When two major wildfires burned through the Sierra Nevada in 2020 and 2021, they took thousands of giant sequoia trees with them. Without prescribed burns, thousands more would have died, according to a new study from the University of California, Davis.
The study, published in the journal Nature Communications, found that giant sequoias treated with prescribed fire in the previous decade were nearly four times more likely to survive 2020's Castle Fire and 2021's KNP Complex Fire than untreated trees within the same groves.
"Prescribed burns are effective at reducing giant sequoia mortality, even for today's extreme wildfires," said lead author Dan Dixon, who was a UC Davis postdoctoral researcher in Professor Yufang Jin's lab when the study was conducted. "If the goal is protecting giant sequoias, prescribed fire is an essential tool available to managers."
About giant sequoias
As their name implies, giant sequoias are massive. Weighing as much as 15 blue whales, these towering, iconic trees can live for more than 3,000 years. They naturally occur exclusively along the western slopes of California's Sierra Nevada.
With thick bark and tall canopies, periodic low-intensity fire is a natural and essential part of what they need to thrive. But images of torched skeletons of the beloved trees during the late summers of 2020 and 2021 astonished many who considered them practically fireproof.
"These trees live for so long and have experienced so much variability of climate, differences in wildfire, drought - they've seen it all," Dixon said. "Having so many die in just two years was unprecedented."
Mapping sequoia mortality from space
Sequoia and Kings Canyon National Parks have operated one of the most extensive prescribed burn programs in the United States, treating sequoia groves since the 1960s, the study said. This program created a mosaic of fire histories across the landscape, with some areas treated with prescribed fire and others that had not burned in more than a century.
In collaboration with researchers at the U.S. Geological Survey and Sequoia and Kings Canyon National Parks, the authors sought to understand the influence of prescribed fire on giant sequoias, which are particularly well-mapped in the parks. They integrated remote sensing data from airborne lidar and high-resolution imagery from PlanetScope satellites to quantify pre-fire fuel structures and to estimate wildfire outcomes for about 26,400 giant sequoias across 19 groves following the Castle and KNP Complex wildfires.
Their analysis showed that prescribed burns conducted within the previous decade reduced the odds of death by about 77%. Trees receiving prescribed burn treatments were four times more likely to survive than untreated trees. Simulations suggested that previous prescribed burns prevented about 1,900 sequoia deaths. Had every tree in the groves been treated, an additional 3,900 would likely have survived, the study found.
Prescribed fire: A giant sequoia conservation tool
The study notes that climate change is increasing the frequency and intensity of disturbances like wildfire in old-growth forests. Even fire-resistant giant sequoias are now vulnerable. The findings demonstrate that prescribed fire is an effective tool for their conservation.
"This is a timely study given the history of fire suppression in these forests and the state's increasing pace and scale of fuel thinning in recent years," Jin said. "By combining scalable, high-resolution remote sensing with a rigorous statistical framework, we were able to reveal insights on the effects of prescribed fires that go well beyond what traditional anecdotes or field surveys can tell us."
The study's additional co-authors are Xiaoli Dong and Andrew Latimer of UC Davis, Adrian Das, David Soderberg and Nathan Stephenson of the USGS' Western Ecological Research Center, and Anthony Caprio of Division of Resources Management and Science at Sequoia and Kings Canyon National Parks.
The study was funded by the NASA Land Cover and Land Use Change Program, California Strategic Growth Council, and the USDA National Institute of Food and Agriculture.
