Increasingly Large and Intense Wildfires Hinder Western Forests’ Ability to Regenerate

As global warming threatens the long-term survival of many forests in the Western United States, a new study suggests that reducing the intensity and size of wildfires would help conifers regenerate after the destructive blazes that have become more frequent in recent decades.

The team of more than 50 scientists analyzed data from more than 10,000 field plots after 334 wildfires to assess how forests regrow after fires. They also compared the effects of the fires to what’s expected from a warming climate. 

The findings, published Monday in the Proceedings of the National Academies of Sciences, show that, for the next few decades, fire intensity is the bigger factor influencing how forests regenerate after a blaze. And they suggest that there is a short-term window of opportunity to help coniferous forests regenerate with pro-active management of wildfires and forests.

The key would be to reduce the overall intensity of wildfires and to minimize the size of the most intense fire patches, said co-author Phil Higuera, an ecosystem and conservation science researcher at the University of Montana. The way to do that is by using intentionally set fires, as well as selective logging, to clear smaller trees out of forests.

Higuera said the biggest advance of the study was the broad-scale assessment of the relative influence of changes in climate and the effects of wildfires, and how they will impact post-fire tree regeneration, in combination with assumptions of how fires will burn in the future. The study covered most of the forested area in Western states, and Higuera said he was somewhat surprised that the modeled effects of fire severity hampered regeneration more than droughts and other  effects of climate change, at least for the next 30 years.

“But the longer you wait, the bigger the warming effect gets,” he said. “Importantly, the projections only go through 2050, and that seems a lot closer than it used to.”

The key take-home message is that, “If we can modify the way that fire burns now, particularly over these next several decades, that will greatly increase our chances of getting seedlings established,” he said. “Then they’re more robust to changes in climate and year to year variability.”

“If we can get seedlings established, that gives us one more forest generation into the future,” he said. “And for this to be a hopeful message, that has to happen in parallel with us addressing climate change.”

The current international climate target is to get to net-zero carbon dioxide emissions by 2050. If the atmospheric concentration of the greenhouse gas levels off, so should the global temperature increase, which could give some forests a chance to adapt to the warmer but stabilizing temperatures in the second half of the century.

Forests in Serious Trouble

The new research underscores the “sobering reality” that many forests across the Western United States are in serious trouble due to climate change and, in some cases, “high-severity wildfire,” said forest and fire ecologist Evan Frost, who was not involved in the study.

The new paper also shows the trend of increasing forest loss across many parts of the West intensifying in the coming decades, said Frost, who heads Wildwood Consulting LLC, a private forest research group.

He said one of the stand-out projections is that conifer forests across 58 to 60 percent of the expansive study area are “unlikely to regenerate following wildfire irrespective of severity with predicted near-term climate change.”

There are big regional differences in forests’ ability to regenerate following high-severity wildfires.

“Overall,  drier and low-elevation forest types such as those found in California and U.S. Southwest are showing more significant regeneration declines, compared to moister, higher elevation forests in the Northern Rockies,” Frost said. 

Another recent study shows that low-elevation forests in California, including vast stands of Ponderosa and Jeffrey pine, are among the least likely to recover after fires. Those findings mesh with the new research, which showed the greatest forest declines at lower elevations and in the Southwest.

Firefighters standing on a snowbank extinguish hot spots near Keystone, Colorado after an unusual high-elevation wildfire burned through in March 2012. Credit: Bob Berwyn

Firefighters standing on a snowbank extinguish hot spots near Keystone, Colorado after an unusual high-elevation wildfire burned through in March 2012. Credit: Bob Berwyn

Coniferous forests rely on seed dispersal to regenerate, and there are only a few ways that can happen. For lighter, winged seeds, the wind plays an important role, and for the heavier seeds of five-needle pine species, like limber and whitebark pines, dispersal by birds and small mammals is critical. Wildfire can affect both, said Higuera.

Larger patches of severely burned forest increase the distance between burned areas that need seeds and the trees that can provide them, so reducing the size of high-intensity burns that kill all the trees can make it more likely those areas will regenerate.

“You can have one surviving large tree in the middle of a big burn, and that’ll be enough to reseed the area,” he said.

Wildfires so severe that they leave no surviving trees with seeds can also wipe out the habitat for some animals that might have helped the landscape recover by bringing in seeds from other areas. Keeping the most severely burned patches of wildfires smaller can also help animals spread seeds to regenerate them.

“We have only a few decades to act, said Alistair Jump, dean of the faculty of natural sciences at the University of Stirling in the United Kingdom. “The paper shows that we can act to reduce current and future risk, but we are limited by our capacity to manage the very substantial areas of forest under threat.”

But even a massive program of forest thinning and prescribed fires won’t eliminate the risk of catastrophic forest loss, he said, “So we also need to ensure that we prepare for very serious negative outcomes for our forests and forest-associated communities over the years ahead whilst working equally hard to avoid them.”

Regional Nuances Are Important

The changes described for the study periods—from 1981 to 2020 for the observations, and from 2031 to 2050 for the projections—were not uniform across the West, with the worst impacts in lower and warmer areas.

“Selfishly, I feel like it gives the Northern Rockies an opportunity to learn and see how things are playing out in other ecosystems,” Higuera said. “But it’s not that we can sit back and say hey, we’re buffered forever.”

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It’s also not a problem unique to the Western United States, said University of New Mexico forest ecologist Matthew Hurteau, another co-author of the paper.

“We see regeneration failure of different tree species around the planet, from alpine forests in Tasmania to the loss of resprouting tree species in Mediterranean Europe because of high fire frequency,” he said. “Climate change is making forest ecosystems more flammable and it is making the post-fire environment more stressful for tree seedlings.”

Like in the Western United States, temperate forests in Europe are beset by multiple threats that decrease forest resilience, and in the Mediterranean region, increasing fire frequency is extirpating some forests, he said.

Frost said it’s also important to recognize that for forest regeneration and persistence, short, acute events may be more influential than the more gradual, steady increase in the global average temperature, like hot droughts and extreme heat waves that exceed tree species’ tolerance. 

“This is consistent with what we’ve been seeing as causative drivers for large-scale tree mortality events occurring, and increasing, in recent years across the Western U.S.,” he said.

Forest Management Can Help, but Slowing Climate Change is Key 

Many foresters and fire scientists say the short time window to slow forests’ decline demands a steep increase in treatments—cutting excess vegetation and setting fires to thin overgrown woodlands—to reduce the severity of wildfires and the size of high-intensity burns.

“We actually know that a combination of thinning and prescribed fire, and in some cases, prescribed fire alone, are pretty effective at changing forest structure (in a way that) subsequently changes the fire behavior after those treatments,” said co-author Marcos Robles, a forest researcher with The Nature Conservancy.

Both options clear spaces between larger, fire-resilient trees so that when wildfires ignite, they burn close to the ground and don’t wipe out all the seed-bearing tree-tops.

“The issue has been the scale of the problem,” he said. “Forest management has not yet been able to reach the pace needed to address that scale of wildfires.”

But he sees opportunity with the 2021 Infrastructure Investment and Jobs Act, which included something like $3 billion to try and respond to the country’s wildfire crisis by treating 50 million acres of forest in the next 10 years, a dramatic increase in the pace and scale of thinning and prescribed burning treatments. 

“So far, fire severity just keeps going up and up and up,” he said. “I’ve seen these patches where you have 100 percent tree mortality, and 10 or 20 or 30 years later, after the fire, you don’t have seedlings.”

The new paper highlights the need for urgency to slow the increase in the number of forest areas that are unable to regenerate, he said.

Frost said it’s an open scientific question, with high levels of uncertainty, whether fuels-focused approaches—burns and thinning operations to reduce the amount of vegetation available to fuel severe wildfires—are capable of successfully mitigating future fire behavior at the large scales required in the face of climate change.

“Many forest scientists and practitioners, including myself, are doubtful that fuels reduction alone will tip the scales toward less intense fire behavior,” he said.

”Unless we get a handle on climate change soon, any management interventions we make are unlikely to do much to help dry forests survive and regenerate.”

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