Using a mobile laboratory, researchers analyzed smoke during the fire and looked for radiocarbon signatures associated with large fuel sources. Old, large-diameter fuels like fallen logs drove the conflagration, they write, causing it to burn at a higher intensity than other fuel sources such as pine needles or leaf litter.

“We’re really in a situation that’s a consequence of both management strategies and climate warming since European-American settlement began in California,” Audrey Odwuor, a PhD candidate at the University of California at Irvine’s Department of Earth System Science and the study’s lead author, said in a news release. “These fuels are building up on the forest floor over periods of decades, which is not typically how these forests were maintained.”

The researchers believe the large-diameter fuel produced longer flames and caused the fire to jump to the forest canopy. Though sequoia trees have largely evolved to coexist with fire, their uppermost layers lack the tough bark that protects them from flames near the ground. Some of the damaged trees are thought to have been thousands of years old.

The research also revealed elevated levels of particulate matter with a diameter of 2.5 microns or less in the smoke. Particles of this size are so small they are easily inhaled and have been linked to health problems such as lung cancer and even premature death.

As human-caused climate change progresses, wildfire severity is expected to rise. The research could inform fire management strategies, especially controlled burns that could eventually lower the intensity of the region’s wildfires, the scientists write.