KYBURZ, Calif. (AP) — On a steep mountainside where walls of flames torched the forest on their way toward Lake Tahoe in 2021, blackened trees stand in silhouette against a gray sky.

“If you can find a live tree, point to it,” Hugh Safford, an environmental science and policy researcher at the University of California, Davis, said touring damage from the Caldor Fire, one of the past decade's many massive blazes.

Dead pines, firs, and cedars stretch as far as the eye can see. Fire burned so hot that soil was still barren in places more than a year later. Granite boulders were charred and flaked from the inferno. Long, narrow indentations marked the graves of fallen logs that vanished in smoke.

Damage in this area of Eldorado National Forest could be permanent — part of a troubling pattern that threatens a defining characteristic of the Sierra Nevada range John Muir once called a “waving sea of evergreens."

Forest like this is disappearing as increasingly intense fires alter landscapes around the planet, threatening wildlife, jeopardizing efforts to capture climate-warming carbon and harming water supplies, according to scientific studies.

A combination of factors is to blame in the U.S. West: A century of firefighting, elimination of Indigenous burning, logging of large fire-resistant trees, and other management practices that allowed small trees, undergrowth and deadwood to choke forests.

Drought has killed hundreds of millions of conifers or made them susceptible to disease and pests, and more likely to go up in flames. And a changing climate has brought more intense, larger and less predictable fires.

“What’s it’s coming down to is jungles of fuels in forest lands," Safford said. "You get a big head of steam going behind the fire there, it can burn forever and ever and ever.”

Despite relatively mild wildfire seasons the past two years, California has seen 12 of its largest 20 wildfires — including the top eight — and 13 of the most destructive in the previous five years. Record rain and snowfall this year mostly ended a three-year drought but explosive vegetation growth could feed future fires.

California has lost more than 1,760 square miles (4,560 square kilometers) — nearly 7% — of its tree cover since 1985, a recent study found. While forest increased in the 1990s, it declined rapidly after 2000 because of larger and more frequent fires, according to the study in the American Geophysical Union Advances journal.

A study of the southern Sierra Nevada — home to Yosemite, Sequoia and Kings Canyon national parks — found nearly a third of conifer forest had transitioned to other vegetation as a result of fire, drought or bark beetles in the past decade.

“We're losing them at a rate that is something that we can’t sustain,” said Brandon Collins, co-author of that report in the journal Ecological Applications and adjunct forestry professor at the University of California, Berkeley. “If you play it out (over) the next 20 to 30 years at the same rate, it would be gone.”

Some environmentalists, like Chad Hanson of the John Muir Project sponsored by the nonprofit Earth Island Institute, said there's a “myth of catastrophic wildfire” to support logging efforts — and he has often sued to block plans to remove dead trees or thin forests.

Hanson said seedlings are rising from the ashes in high-severity patches of fire and the dead wood provides habitat for imperiled spotted owls, Pacific fishers and rare woodpeckers.

His research found forests always had dense patches of trees and some severe fires, Hanson said, contending that increasingly large ones result from weather and climate change, made worse by logging practices.

“If everything people are hearing was true there would be a lot more reason for concern,” he said. “But the public is being gaslighted.”

However, others are concerned failure to properly manage forests can result in intense fire that could harm wildlife habitat, the ability to store climate-warming carbon in trees and the quality of Sierra snowmelt that provides about 60% of the water for farms and cities.

Burn scars are more prone to flooding and erosion, and runoff becomes tainted with ash and sediment.

“Areas where mixed conifer burned at high severity, those are all areas that are vulnerable to total forest loss,” said Christy Brigham, chief of resources management and science at Sequoia & Kings Canyon National Parks. “We have no idea what that means for wildlife habitat, for water cycling, for carbon storage. And that’s not even getting into the things we love about forests."

After wildfires in 2020 and 2021 wiped out up to about a fifth of all giant sequoias — once considered almost fireproof — the National Park Service last week embarked on a controversial project to help the mighty trees recover with its largest planting of seedlings a single grove.

CHANGING FOREST LANDSCAPE

Many researchers say the canopy of the Sierra Nevada has changed dramatically since heavy Gold Rush logging.

Before the mid-1800s, fire sparked by lightning or set by Indigenous people burned millions of acres a year. It kept undergrowth in check, allowing low-intensity flames to creep along the forest floor and remove smaller trees competing with big ones.

"The inviting openness of the Sierra woods is one of their most distinguishing characteristics,” John Muir said, describing how a horse rider could easily pass through the trees.

But after settlers drove out Native Americans and logged forests, fighting fires became the mission to protect the valuable trees — and, increasingly, homes built deeper into wildlands. In 1935, the U.S. Forest Service established a policy to knock down any fire by 10 a.m. the next morning.

That has allowed forests to become four to seven times more densely wooded than they once were, Safford said. While many larger, fire-resilient trees like ponderosa and Jeffrey pines were logged for lumber, smaller trees that are not so fire resistant have thrived. They compete for water and their low branches allow fire to climb into the canopy of taller trees, fueling devastating crown fires.

“John Muir would not recognize any of this,” Safford said, gesturing at a stand of tightly packed dead trees during the tour last October. “He wouldn’t even know where he was.”

A TINDERBOX TAKES OFF

The Caldor Fire, which destroyed 1,000 structures while burning across the Sierra Crest and into the Tahoe basin, torched forest that hadn't seen flames in over a century, Safford said. Years of drought fueled by a warmer climate had made it a tinderbox.

Swaths of Eldorado National Forest burned at such intensity that mature pines went up in flames and their seeds were killed. Unlike species such as giant sequoias and lodgepole pine that drop their seeds in fire, the dominant pines of the Sierra can't reproduce if their seeds burn.

Manzanita and mountain whitethorn — chaparral typical at lower elevations in California — take root in ashes and can dominate the forest.

Studies have found that repeated fires or other disruption provoke such shifts in ecosystems.

A March study of 334 Western wildfires found increasing fire severity and drier conditions after fire made the dominant conifer species less likely to regenerate and it concluded the problem is apt to worsen with climate change.

Along U.S. Highway 50, where the Caldor Fire had continued burning out of control toward Lake Tahoe, Safford parked his SUV and scrambled up a rocky knoll to point out a slope barren of trees. Forest there had been burned in 1981 and was replaced with chaparral.

The Caldor blaze, allegedly caused by a reckless father and son, is likely to reinforce that condition, Safford said. And whether the severe burn recovers will depend largely on whether another fire tears through in coming years, he said.

TOOLS FOR TREATING FORESTS

To tackle the problem of huge wildfires, the federal government, which owns nearly 60% of California’s 51,560 square miles (134,00 square kilometers) of forest, agreed with the state in 2020 to jointly reduce fuels on 1,560 square miles (4,040 square kilometers) a year by 2025.

While a fraction of the land needing treatment, it's considered a promising development after years of inaction, though not without controversy.

Fire scientists advocate more deliberate burning at low-to-moderate severity to clear vegetation that makes forests susceptible to big fires.

But the Forest Service has historically been risk averse, said Safford, the agency's regional ecologist for two decades before retiring in 2021. Rather than chance that a fire could blow up, officials have generally snuffed flames before they could deliver benefits of lower-intensity fire.

Weeks before the Caldor Fire, the Forest Service had been monitoring a lightning fire south of Lake Tahoe, while dealing with more pressing ones. But when the small fire took off, causing millions of dollars in damage, politicians blasted the agency for not doing more. Officials quickly said they would no longer let some naturally ignited fires burn that season.

With more than $4 billion in funding from the Bipartisan Infrastructure Law and the Inflation Reduction Act, the Forest Service plans to ramp up forest thinning in places where the wildfire threat to communities and infrastructure is most immediate.

That will include cutting smaller trees, as well as setting intentional fires to clear accumulated forest litter.

BATTLELINES OVER THINNING

Last fall when Safford led two graduate students up a rutted fire road through charred forest, they came upon a patch of life where large pines and cedars towered overhead and seedlings sprouted.

A “nirvana” is what Safford called it. Smaller fire-intolerant trees had been harvested and other vegetation removed before the fire. The space between the trees allowed the fire to creep along the ground, only charring some trunks.

A coalition of Sierra-based conservation groups wrote congressional leaders in 2021 urging more federal funding for fire resilience. Their letter cited “broad consensus among fire scientists, land managers, firefighters” to increase thinning and prescribed fire.

Susan Britting, executive director of one of the groups, Sierra Forest Legacy, acknowledged any cutting triggers skepticism because loggers historically took the largest, most marketable trees. But she said thinning trees up to a certain diameter is acceptable, though she prefers prescribed burning.

“In my experience, things like logging, tree removal, even reforestation, those things happen,” Britting said. “The prescribed fire that needs to happen ... just gets delayed and punted and not prioritized.”

The goal of prescribed burns is illustrated by a large green island on a fire severity map of the nearly 350-square-mile (906 square kilometers) Caldor blaze. The green area, representing low fire severity, corresponded to where a fire was set among older trees in 2019.

The chance of a deliberate burn escaping its perimeter — as happened last year in New Mexico's largest fire in state history — remains a big challenge to the strategy.

While managed fire and prescribed burns are widely supported by scientists and environmental groups, thinning is controversial and often faces court challenges.

In a 2020 letter to Congress that opposed logging, The John Muir Project's Hanson and more than 200 climate and forest scientists said some thinning could reduce fire intensity but those operations often take larger trees to make it economically worthwhile.

Safford — now chief scientist at Vibrant Planet, an environmental public benefits corporation — acknowledged larger trees have been logged in the past but said that's not now envisioned in thinning projects aimed at making forests healthier.

Even with chainsaws, we won't be able to cut our way out of the problem, he said. Two-thirds of the rugged Sierra is inaccessible or off-limits to logging, so fire will have to do much of the work.

But there's a backlash against fire as as a management tool. Homeowners are anxious prescribed fires will jump perimeters and destroy houses. Similar fears lead fire agencies to tame moderate fires that can clear forest floors.

“It’s the classic wicked problem where any solution you derive has huge implications for other sides of society and the way people want things to be,” Safford said. “So I’m afraid what’s going to happen is at some point we’ll burn all of our forests.”