Key takeaways
- From 1985-2024, high-severity forest fire increased thirtyfold in California, outpacing restorative fire, a UCLA study found.
- Increases in fire severity correlate with drying trends linked to climate change, especially in densely forested areas.
- Severe fires kill trees instead of giving them a chance to recover, changing the face of California's landscape.
Forest fires now burn ten times more acreage annually than in 1985, while wildfire severity has gotten even worse. In California, 30 times more acreage burned from high-severity, forest-killing fires, according to new UCLA research.
In the 1980s and 1990s, California's forest fires burned mostly at low or moderate severity, generally benefiting ecosystems. But as fires have grown in size, severe fires causing widespread tree death have overtaken beneficial fire as the most common fire type in California's forests.
The study, published June 22 in the journal PNAS, found that the changes are tied to the increasingly warm and arid environment. These aridity-driven changes were also stronger in more densely forested areas, said UCLA bioclimatologist and senior author Park Williams.
"These high-severity, forest-replacing fires used to be uncommon, and now it's the dominant fire type," said Williams, a UCLA professor of geography. The research showed that the yearly forest area burned by severe fires increased at a much faster rate than the area burned by beneficial, lower-severity fire from 1985 to 2024, even as both fire types increased. "Fire is a natural process that can be healthy for ecosystems, but most of California's tree species have a difficult time coming back following fires that kill huge swaths of trees."
The biggest fire year in terms of forest area burned in the Western U.S. in modern history was 2020, and then 2021 quickly became the second biggest year on record, Williams said. CalFire data showed that 8 of the 10 largest fires to take place in California's last 100 years all happened within the last 10 years.
Low-severity blazes started to be outnumbered in California in 2012, said lead author Mitchell Hung, an earth-systems researcher who completed the fire study as a graduate student at UCLA.
"From 2012 to 2024, high-severity fire beat out low-severity fire every single year," said Hung, now a doctoral student at Stanford. "There's a clear trend."
Burn severity has quantitative measures, such as how much tree canopy burns away, or how badly the soil chars. To Hung, it also has a telltale look that he associates with painful trips through Yosemite National Park.
"I've driven through Yosemite a fair number of times, where often I'll round a bend and see this huge field of burned snags," Hung said. "Post-fire snags are the remnants of trees after everything has burned away but the trunk. They're like tree gravestones."
Why this is happening
The researchers found two main causes for the increase in fire severity. The first was fuel density. The UCLA study showed that severe burning increased the most rapidly in forests with the highest biomass density — that is, the densest plant life, with understories often full of highly flammable brush. This suggests fire prevention efforts, which have allowed many of California's forest areas to accumulate high fuel densities, have contributed to the current problem, the researchers observed in the paper.
"We all remember Smokey Bear — 'Only you can prevent forest fires'," said Hung. "That had an inadvertently damaging effect on these ecosystems, which evolved alongside more frequent, less severe fires to keep them healthy. Preventing a fire often just prevents it in the short term, but kicks the can to a later point when there's even more fuel to burn."
With the increase in fire severity causing extensive tree death rather than benefiting the forests, California's forest ecosystems are vulnerable to shifting toward grass and shrub plant types. When severe fires open up massive gaps in a forest and bake the soils, the nearest living seed source is often too far away for a new generation of trees to quickly regrow, Williams said.
"The shift in fire types means land cover is changing," Williams said. "Are we going to get rapid regeneration of dense forests, or will we go decades or longer without forests returning? Changes in vegetation cover will then circle back to affect future fire. A shift from forest to grass would alter how tightly connected future fires are to droughts, heat waves or wind events. If we want to understand how fire will continue to change in the future, we need to understand how fire is changing vegetation ecosystems."
As forests burn, the state also loses the benefits that forests provide, such as cleaning the air, regulating the climate, supporting water management by reducing storm run-off and flooding, and even economic benefits like the timber and tourism industries, Hung said. In contrast, high-severity fires generate massive amounts of air pollution and increase flood risk.
"The loss of these forests isn't just, 'I can't take a pretty picture,'" Hung said. "There are profound socioeconomic impacts. Real dollars are being lost each year due to high-severity forest fire. Studies suggest the loss of forests will put more stress on water management, which has already come under strain in recent years from prolonged drought."
Environmental dryness is the second driver of increasingly severe fires, the researchers found. A measure called the vapor-pressure deficit measures the gap between how much moisture the air can hold versus how much moisture it has. The bigger the deficit, the drier the air, and the more water the air can absorb from plants and soil.
"Climate change has made the atmosphere warmer, and a warmer atmosphere has a larger water-holding capacity," Hung said. "When the weather is hot and dry, the vapor-pressure deficit is high, which leads the atmosphere to act like a sponge, soaking up surface water. In general, the warmer and drier the atmosphere, the more high-severity fire we saw over the last 40 years."
What can be done to help
The researcher's conclusions show that the state can make some headway in protecting California's forests with changes in forest management, such as doing more manual clearing of underbrush and conducting more prescribed burns, Hung said.
"Among the largest drivers of burn severity are the warming and drying of the atmosphere, which no amount of forest thinning can change," Hung said. "But even though forest management alone can't solve the problem statewide, for individual locations, making good forest management decisions can help alleviate the risk of high-severity fire."
The research was supported in part by the MacArthur Foundation, the Moore Foundation, USGS, the National Park Service, and the UCLA Sustainable LA Grand Challenge. This paper is a contribution of the Western Fire and Forest Resilience Collaborative .
