Forest exhibits resilience after California mega fire

UNIVERSITY PARK, Pa. — In 2019 and again in 2021, Penn State researchers in the Department of Geography walked a series of 1,000 square foot plots in California’s Lassen Volcanic National Park. The goal was to see how the forest that’s hands-off to residents and most fire-suppression measures had recovered — or not — from naturally occurring forest fires as far back as the mid-1980s.

As the team examined the stratified sample of plots in 2021, their field campaign went up in smoke — literally — with the Dixie fire, a nearly 1-million-acre blaze that became the second largest in California history. In the 104 days before it was contained, the fire ravaged five counties, including the entire swath of land researchers were canvassing.

It was devastating to the forest. But Alan Taylor, a professor emeritus of geography who has been researching California forests since the late 1980s, saw an opportunity. The results of that opportunity were published in Forest Ecology and Management.

“The Dixie Fire rolled through and just blew our research project up,” Taylor said. “But it then gave us a chance to see how these massive and ever-increasing mega fires are impacting the forest. We wanted to know if and how the forest would recover after this extraordinary event.”

The team returned in 2022 with a fresh research objective: to see how the Northern California landscape responded to extreme wildfire, and study how prior fires influenced chances of forest recovery. That research, detailed in the team’s recent paper, found that despite more than 40% of the Dixie fire burning at high severity — meaning a total loss of the forest canopy — the forest showed signs of regeneration.

Researchers examined post-fire survival of tree regeneration, understory and ground fuel conditions such as downed logs and vegetation that they had measured the year before the Dixie fire. Just after the Dixie fire, 32% of the plots had at least one seedling, and about half were considered “stocked,” a label the U.S. Forest Service applies when a plot is deemed likely to make a full recovery. Of all seedlings in the plots in 2021, 19% survived the Dixie fire.

The researchers developed statistical models to identify which factors contributed to seedling persistence. The analysis indicated that plots on steeper and wetter sites, farther from forest edges, with lower ground cover, and burned twice by previous fires rather than once before the Dixie fire stood a stronger chance of having seedlings survive the Dixie fire. Moreover, regeneration was less likely to persist where more ground cover was consumed by the Dixie fire because that causes fires to burn hotter.

“What we’re seeing in California are areas that haven’t burned in as much as 100 years because of fire suppression practices beginning to burn,” Taylor said. “That’s resulted in massive amounts of fuel for these fires.”

U.S. Forest Service was founded in 1905 and almost immediately began fire suppression measures, according to Forest History Society. Since the 1970s, policies have shifted in response to the positive role fires play in forest ecology and reducing fuel. Still, about half the forest service’s budget goes to fire suppression.

“In the past few decades, there has been a large increase in areas burned by wildfires, and these wildfires are now burning over areas that burned before,” Taylor said. “Factors contributing to the persistence of forest regeneration in this overlapping mosaic of reburns is poorly known, and this research provides insights on where forest management activities could be focused to improve persistence of regeneration through reburns and new regeneration after fires.”

Despite the extreme temperatures exhibited during the Dixie fire, researchers found only about half the fire’s path resulted in a total loss of tree canopy.

“We’re seeing more high severity fires,” said Lucas Harris, a postdoctoral scholar at the University of Vermont and a co-author who worked on this research as a postdoctoral scholar at Penn State. “It’s a really important question whether forests are able to recover from these fires, and this research shows in many areas that they are.”

Dani Niziolek, now a data scientist at the Great Basin Institute working with the National Park Service, surveyed plots following the Dixie fire while earning her master’s degree in geography at Penn State. She expected to find a total loss of regeneration and said she was surprised at the resilience of the forest.

“It was great to visit these plots and to see not only was there still something intact, but in fact, elements of forest regeneration had persisted through this event,” said Niziolek, a lead author on the research.

Niziolek said the research offers new tools for forest managers by showing areas with the greatest chance for regrowth. It also highlighted how lessening fuels at forest edges and in large areas of high severity fire where fire-killed trees have fallen to the forest floor can lead to greater rates of forest recovery.

“When you have a landscape in which you’re concerned about mega fire, which have become more common, the use of smaller scale, controlled fires under moderate weather conditions could offer very real protective benefits when you inevitably do have a larger scale disturbance,” Niziolek said.

This research was funded by the National Parks Service and Penn State.