To lessen the risk of catastrophic wildfires, California’s forests need more routine burns. This message has been echoed for years. Relentlessly putting out even small wildfires in the Golden State and other parts of the western U.S. has long deprived the regions of beneficial flames, resulting in the buildup of dense vegetation and dry leaf litter. The accumulation of this fuel, experts say, is creating a dramatic rise in megafires.

More ironic is new research that shows that massive fires do not necessarily reduce the incidence of big future blazes in the same places. In California alone, recent large fires have burned within the footprint of previous ones, including the 2012 Chips Fire, the 2013 Rim Fire and parts of the 2018 Camp Fire. In less than a decade, some of the burned expanses from this year’s megafires could burst into intense flames again. In some ecosystems, high-severity fire can beget more high-severity fire, furthering the loss of forest.

A typical wildfire is a patchy mix of low-severity areas—where flames cruise close to the ground, eating up pine needles, debris and smaller plants—and areas of high severity, where embers climb into the canopy, torching entire trees. Some islands of land are usually left unburned, too.

But the megafires of late have a larger proportion of high-severity areas. Susie Kocher, a forester at the University of California Cooperative Extension, says researchers estimate that prior to the advent of official policies going back 100 years, which dictate that all wildfires should be put out, only 5 to 10 percent of a typical blaze in California’s Sierra Nevada range would burn at high severity. Today, as forests have become dense with trees and fuels, that proportion is between 40 and 60 percent for fires that break out after initial containment efforts. In 2014 about half of the King Fire burned at high severity and killed all the conifers across a 40,000-acre area in the central Sierras east of Sacramento. “That’s way outside of what we think would have been natural,” Kocher says.

The extent of high-severity burn has important consequences. For one, because conifer seeds in pine cones only spread a short distance from their mother trees, it may take a long time, or even be impossible, for ponderosa pines and other evergreens to move back in. The majority of a large burned landscape could remain devoid of conifers for decades. The mixed-severity fires that burned for thousands of years spared more mothers, allowing the high-severity patches to reseed.

A high-severity fire also does not preclude big subsequent blazes. Ecologists who monitor field plots in the Sierra Nevada have observed that high-severity fires tend to produce more high-severity fires. When an intense blaze rips through most of the trees, it leaves behind a scorched and shadeless area. Shrubs easily sprout in the resulting surplus of sunlight. Remaining dead trees fall over. In the first few years, the shrubby expanse is not particularly flammable because young plants contain a lot of water. Over time, the bushes accumulate dead twigs and leaves, and desiccated branches fall from the scorched trees. If there is a spark around eight or more years later—lightning, a downed electrical wire, a fugitive campfire—the shrubs and fallen wood can sustain another large fire, providing a continuous blanket that can easily spread flames. “Shrubs naturally burn at higher severity,” says Michelle Coppoletta, a Forest Service ecologist who has documented this phenomenon. “You’re getting this perfect storm of fuel that can carry high-intensity fire.”

This process may have partly driven the 2018 Camp Fire, the disastrous conflagration that destroyed the town of Paradise, Calif., and killed dozens of people. The landscape where the fire started had also burned in the years previously, says Don Hankins, a pyrogeographer at California State University, Chico, and a Plains Miwok cultural practitioner. “That’s partly why the Camp Fire spread so quickly. It was moving through areas that had a lot of regrowth of shrubs and grass.”

Back-to-back fires can completely convert a landscape. The large, high-severity regions “often reset to chaparral [shrubland] or sometimes grassland,” says Wolfy Rougle, forest health watershed coordinator at the Butte County Resource Conservation District in Northern California. “So now you’ve got this high-severity patch that continues to burn that way.” The recurring fires eat away at neighboring forests, too.

A large-scale conversion also has implications for carbon storage and biodiversity. As Coppoletta puts it, the result is a homogenization of the landscape. What was at one time a mosaic of conifers, shrubs and meadows is comprised of just a few shrub species. Much of the biodiversity associated with the prior mosaic can be lost. Furthermore, “that chaparral or brush-field vegetation is not going to really build carbon over the long term the way the forest would because it tends to burn all at once,” Rougle says.

Fire-driven conversion is not unique to forests. Hankins points out that fire in California chaparral and oak woodlands can give invasive annual grasses a chance to move in. Nonnative annual grasses spring up quickly with winter rains but dry out by June. If there is a spark, “it’s always going to be ready to burn,” Hankins says. Meanwhile, although the oaks can tolerate some fire, he says, “they haven’t necessarily evolved to survive these [severe] fires and this frequency of fires. So we’re losing oak forest at the same time.”

Land managers have two primary tools to try to stop severe forest fires from recurring: thinning and prescribed fire. Thinning some dead and smaller live trees and other vegetation can reduce fuel loads. Carefully managed burns can prevent the growth of dense canopy and fuel buildup. But there is still debate about where intervention is feasible and which strategies to use, Coppoletta says.

Controlled fire can reverse the trend to severe fires in different ways. At the Big Chico Creek Ecological Reserve, Hankins has researched the effect of Indigenous prescribed burning practices on an area that was dominated by star thistle and other invasive plants. By timing fires early in the season, when the thistle was just popping up, burn crews were able to allow native bunchgrasses to return. Those grasses are perennial and deep-rooted. They stay green for longer and are thus less susceptible to carrying a fire. “We’ve been able to shift the system to be relatively free of some of these species that we don’t want in the landscape,” Hankins says. “That’s all through very carefully timed fires.”

Rougle is part of an effort to reduce the potential for future intense reburns within the Camp Fire footprint. “We’ve only got maybe three years to reburn this chaparral and brush that’s sprouting up to try and keep it from becoming 10-foot-tall brush,” she says. The researchers are not necessarily trying to restore the forest that stood before. Instead they want to bring back patchiness to the landscape, breaking up vegetation both for fire safety and biodiversity. “We want fire to give us a healthy mosaic of habitat types, with a lot of edges. We want it to look very messy—high-severity [fire] over here, low-severity [fire] over there,” Rougle says. This “pyrodiversity,” she says, helped create California’s tremendous biodiversity.

Officials are still mapping how much of the record-breaking four-million-plus acres that burned in California this year succumbed to severe fire. It looks like the August Complex Fire mostly burned at low severity, removing dry fuels but leaving trees intact. Others, such as the Creek Fire on the Sierras’ western slope, burned hot, fed by standing trees already dead from drought and insects.

“Ultimately, all of these lands are going to need management within anywhere from two to 10 years —and probably closer to two to five years after a fire—to maintain that reduction in fuel,” says Kate Wilkin, a fire scientist at San José State University. As climate change makes California hotter and drier, increasing the propensity for monstrous fires, this need will only grow.