire event. “As we see more extreme fire weather, some people wonder whether pre-fire treatments still matter, or does intense fire behavior override this work,” said coauthor Saba Saberi, a doctoral student at UC Davis. “But we see it clearly moderates fire effects. We saw three times the tree survival in areas that were treated. This was a notable effect. But if you didn’t burn those piles, the effects on the forest weren’t different from not doing the treatment at all.”
More than one quarter of California’s population lives in wildland-urban interface zones where wildfire risk is high; nine out of ten of America’s most destructive wildfires have occurred in California with insured losses reaching tens of billions of dollars.
Historically large fires had been rare around Lake Tahoe until events such as Gondola and Showers Fires (2002), Angora Fire (2007), Tamarack Fire (July 2021), Dixie Fire (August 2021), and Caldor Fire (August 2021). The Caldor was only second recorded wildfire crossing Sierra Nevada crest—preceded by Dixie just one day before.
The research received funding from several sources including Southern Nevada Public Lands Management Act funds; League To Save Lake Tahoe; The Tahoe Fund; UC Davis; and U.S. Forest Service Region 5.
