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Wildfires across the US are changing forest landscapes — and the tipping point may be on the horizon

As Monica Turner trekked across the western hills of Yellowstone National Park, the cold rain was inescapable. Even the constant movement of putting one foot in front of the other as she fought through a slew of dense branches could not warm her body. It was easy to forget it was July.

CNNTaylor Romine查看原文 ↗
美国各地的野火正在改变森林景观——而临界点可能即将到来。

Courtesy Monica Turner

As Monica Turner trekked across the western hills of Yellowstone National Park, the cold rain was inescapable. Even the constant movement of putting one foot in front of the other as she fought through a slew of dense branches could not warm her body. It was easy to forget it was July.

Four years later in 2017, Turner made the same journey. But instead of fighting the once compact forest, she was met with a desolate clearing as the sun pounded above her. After the Maple Fire the year before, all that was left of the towering trees was a smattering of survivors looking over a sea of skinny, pencil-like sticks sprouting from the charred dirt.

In Turner’s time as a researcher, she has seen massive destruction and its subsequent regrowth as she studied the impacts of Yellowstone’s 1988 fires, which left a crown jewel of the national park system scorched. This made the level of destruction Turner witnessed in 2017 even more shocking — especially as the area had burned before without this level of loss.

Researchers across the US are seeing a similar trend — forests that are historically adapted to fire are now often being obliterated by the fast-moving, high temperature wildfires that are becoming increasingly more common with climate change . These researchers are grappling with what it will mean for our world if the trees that are the centerpiece of these landscapes we love don’t recover like they once did.

The fire itself is not inherently the problem. Every scientist CNN spoke with emphasized the need for fire on the landscapes they researched. But the lack of prescribed burning, a practice long used by indigenous people to clear out dead plants and to keep landscapes healthy, has left these trees vulnerable . It means their futures — when they encounter hotter, more vicious fires — are uncertain.

As these ecologies start to shift, it’s unclear if or when these different areas around the US will hit a tipping point where the trees won’t come back. Instead, researchers say it’s more likely there will be a gradual conversion over time, with once towering forests becoming robust scrublands or large fields of grassland.

Here’s how some places around the country are changing, and how one state is succeeding in preserving its native wildlife.

As Turner stood on the dirt field, the air was hot — much hotter than during her previous 2013 visit. Cool summers and rain had been the norm for Yellowstone during her years of research. The heat was so intense this time that a field assistant was hospitalized for dehydration during the trip, said Turner, who works at the University of Wisconsin-Madison as the Eugene P. Odum Professor of Ecology and a Vilas Research Professor.

The ash coating the former forest floor reminded her of the ash she walked through during the 1988 fires — ash that covered her face and caked her socks. For a long time, Turner thought the wildfires from that summer 38 years ago represented how forests could and would recover. Despite miles of charred trees, the research in her field showed the forest would come back, and with a high density of trees, she said. But Turner said she didn’t appreciate “how much things could change in a relatively short time.”

Bettmann Archive/Getty Images

“We could see what we call ghost logs, which is where there had been large logs on the ground, and they were completely combusted,” she said. “And then you can see their shadows where the white ash is left behind from the hot temperatures.”

Only about 5 percent of the tree seedlings have grown back so far from the 2016 fire, compared to what burned. In place of the trees, grasses blow in the exposed meadow, now dotted with wildflowers that commonly sprout after a wildfire. Turner said she is seeing “a slow but detectable shift” to plants that are “more tolerant of warmer, drier conditions” and can grow back more easily if more wildfires come.

Slowly but surely, these changes could impact visitors to the park, Turner said. People like trees, for both how they look and the practical element of shade during hikes, she said, and the changes the forest is undergoing could impact how and when people visit the park.

The repercussions are seen in more unexpected ways as well — at one point, the park had a temporary ban on fishing, including catch and release, because the fish were already too stressed out from low water levels and the high water temperatures that were partially due to the lack of shade, she said.

“What will be the consequences for lots of other values that people care about in that region?” Turner said. “That could be wildlife habitat. It could be water availability. It could be shade if you’re hiking.”

But ultimately, it’s not yet clear what the future Yellowstone will look like.

About 100 miles northeast of California’s state capital, Derek Young stood in the middle of a clearing in Plumas National Forest. The carcasses of burnt trees that surrounded him felt like a graveyard, the remains left in the wake of the 2021 Dixie Fire.

An occasional bird would chirp in the distance, but the forest emanated an eerie stillness, said Young, a research ecologist at the University of California, Davis. Fifteen years ago, Young started studying these landscapes as a PhD student. Many of them are now unrecognizable compared to the thriving forests they once were.

“It’s really actually crazy to how much change has happened,” he said.

Courtesy Derek Young

Many of the areas Young studies are changing from dazzling emerald trees to fields of shrubs — even years after the wildfires.

The shrubs, ranging from deer brush to white thorn shrub, are incredibly good at resprouting, Young said. While fire might lop off the shrub tops, their roots remain, helping the plants return quickly, he said. It’s stiff competition for trees to endure, which need specific conditions to grow back compared to the hardy, drought-tolerant shrubs, he said.

In just a few years, the shrubs can grow to be as tall as a person, covering the entirety of an area, Young said. “It can really feel like you’re swimming in a sea of shrubs. You can’t see more than a few feet ahead of you.”

There are a couple of factors required for trees to successfully reemerge. Some species of trees can grow in the shade, but those species are also more likely to be destroyed during a high temperature fire, he said. Others need the heat of a fire to release their seeds, while many others don’t, Young said. Those seeds then need to land in an area where they can grow and survive, which can be impacted by wind and wildlife, he added. A lot needs to happen in a short window of time in order for them to regrow.

As he sees these landscapes shift, it’s hard to weigh if this is inherently a bad change — although he noted the important services forests currently provide humans, namely capturing and holding carbon dioxide through carbon sequestration.

“As forests burn up, they release carbon, contribute to climate change, which makes it more likely for more fires to burn and convert more forests,” he said. “You could say, if you care about humanity’s well-being, that that’s actually a reason to favor maintenance and restoration of forests in place of post-fire shrub fields.”

“I do sometimes try to think from the perspective of like, what if there were no humans, and there is no society to decide what our values are,” he said. “In that scenario, I think the sort of definitive answer is the most valuable ecosystem is the one that is adapted to the site in the absence of all the human intervention.”

But he notes this change to shrubs is not happening in that theoretical vacuum without humans.

“It’s not that the system naturally wanted to be shrubs. We created that condition by suppressing fires for 100 years,” he said.

On the border where the Great Plains meet the Rocky Mountains, the flat-top mesas of Sugarite Canyon State Park frame the indigo blue lake nestled at the bottom of the canyon.

But the Track Fire had interrupted this picturesque scene, charring the thick tree trunks that peppered the canyon wall in New Mexico. “You look up on the hill slopes, and it’s just these charred stems of old Douglas fir trees,” said Chris Guiterman, a research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

Guiterman went to the park to research how forests recover from wildfires, inspired by the 2011 Las Conchas Fire, which burned over 156,000 acres in what was at the time the largest wildfire in New Mexico’s history, according to the National Park Service . As the areas impacted by the fire started to recover, new life sprouted where the trees had previously been – the thorny New Mexico locust, bright green thickets of Gambel oak, clusters of dandelions and other types of herbs and shrubs, Guiterman said.

Like California, many areas of the southwest are also seeing forests slowly transform into shrubland — specifically, many pine-dominated forests changing to drought-resistant oak shrubs, Guiterman said.

Those shrubs, in some circumstances, can act like kindling during wildfires, adding more fuel to an area that is already in extreme drought . This contributes to the high intensity of fire, in turn killing more trees that had previously survived, he said.

Many of the areas Guiterman studies have a long history with fire. In 2011, the Wallow Fire burned over half a million acres of forest in New Mexico and Arizona, the biggest wildfire in modern Arizona history . But Guiterman’s research shows large-scale fires happened in this area about every 20 years going back millennia. But what has changed recently is the growing intensity of these fires.

“The fires in the path that we recorded, the trees survived those fires to record the scar and kept going,” he said. “But now those forests, like the trees that we sampled, were dead.”

These intense wildfires are fundamentally changing the landscape, but in a way that he says isn’t inherently good or bad, but just different.

“In Sugarite, you feel that sense of loss,” Guiterman said when talking about losing 400-year-old trees. “At the same time, there’s beauty to it because you have all these vibrant colors and everything (from other plants). But instead of wanting to wander aimlessly through this (forest) and have a picnic, it’s this thicket of shrubs, and you can’t leave the trail.”

While for many the charred remains of trees and dirt feels like the end of life, to Rae Crandall, it is just a beginning.

After a fire, a pine savanna ecosystem can look “almost impossibly black,” she said. ” But the blackened landscape is anything but lifeless at the University of Florida’s Austin Cary Forest , which is used for Crandall’s research.

The gray spiny fence lizards blend into the singed ground, the movements of the dark scales of the pygmy rattlesnakes can look like an optical illusion.

Within two to three months of a prescribed fire, the scene is transformed. The previously burned landscape transforms “into a sea of grasses, wildflowers, butterflies, and other species that depend on fire to persist,” said Crandall, an associate professor of fire ecology at the University of Florida and the principal investigator at the Southern Fire Exchange. When you hear her describe it, it feels like she is describing a small miracle — even though it happens all the time around here.

Courtesy Rae Crandall

Like many places around the country, ecosystems in Florida have adapted to fire over time, and it’s needed to help these places thrive, according to Crandall. While there might be different burning needs for different areas, like Everglades National Park versus Big Cypress National Preserve, both do better with occasional fire, she said.

“Fire has been shaping those landscapes for millennia,” Crandall said, adding that they are “so, so resilient.”

Florida has about 2 to 2.5 million acres of land undergo prescribed burns per year, according to Rick Dolan, the director of the Florida Forest Service, which issues about 85,000 burn authorizations a year. Not only does prescribed fire have an ecological benefit, it also “helps enhance wildlife and habitats for wildlife,” he said. It also means there is less plant material to burn when an unplanned fire comes through the area.

The state has unique conditions that allow for an easier prescribed burn culture, such as distinct wet and dry seasons, flat topography and a long history of private landowners burning their land to maintain it, Crandall said. And while they have these things on their side, state officials being proactive about prescribed burning has put Florida in a better position than other areas of the country.

“We don’t see a tipping point,” Crandall said. “Very rarely do we have any sort of catastrophic fire that’s going to fundamentally change the plant and animal communities that are there.”

One of the biggest hurdles the state faces is communicating to the public the importance of these fires.

“Florida is growing, and people that are native Floridians understand prescribed fire because they’ve grown up with it, you know, their whole lives,” Dolan said. “The challenge that we have is when folks move into the state, and we are growing, you know, from areas of the country that maybe don’t do prescribed burning. And I think education is the key.”

Crandall said this isn’t a one-size-fits-all approach to burning: “We’re evaluating the land. We are writing a prescription like the doctor would for a patient.”

One of the remarkable parts of the low intensity fire used during prescribed burns is that even when something is left blackened on the plains of the pine savannas, it isn’t irreversibly damaged — the same plants return and life goes on, said Crandall.

“By fall after a prescribed burn, it can be difficult to take a step without brushing against a flowering plant or sending a butterfly or grasshopper into the air,” she said. “Gopher tortoises graze on the tender new growth. And beneath all that apparent renewal is remarkable continuity.”

Many of the trees that these researchers are studying don’t seem to have a strong chance of returning. In the hot and progressively drier areas of the Southwest, the conifers — such as ponderosa pines or Douglas fir — are having difficulty growing back compared to their hardy competition.

“The conifers just don’t really have much of a chance to get back in there,” Guiterman said. “The oaks (shrubs) are going to hold it, and they’re very persistent.”

While Guiterman can’t say these areas that have turned into shrub won’t ever go back to the forests they previously were, climate change has made it more likely that the shrubs are here to stay. But despite what seems like a monumental change, life here continues.

“I remember we were eating lunch and we watched an osprey dive into the lake, pick up a trout,” he said, which was an amazing moment to witness on its own. But then, they saw a bald eagle chasing the osprey until it dropped the fish, then the eagle stole the fish. The moment illustrated how despite all the change, life keeps going, he said.

“There’s probably other loss because of the fire, but I couldn’t feel it in that moment,” he said.

In Yellowstone, it’s hard to say when the landscape will change from forests to grassland permanently. It’s unlikely there will be a single tipping point, Turner said. “There’s going to be multiple of them,” she said. “The forest is responding to many things at the same time, and so it’s that’s why it’s just difficult to predict.”

Turner acknowledges that while we can observe what is happening now, we have no idea what the future holds, and we might be surprised by the landscape’s resilience.

“The Greater Yellowstone area is our largest intact ecosystem in the lower 48 states,” Turner said. “I have been surprised at some of the things that we have seen there. Where again, we thought we thought everything was dead and everything recovered. We thought the soils would be sterilized, and they weren’t.”

“I think it’s going to be very interesting to see how this ecosystem adapts to the changes that are coming, and they are coming,” she said. “There may be more capacity to adapt than we give it credit for.”

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