The firefighters moved through the ponderosa pine forest in darkness, intentionally setting fires with drip torches as they moved down a hill near Mormon Lake, about 40 miles south of Flagstaff, Arizona.
A few trees burst into flame, mimicking the report of a .22, but the night was cool and moist and most of the flames stayed beneath the trees.
Shaula Hedwall, a U.S. Fish and Wildlife Service biologist who oversees recovery of the threatened Mexican spotted owl in most of the Southwest, proposed the fire to test how controlled burns and forest thinning will affect the species.
She’s testing the hypothesis that thinning and burning will protect the long-term viability of the bird’s habitat from severe wildfires by reducing the available fuel and the density of trees after a century of fire suppression.
Still, Hedwall was nervous as she watched the line of flames creep down the hill.
“You know the habitat is working now, and you’re hoping that you’re doing something that’s going to not only keep it working but also make it more sustainable in the future,” she said. “But you don’t quite know yet.”
As extreme wildfires become more common across the West, monitoring shows they can kill the tall, old forests where Mexican spotted owls live. So the USFWS has proposed using prescribed burns and forest thinning to reduce the wildfire danger, knowing the work could also potentially harm owl habitat by reducing the canopy cover or removing large trees.
Many scientists say it’s worth risking the negative effects to protect the species from fire, but others argue that thinning could do more harm than good if government agencies aren’t careful.
The Mexican spotted owl, one of three subspecies of the spotted owl, lives in tall pine forests with lots of canopy cover and dry canyon lands across the Southwest. In 1993, it was added to the list of threatened species under the Endangered Species Act, primarily because of habitat loss caused by logging those bigger, older trees.
The Fish and Wildlife Service wrote a recovery plan that prohibited most logging around Mexican spotted owl habitat, but in 2012 the agency revised that plan to account for a new threat: high-intensity wildfires.
Many wildlife species in the West evolved to survive – and in many cases, take advantage of – naturally occurring low- to moderate-severity fires. But in the last few decades, intense fires have become so frequent that biologists worry certain species won’t be able to cope.
Cutting trees and igniting prescribed burns could help mitigate that risk, but there’s not much research on how those actions will affect individual species.
Hedwall’s study is one of the first to test the effects of thinning and burning experimentally. She’s monitoring her study sites to see if burning or removing trees and then burning will affect whether owls will live or reproduce there. Her results could help land managers minimize the impacts on owl habitat while protecting the species from fire.
But that will take more tests, and time.
“We don’t have good information,” she said. “Even this study that we’re finally getting implemented has a pretty small sample size, which is why I have two other projects that will be following closely behind.”
Hedwall said management agencies are trying some treatments based on observational results, but those studies don’t offer as much as rigorous experiments with large sample sizes. Without those kinds of results, managers must rely on case studies.
Serra Hoagland, a researcher with the U.S. Forest Service’s Fire Lab in Missoula, Montana, examined one of the New Mexico case studies. She used satellite imagery to compare owl habitat in the Lincoln National Forest with owl habitat on the nearby Mescalero-Apache Indian Reservation.
She found that the places owls live on the Mescalero-Apache look different than the places owls live in the national forest. That might mean owls can live across a wider range of habitats than previously known.
“The tribe and the (Bureau of Indian Affairs) have maintained a pretty active [thinning and burning] forest management program,” she said. “Here we have places that have undergone a moderate amount of treatment within near proximity to owl sites. … So, this is the first look at how are those owls responding post-treatment.”
She’s cautious about reading too much into the findings. It’s just one example from one part of Arizona. But right now, these case studies are all that land managers have to manage fire in owl habitat.
Without more information, Joe Trudeau, an ecologist for the Center for Biological Diversity, said government agencies need to be careful when they cut trees or use fire in owl habitat. He said so far land managers have done a good job of applying these treatments judiciously, but the recovery plan only offers general guidelines and he hopes managers will continue to use thinning and burning strategically.
In many cases, he said, cutting down trees can protect owls from wildfire, but in others, it’s best to let nature take its course.
“It’s important that in the debate we don’t just boil it down to one or the other, and it’s important that we don’t just take sides based on our historical allegiances,” Trudeau said. “There’s a broad range of conditions on the landscape and it’s important that we treat each area independently and specifically for the needs of that area.”
He worries that the new recovery plan actually opens the door to the type of logging that would harm the owls and their recovery.
The current recovery plan argues that thinning forests could have some negative effects on the owls, but that the protection it provides from wildfire outweighs the negatives.
Monica Bond, a scientist at the wildlife research and advocacy organization the Wild Nature Institute who has studied several subspecies of spotted owls, questions whether that’s true.
She’s not convinced that severe wildfires are such a big threat to owls, arguing they evolved in a landscape with frequent fires and can likely survive these fires as well.
She studied the responses of California spotted owls – a separate subspecies – after the 2013 Rim fire in Yosemite National Park, and found that most owls did not abandon their nest sites. In another study in Southern California, she found that owls were less likely to inhabit areas where large, severe fires had burned, but the effect wasn’t statistically significant.
Joseph Ganey, a research scientist with the Forest Service’s Rocky Mountain Research Station, said the effect of wildfire on owls likely varies depending on how and where the fire burns, but he still thinks it’s a good idea to thin forests.
In a recent review paper, Ganey and his co-authors calculated that between 2000 and 2014, huge, high-severity wildfires destroyed enough Mexican spotted owl habitat to cover the island of Hong Kong.
He believes the long-term benefit of protecting owls from fire probably does outweigh the short-term cost of cutting down trees.
“I would actually take that further and argue there may be treatments we can develop that would reduce the fire risk without a major cost to the owl,” he said.
However, Ganey cautioned that thinning forests without cutting down big trees is expensive. The larger trees yield valuable timber, which can sometimes pay for the work, but the small trees aren’t worth much.
Controlled burns are cheaper, but some areas of owl habitat haven’t seen fire in decades and it’s not always safe to burn without cutting trees first.
Meanwhile, land managers don’t have the option of bowing out while scientists search for answers because not acting is also a management action. They must weigh the costs of severe wildfire against the potential damage caused by forest thinning without knowing how either affects the owls.
Matt Blois is a freelance reporter based in Missoula, Montana. He writes stories about science and the environment. You can find more of his stories in Science, Hakai and SciDev.