UC DAVIS: Lessons from the Caldor Fire’s Christmas Valley ‘Miracle’

In what came to be called the “Christmas Valley miracle,” the Lake Tahoe Basin communities of Christmas Valley and Meyers were spared in late August 2021 when the massive Caldor Fire entered the basin, burning more than 222,000 acres and forcing roughly 30,000 people to evacuate during one of the hottest, driest summers on record. Outside of the Lake Tahoe Basin, the fire destroyed over 1,000 structures, many of them homes.

Decades of fuel-reduction treatments conducted by federal, state and local land managers to protect people’s communities well before the fire are widely credited for the “miracle.” But forest ecologists from the University of California, Davis, wanted to understand which treatments worked best to protect the forest. Such questions are increasingly important for the Lake Tahoe Basin, which is one of the most wildfire-threatened landscapes in the West.

Their study, published in the journal Forest Ecology and Management, produced three key findings:

1. Trees in treated areas were three times more likely to survive fire than trees in untreated sites.
2. The most effective fuel treatment was mechanical- and hand-thinning areas across multiple years, followed by mastication, which uses heavy machinery to grind trees into chips and mulch.
3. Hand thinning and piling the leftover fuels — if the piles were then burned — was also effective. However, unburned fuel piles led to higher fire severity and tree mortality than untreated sites.

“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 in the Environmental Science and Policy department. “The presence of thousands of fuel piles in the Caldor Fire caused higher fire severity than we expected to see.”

Unburned fuel piles a problem

Safford, whose home in Meyers was within a few hundred feet of the Caldor Fire, observed a similar issue during the region’s 2007 Angora Fire, but the extent of fuel piles across the landscape in 2021 was much greater than in 2007. Lake Tahoe Basin Management Unit staff suggested in 2021 that as many as 250,000 unburned piles were present on U.S. Forest Service lands in the basin. Thinning is not enough if unburned fuel piles remain.

“You’ve got to sweep up. If you’re finished cooking, you have to clean the kitchen,” said Safford, acknowledging barriers to that clean-up work. “The Forest Service has always had major capacity and funding issues, and these have only gotten worse since the change in administration in D.C.”

The researchers were surprised that the most successful treatment types in the Caldor Fire did not include controlled fire. Safford said prescribed burns are highly effective in cleaning surface fuels, but the success of non-fire treatments warrants a deeper look.

“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 co-author Saba Saberi, a doctoral student in Safford’s lab 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.”

Fires in the Lake Tahoe Basin

More than a quarter of Californians live in the wildland-urban interface, where threats of wildfire are high. Nine of the 10 most destructive wildfires in U.S. history have happened in the state, with insured losses totaling in the tens of billions of dollars.

Large forest fires were relatively uncommon in the Lake Tahoe Basin for much of the 20th century. That changed in the 2000s with the arrival of the Gondola and Showers Fires in 2002, and the destructive Angora Fire in 2007. In 2021, the Tamarack Fire threatened the basin from the south in July, quickly followed by the Caldor Fire a month later. The Caldor was the second recorded wildfire to burn across the Sierra Nevada crest, preceded only by the Dixie Fire, which crossed that threshold only a day earlier.

The study was funded by the Southern Nevada Public Lands Management Act, the League to Save Lake Tahoe, The Tahoe Fund, UC Davis and U.S. Forest Service Region 5.