Five miles down a bumpy dirt road in Plumas National Forest, at the northern end of the Sierra Nevada mountain range, lies a paradox. On one side of the road, a scorched wasteland of burnt toothpicks covers the Earth, a stark reminder of the devastating Dixie Fire that swept through this area four years ago. Not a single living tree remains. But a few hundred feet away, a stand of hardy, healthy ponderosa pines shoot into the sky, their branches reaching toward the sun. Small burn marks on the trunks of these trees are the only evidence of that brutal fire, which devoured nearly 1 million acres across five counties in 2021, making it California’s largest single wildfire to date.

The most commonly used methods to protect forests against megafires today include either thinning the trees or setting prescribed burns, and sometimes both. But some have begun to question whether controlled burns are effective during increasingly intense droughts, given the risks that such burns could spiral out of control. And while plenty of research has looked at how thinning and burns influence fire severity, few efforts have studied how these treatments affect the longer-term survival of individual groves of trees.

When the researchers analyzed the data from their plots, what they found is that thinning trees alone is not much better than a total lack of forest management. This is possibly due to the fact that some methods of tree thinning actually can increase fire risk because they leave behind dry branches that serve as tinder. But prescribed burns clear away that fuel, making a combination of thinning and burning the winning approach, the researchers found. It didn’t matter if the combination of treatments had been tried five years or 20 years prior. When thinning was followed by burning, the researchers saw much lower levels of both tree mortality and canopy torching. To save trees, it appears, you have to both cut them and burn them.