Two devastating wildfires in California’s Sierra Nevada mountains in 2020 and 2021 killed thousands of giant sequoia trees. A new study from the University of California, Davis suggests that losses would have been even worse without prescribed burns.
Published in nature communicationsa study found that giant sequoias exposed to prescribed fire in the past 10 years were nearly four times more likely to survive the 2020 Castle Fire and 2021 KNP Complex Fire than nearby untreated trees.
“Prescribed burns are effective in reducing mortality in giant sequoias, even in today’s extreme wildfires,” said lead author Dan Dixon, who was a postdoctoral fellow at the University of California, Davis, and a research associate with professor Yufan Jin at the time the study was conducted. “If the goal is to protect giant sequoias, prescribed fire is an essential tool available to managers.”
ancient giant made for fire
Giant sequoias are one of the largest and longest-living creatures on Earth. A single tree can weigh as much as 15 blue whales and can survive for over 3,000 years. In the wild, it grows only on the western slopes of the Sierra Nevada Mountains in California.
These giant trees evolved in landscapes formed by periodic low-intensity fires. Thick bark and a raised canopy provide protection, and small fires clear vegetation, recycle nutrients, and create conditions conducive to the growth of new sequoias.
Therefore, many people once thought that mature sequoias were almost fireproof. That perception changed dramatically after the 2020 and 2021 fire seasons, when images showed entire groves filled with burnt remains of trees that had survived for centuries.
“These trees are very long-lived, they’ve experienced climate changes, wildfire differences, droughts, they’ve been through it all,” Dixon said. “It was unprecedented for so many people to die in just two years.”
Naturally tested by decades of prescribed combustion
Sequoia National Park and Kings Canyon National Park have maintained some of the most extensive prescribed fire programs in the country since the 1960s, according to the study. Over time, this created a patchwork of groves with very different fire histories.
Some sections were relatively recently treated with controlled burns. Other areas had not seen a fire in more than 100 years, so dead trees, shrubs, and other combustible materials had accumulated.
This contrast gave researchers a unique opportunity to measure how previously prescribed burns affected giant sequoia survival during extreme wildfires.
Scientists from the University of California, Davis, the U.S. Geological Survey, and Sequoia and Kings Canyon National Parks combined several types of remote sensing data. They used aerial lidar, which measures forest structure with laser pulses, and high-resolution PlanetScope satellite imagery.
The research team examined pre-fire fuel conditions and estimated the impact on approximately 26,400 giant sequoia trees in 19 groves affected by the Castle and KNP complex wildfires.
Prescribed fires reduce the risk of death
The results showed surprising survival benefits. Prescribed burns performed within the past 10 years have reduced the chance of giant sequoia death by about 77%.
Trees in treated areas were about four times more likely to survive than trees in untreated parts of the same grove.
Computer simulations suggest that pre-prescribed burns could have prevented about 1,900 giant sequoia deaths. The researchers also estimated that if all the sequoias in the affected orchards had received similar treatment, an additional 3,900 trees might have survived.
An important tool for sequoia conservation
Climate change is increasing the frequency and severity of disturbances affecting native forests, such as drought and extreme wildfires. Giant sequoias are naturally resistant to common fires, but studies have shown that even this highly durable tree becomes vulnerable when the flames get intense enough.
The findings provide strong evidence that prescribed burning reduces the amount of fuel available and helps protect redwood forests during large wildfires.
“This is a timely study given the history of fire suppression in these forests and the increased pace and scale of state fuel thinning in recent years,” Jin said. “By combining scalable, high-resolution remote sensing with a rigorous statistical framework, we were able to uncover insights into the impacts of prescribed fire that go far beyond what can be gained through traditional anecdotes or field surveys.”
Additional co-authors of the study are Xiaoli Dong and Andrew Latimer of the University of California, Davis, Adrian Das, David Soderbergh, and Nathan Stevenson of the USGS Western Ecology Research Center, and Anthony Caprio of the Sequoia and Kings Canyon National Parks Division of Resource Management Sciences.
This research was funded by the NASA Land Cover and Land Use Change Program, the California Strategic Growth Council, and the USDA National Institute of Food and Agriculture.
