Refugia dot a hillside in the western Cascades after the 2020 Holiday Farm Fire, one of the largest blazes in Oregon’s history.
Refugia dot a hillside in the western Cascades after the 2020 Holiday Farm Fire, one of the largest blazes in Oregon’s history. Credit: Meg Krawchuk

The Science of Resilient Forests

Cover of January 2023 issue of Eos

Forest refugia are the oases of the woods, lush green patches that evade wildfires by quirks of topography, moisture, or the unpredictable wind and weather conditions during a fire. When fires subside, these patches of surviving trees and other vegetation repopulate nearby acreage as winds disperse seeds and spores across a charred landscape. Some refugia have persisted through fire after fire. Now, climate change threatens even these longtime survivors.

“The tricky thing is that as climate conditions continue to get hotter and drier and fire weather conditions tend to get a little more unpredictable and more extreme, the predictability of whether those persistent fire refugia will exist through future fire events is pretty uncertain,” said Sebastian Busby, a fire ecologist with the U.S. Forest Service (USFS). Even the definition of refugia has expanded. It once meant an untouched oasis of persistent refugia but now can mean an area that is simply less burned than its surroundings or an ephemeral refugium that outlasts one fire but is burned by the next.

Refugia occur in many ecosystems, ranging from a few meters of rocks and plants to sizable stands of unburned trees, and aren’t limited to forests. They can appear, for example, in climate-resistant patches of many ecosystems, such as deep lakes that are slow to warm, cold groundwater inputs, deep snowdrifts, and cold air pools in valleys. Any remaining trees, in the case of fire, can be considered refugia. And many, if not all, fire refugia are now also considered climate change refugia because of the link between our warming planet and hotter, more intense fires.

Diagram of different types of refugia (including forests and wetlands) and how they respond to climate change
Climate change refugia are “areas that remain relatively buffered from contemporary climate change over time and enable persistence of valued physical, ecological, and socio-​cultural resources,” according to the U.S. Forest Service. Credit: USFS

“We have more tools to manage forest refugia than other refugia,” said Toni Lyn Morelli, a research ecologist with the U.S. Geological Survey. Unlike other climate change–resistant spots, “forest stands can create their own microclimates that create refugia,” Morelli added.

Scientists and forest managers alike are scrambling to define new best practices for both human-driven and hands-off management strategies as climate change shifts the role of these surviving pockets of trees. Fires today are harder than ever to predict. Warmer temperatures around the globe are now accompanied by longer, hotter fire seasons fueled by drought-stricken soil and vegetation in areas that were once too damp for flames to spread.

Although fire is an inherent and necessary part of many forest ecosystems, scientists don’t yet agree on the best approach to managing fire and refugia. Some feel that the less intervention, the better, whereas others hope to guide forests to become more adaptable to the impacts of climate change, including fire. Although news cycles tend to focus on extinguishing large fires, bigger fires aren’t necessarily bad; regions are characterized by unique patterns, or fire regimes, that vary in severity, frequency, and extent. Some forests, like the high-elevation, boreal forests of the Rocky Mountains, need high-severity fire to function. But high-severity fire in a region where this is not part of the typical fire regime is more troubling. Human impacts on fire’s ignition and spread complicate the equation, adding new variables that influence when, where, and how fires start and grow.

Piecing Together the Best Approach

“Keeping refugia on the landscape may actually inhibit adaptation because you’re anchoring down a community that needs
to shift.”

Protecting existing trees and refugia at any cost threatens to throttle natural changeover to species that are more fire resistant, according to Oregon State University fire ecologist Meg Krawchuk. Despite specializing in refugia, she also knows that saving refugia at any cost is not the answer. “Keeping refugia on the landscape may actually inhibit adaptation because you’re anchoring down a community that needs to shift,” she said. “We think about fire as the ultimate catalyst that can potentially facilitate change by removing competition and allowing a playing field for adaptation to happen.”

Managing forest refugia in the face of the climate crisis, Krawchuk said, means constructing a patchwork of approaches tailored to each forest. For example, old-growth forests might demand a resistance approach that prioritizes preserving as much of the original ecosystem as possible. In other regions, helping ecosystems rebound from fire could mean a healthy balance of allowing fire to progress naturally while also supporting recovery. Finally, recognizing that forests evolve, some regions might best be allowed to transition to new species with an eye to keeping various ecological roles intact (think one fire-resistant conifer species replacing another, less resistant species). Such a transition might mean letting even persistent refugia burn. Krawchuk said an approach using all three methods is best.

First, however, scientists must learn to predict how forests will respond to fire and where refugia will form. Only then can they effectively address the next controversy: whether, and how, to respond.

Far from Humans, Refugia Are More Resilient

Researchers in northeastern Siberia’s larch forests found that wetness and terrain steepness predicted refugia, even during record-setting heat waves and ground and surface fires in 2020. After analyzing 2 decades of burns in these gently rolling landscapes, scientists found little change in refugia size and density. These regions are underlain by continuous permafrost, and refugia protect this frozen subsurface layer from thawing after fires, according to ecologist Anna Talucci, a postdoctoral researcher at Woodwell Climate Research Center in Falmouth, Mass.

Siberia’s remote fires have created a patchily burned postfire landscape, a sign of healthy fire. Less burned patches, she said, which coincide with low-lying, wetter areas or trees growing on steep slopes, are more likely to yield refugia whose seedlings take advantage of wiped-out competition—smaller plants that might otherwise take root in the same soil as seedlings—to regrow after fires.

But Siberian larch forests don’t burn the same way that North American boreal forests do. “The fires that we were looking at are not in mountainous regions,” Talucci said, noting that the region’s pattern of frequent, smaller surface fires is governed by conditions on the ground. “And so it’s really those microsite characteristics that seem, in the context of this study, important for influencing fire refugia.” Fires in northeastern Siberia tend to be ground or surface fires, which wipe out low-level plants and shrubs that would otherwise compete with larch seedlings. Ground fires, which can smolder in the earth for months, spread slowly but can be harder to extinguish than surface fires that consume dry and dead vegetation just above the ground.

In contrast, North American boreal regimes include crown fires that blaze destructively through an interlaced network of treetops, leaving behind few refugia. Some high-intensity fire is needed in these regions, and boreal forests can adapt to these fires over time, so not all larger fires are ecologically catastrophic. But crown fires coupled with shifting conditions due to climate change have led to extreme blazes in recent years.

North America and Australia’s Unruly Blazes

Aftermath of the 2012 High Park fire in Colorado.
In Colorado’s Cache la Poudre River valley, the 2012 High Park Fire was ignited by lightning. Credit: Meg Krawchuk

Fire weather, a factor in all types of fire, is governed by a triad of factors: drought and dryness, temperature, and wind, which can quickly fuel an inferno when paired with an ignition source like lightning or a campfire left unattended. In North America and Australia, extreme conditions have contributed to historic burns in recent years.

In the 1990s, wildfires burned an average of 526,000 to 2.5 million hectares (1.3 million–6.1 million acres) in the United States each year. In the past decade, that average has topped 4 million hectares (10 million acres) several times. As of mid-September 2022, 2.7 million hectares (6.7 million acres) have burned in the United States. Fires that have become bigger and hotter, combined with population growth, have led to more and more people living in fire-prone regions. As fires began sooner each year and ended later, California’s Department of Forestry and Fire Protection stopped measuring fire seasons in favor of fire years, and USFS followed suit.

The 2019–2020 Australian “Black Summer” megafires burned 14.3 million hectares (34 million acres), affecting approximately 80% of Australians and causing 34 deaths, along with the loss of more than 2,700 homes.

The charred effects of bushfires at The Spectacles Wetlands, Western Australia.
Bushfires devastated The Spectacles Wetlands, part of a parkland just south of Perth, Australia, in 2019. Credit: Calistemon/Wikimedia, CC ­BY-SA 4.0

Extreme Fires, Unpredictable Refugia

“The more extreme the fire weather conditions, the less effective the topographic controls are in impeding fire completely but also in moderating the burn intensity,” or how much vegetation is consumed by fire, said ecologist Brendan Mackey, who directs the climate change response program at Griffith University in Queensland, Australia. Instead, enormous, intense fires can be accompanied by similarly extreme weather patterns. Even normally protective features like pockets of moisture on a north facing slope on the U.S. West Coast can be overpowered by the outsized temperatures of recent fires. In other words, extreme fires become so hot that they can overcome these protections.

This extreme behavior means that it is harder to predict how fires will ignite and spread as well as where and when refugia will be left behind. Not only does this make it more difficult for scientists to agree on how to manage fires, even if they did agree, the unpredictable nature of contemporary burns makes any strategy other than simply leaving fires alone that much harder to implement.

In addition to dryness, temperature, and wind, “we’ve been joking that that’s sort of the fourth axis of what we refer to as the fire behavior triangle: fire itself.”

When it comes to predicting fire behavior and resulting refugia, there’s something of a “Goldilocks effect,” according to Krawchuk, in which moderate fires—as well as refugia in areas with moderate topographic variation—tend to be more predictable, whereas very mild or very intense fires, increasingly, are not. In fact, hotter, more intense blazes can create their own weather, including wind, thunderstorms, massive clouds, and even tornadoes. In addition to dryness, temperature, and wind, “we’ve been joking that that’s sort of the fourth axis of what we refer to as the fire behavior triangle: fire itself,” Krawchuk said.

Meanwhile, efforts to understand and manage refugia are still in their infancy. “We are just beginning to conserve refugia,” Morelli said. “I think there are a half dozen projects in the world on it.” These range from studies of whitebark pine in Wyoming, to refugia in Canada’s high-latitude boreal forests, to climate-sensitive Joshua trees in Southern California.

Seed dispersal and other phenomena that help repopulate forests are complex events that are still being mapped and studied. Even in areas with fire regimes not characterized by crown fires (like Siberia), it’s become harder to predict whether and where forest regeneration might occur and thrive as permafrost ebbs. Burned parts of even healthy, patchy forests that vary in burn severity may be too far from refugia for seedlings to spread and thrive. Conifer reproduction, for example, relies on the timing of masting events in which trees produce a bumper crop of seeds. These seeds can travel only as far as the wind takes them, often just 100 meters (328 feet) or so from live trees. (One species of Siberian larch has a mean dispersal of just 15 meters, or about 49 feet.) When seedlings can’t establish themselves quickly in burned areas after masting, hardier vegetation moves in to take advantage of newly exposed mineral soil. Late-arriving seedlings face fierce competition, which slows repopulation.

How Humans Can Help

A recent review of 49 studies of postfire forest regeneration in the United States found that only six of those studies directly analyzed climate metrics like moisture deficits or seasonal precipitation. Some studies did indirectly take such climate change outcomes into effect, but drawing direct links between refugia and, say, droughts could help scientists learn precisely when to intervene.

“I don’t believe we can’t find predictable refugia just because of climate change,” Morelli said.

Scientists are still figuring out when and how to support forests after fires. Updating models for how far trees can disperse seeds and adding nuance to debates about thinning forests, prescribing controlled burns, and incorporating Indigenous practices could help as fire-prone regions set new records.

Scientists are still learning to predict refugia after very large fires—and figuring out when and how to intervene.

“We as scientists need to come up with and develop tools that are user friendly for [forest] managers in order to do these more complex calculations,” Busby said.

Tough budget choices might be on the horizon, too. Although it’s tempting to prioritize saving old-growth forests and vulnerable stands of trees, balancing these efforts with protecting refugia that encourage biodiversity and regrowth could become equally important.

“We’re always going to focus on preventing fire in fire-prone regions,” Morelli said. But “some places are going to have to burn, either because we can’t stop it or because the only way we’re going to get through this period is [to accept that] some burning is going to have to happen.”

Even in forests where high-intensity fire is part of the typical pattern, too many larger fires without a healthy balance of smaller, brush-clearing blazes can wipe out refugia, according to Krawchuk. Forest managers in areas like New Mexico and Arizona that historically had patterns of small, brush-clearing fires are now paying the price for decades of fire suppression; these states may need to learn to rely on smaller, “pocket” forest refugia, Krawchuk said.

Longer, more severe fire seasons have also stretched the duties of forest managers who are responsible for various agencies’ firefighting efforts, as well as managing forest conditions that predicate fires. That increasing burden makes it tough for ecologists’ research to make its way to exhausted managers, who have less and less time to digest new best practices when fire seasons wind down.

“They’re usually grieving the loss of their landscape or perhaps even worse, and then they’re having to go back into project planning,” Krawchuk said, noting that she does not work with managers directly.

Some forest managers say they are working with scientists to incorporate the latest research into their work. Monte Williams, a USFS forest supervisor at Colorado’s Arapaho and Roosevelt national forests and Pawnee National Grassland (ARP), said his region is “luckier than most” because of its proximity to a local university and research station. “Researchers and scientists influence and often directly participate in the planning and execution of the work on the ARP,” he said. Local practitioners, in turn, provide feedback to researchers.

Still, learning when and how to intervene in the face of less predictable fires is crucial. Cultural burning practices from Indigenous groups like the Karuk, Hoopa Valley, and Northern Sierra Miwok tribes in California and the Noongar people, among many others in Western Australia, are another option. They prescribe controlled, predictable “cool burns” that clear brush and promote biodiversity. These practices largely stopped when Indigenous Peoples were removed from their lands during colonization, but recently, new partnerships have reintegrated this Traditional Ecological Knowledge.

In the Cascades, arid regions required human intervention to regrow, but wetter areas fared worse when scientists stepped in.

Given the uncontrollable nature of megafires, not everyone agrees that humans should intervene at all. “Minimizing human perturbation overall in a forest is becoming a necessary contingency given we can’t really predict where refugia will be next time,” Mackey said. He believes that many current interventions are outdated and hurt more than they help because, for example, controlled burns may predry regions that wind up burning anyway.

Researchers from Portland State University in Oregon were curious to see whether some human interventions are worse than no action at all. They analyzed forest refugia and regrowth after a fire. Sometimes, they found, replanting by humans helps forests regrow. But when humans step in unnecessarily, the outcome is worse than if they’d done nothing. Busby led the study, comparing the regrowth of wet, mesic forests with drier, desertlike regions in the Cascade Mountains. He and his team found that arid regions required human intervention to regrow, but wetter areas fared worse when scientists stepped in. Human-planted trees in wet areas filled in alongside natural regrowth, forming doubly dense tree stands—a kind of superfuel for future fires.

“There’s really no need to replant those areas even though all of the prior trees were wiped out,” Busby said of his study, which was specific to the Cascades, a wet, high-elevation forest. “There needs to be a little more evaluation done around when it is actually necessary to replant versus having this warm and fuzzy feeling of ‘we’re going to help the forest and go out there and replant’ when it might actually not be necessary at all.”

No forest is immune to climate change, even if some have avoided crude practices, like fire suppression, or controversial ones, such as salvage logging, Mackey said. And if we can’t currently predict the next refugia, is simply minimizing human impacts our best bet? Krawchuk has focused her research on figuring this out. As she’s zoomed in on climate change impacts, she’s defined climate change slow lanes, including forest refugia, that seem to resist them.

“There’s a geography that we need to understand more clearly of the variability in the speed of the change that climate change is painting across our landscapes,” she said. And if parts of the landscape can take care of themselves, there are also refugia that are teetering toward losing their protective qualities. After generations of fire interventions, fire ecologists like Krawchuk can only race against climate change with new research, hoping the right management practices will encourage forests to burn, regrow, and evolve.

—Robin Donovan (@RobinKD), Science Writer

Citation: Donovan, R. (2022), Last tree standing, Eos, 103, Published on 22 December 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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