On Wednesday, the U.S. Forest Service’s Grand Mesa, Uncompahgre and Gunnison national forests confirmed that a prescribed burn started the Simms Fire near Montrose, a 313-acre blaze that destroyed one home.

Five days earlier, the agency announced that a U.S. Forest Service burn pile ignited New Mexico’s Calf Canyon Fire on April 19, which then merged with the Hermits Peak Fire to form the largest wildfire in the state’s history, destroying more than 350 homes.

The Hermits Peak Fire erupted from a 1,200-acre Forest Service prescribed burn that escaped control near Las Vegas in north-central New Mexico.

The spate of burn-induced wildfires led Forest Service Chief Randy Moore to suspend the agency’s prescribed fire operations to conduct a 90-day review of its protocols and practices.

Yet, amid renewed awareness of the risks of prescribed fire, forest ecologists and biologists say fire serves an irreplaceable role in Southwest Colorado’s mixed conifer and ponderosa pine forests, and that limiting the use of fire would do more harm than good.

As agencies and initiatives increasingly employ both thinning and prescribed fire in the names of forest health and fire mitigation, scientists are drawing a line between the two and their benefits, even as climate change increasingly makes the application of fire more difficult.

“Mechanical thinning is not so much to restore the forest, it’s fuels mitigation,” said Julie Korb, a professor of biology at Fort Lewis College who studies fire and forest ecology. “The key thing is to label (forest) treatments accordingly. Is our end goal fuels mitigation? Is it ecological restoration? Is it increasing habitat (and) species diversity? Is it increasing the function and the health of that ecosystem? Those are kind of the big questions that we need to look at.”

In its January report “Confronting the Wildfire Crisis,” the Forest Service called for renewed efforts tackling increasingly large and destructive wildfires and restoring forest health in the West.

The Forest Service’s new “wildfire crisis strategy” presented in the report includes the treatment of an additional 20 million acres of national forest land and 30 million acres of forest across other federal, state, tribal and private lands using a combination of thinning and prescribed fire.

But according to forest ecologists and biologists, the two techniques of prescribed fire and thinning should not be conflated.

Thinning is often separated into two categories – hand thinning and mechanical thinning. In hand thinning, crews use chain saws to cut down trees, selecting them based on things like tree size and density.

Mechanical thinning attempts to do the same thing but relies on machines such as feller bunchers, which look like excavators with a large sawing attachment to quickly cut trees. In mechanical thinning, scale and economics are often more of an imperative.

According to the Forest Service, thinning can benefit forests by recreating their natural structures through the regulation of density, age and species distribution. Doing so reduces stress on trees and makes them less susceptible to disease and insects like bark beetles.

But thinning, particularly mechanical thinning, can also affect forests by encouraging the growth of nonnative species and compacting soil if not done with care.

“Prescribed burning has very minimal soil disturbance, whereas some of the mechanical treatments can actually have pretty widespread soil disturbance,” said Michael Remke, a lecturer of biology at FLC, who specializes in forest and soil ecology.

Prescribed fire can also be broken down into two types. As the name suggests, pile burning involves the ignition of collected and previously cut vegetation, while broadcast burning is what one thinks of when they see wildland firefighters igniting a controlled blaze with a drip torch.

In Southwest Colorado’s fire-adapted mixed conifer and ponderosa forests, prescribed fire and naturally occurring wildfires have a number of benefits that thinning cannot replicate.

Fires recycle nutrients like nitrogen by expediting the decomposition of plants and trees.

“All fire is rapid decomposition, so all those nutrients that are bound in plants and trees and needles then breakdown from the fire and you get these big nitrogen pulses,” Korb said. “You take nitrogen that’s unavailable to plants and you make it available as forms of ammonium and nitrate. It’s just like people do with their garden that they have to work the soil (and) add nutrients to it and that’s what fire does.”

Fire creates habitat and spurs regrowth. Trees that are left dead or fallen serve as habitat for birds and small mammals while also providing shade for tree seedlings to grow, Remke said.

Fire also allows forest managers to treat forests more quickly and at larger scales.

But perhaps the most important advantage of fire is its ability to naturally restore the diversity of forest ecosystems, what scientists call “heterogeneity.”

Forests vary according to local terrain and climates with differences in moisture, sunlight and soil, among others, determining the tree and plant species that could thrive there. That diversity made forests more resilient to wildfires, insects and pests.

Fires historically regulated that diversity by disturbing the forest and resetting the distribution of trees and plants.

Though forest managers attempt to replicate that process with thinning, the complexities of mixed conifer and ponderosa pine ecosystems, which have evolved with fire over eons, make it difficult if not impossible to replace fire.

“The reason why ecologists want fire back in those systems is because when we talk about ecological restoration we talk about restoring ecosystems structure and function,” Korb said. “In order to return the function of that ecosystem, you need to have fire as part of that management plan.”

At its core, the debate between thinning and prescribed fire is one of purpose.

Thinning can achieve what it is intended to do, which is reduce wildfire risk, but it cannot be a replacement for fire and does not address forest restoration, said Bill Baker, an emeritus professor of ecology at the University of Wyoming who wrote the book “Fire Ecology in Rocky Mountain Landscapes.”

“If the goal is restoration, the problem mechanical thinning has is that we don’t have a very good understanding of what those forests really looked like, historically,” he said.

Ecologists believe that mixed conifer and ponderosa pine forests evolved a particular organization called the “ICO structure.” ICO stands for individual, clumps and openings, meaning that forests evolved to have areas with gaps between individual trees, areas where trees of different sizes were clumped closely together and openings where there were no trees.

That structure represented a “healthy” forest.

But after European colonization and the drastic reshaping of the West’s forests, in part for timber production, ecologists and forest managers have little sense of what that looked like locally in Southwest Colorado’s forests.

“There’s no question that mechanical thinning has a lot of problems in terms of just recreating that (ICO structure), because first of all we don’t know much about the details of it,” Baker said.

Conversely, fire underpinned those forest ecosystems, so reintroducing fire will help them to revert to their historic structure and “restore” them, Baker said.

With cautious hand thinning and mechanical treatments, forest managers can replicate some of the effects of fire, however, it’s more difficult to do so.

“It just takes more intentionality and effort to create that variability that a fire naturally does on itself,” Remke said.

Remke argues that limited intervention in wildfires when possible can often do the most good for restoring forests. He pointed to the 416 Fire, which achieved many of the outcomes that ecologists and forest managers would want, including creating openings and restructuring tree density.

“We don’t always need interventions for forest health,” he said. “Even though it sometimes seems like the most appropriate conservation tool is to intervene, I would say stepping back and recognizing that forest processes have operated in these ways for a long time can also be a really effective conservation tool.”

For Remke, Baker and Korb, there is simply no replacement for using fire.

“All we have to do is reintroduce it (and) put it back in these forests and over time they will revert back to this (ICO) structure,” Baker said. “It may take decades, but it is essentially the force that structured them originally and it will do it again.

“We’re much more likely to be able to restore these forests in a way that will allow them to persist if we use the natural force, which is fire, to reset them and restore them.”

When Moore announced the Forest Service’s burn pause, Colorado’s 3rd Congressional District Rep. Lauren Boebert immediately criticized the decision, saying the suspension of prescribed fire would harm Colorado’s rural communities.

According to Moore’s news release, 99.84% of the agency’s prescribed burns are successful and safe, but more than 90% are conducted between September and May.

That’s in part because of safety concerns.

Forest managers do not perform prescribed burns when the weather is too hot, conditions are too dry or firefighting resources are insufficient. The Bureau of Land Management postponed the prescribed burn on Animas City Mountain north of Durango this spring because of dry and windy conditions that made burning risky.

“We only use fire when it’s safe for the public and for the resource,” said Scott Owen, spokesman for San Juan National Forest. “And we honestly haven’t had many days where it is safe for us to do prescribed burning over the last couple years.

“We want to make sure that we’re able to contain that fire no matter what, and we don’t ever want to put the public in a situation where it escapes,” he said.

Yet, according to Remke, Baker and Korb, forest managers and the communities they serve need to discuss how thinning and prescribed burns and the issue of forest health fit into safety, especially as climate change makes it increasingly difficult to carry out prescribed burns and exacerbates wildfires.

A study published last month in the journal Science of the Total Environment found that the window for prescribed burns in the winter and spring was decreasing by one day per year in parts of California.

A significant body of research also suggests that climate change is already making wildfires larger, more frequent and more severe, and will continue to do so.

“If there’s anything that we can expect right now, it’s more fire and that means that fire preparedness and (our) readiness to accept fire on the landscape needs to be higher than it’s ever been before,” Remke said.

Thinning can be an important tool in the wildand urban interface, or WUI, where fires are started most frequently and can be most destructive for communities, Korb said. The hope is that the thinning changes fire behavior and in doing so mitigates some of the risk to communities.

However, Baker is less sure that thinning will have its intended effect as climate change continues to dry out Southwest Colorado’s forests.

“Part of the problem is that as the fuels dry out, they become more available to participate in the burn,” he said. “Even though you maybe reduced the amount of fuel (through thinning), you still have fuel there. If all of it becomes available to burn because it’s dried out, you still can have quite a blaze.”

Prescribed fire, as a driver of natural selection, is the best way to increase community resiliency against wildfires, Baker said.

“The argument has been made, and I think this is a very sound argument, that it’s much better to use fire to restore these forests because future forests need to be adapted to more fire,” he said. “Fire is the force that can right now go through and select the trees that are unlikely to be able to survive future fire. We can’t do that mechanically.”

After a spring in which prescribed fires have started multiple fires, forest managers and communities in Southwest Colorado must decide what risks they are willing to take and the role they envision for fire in restoring forests and creating resilient communities.

“It’s like being a lion tamer,” Baker said. “Fire is a huge, dangerous force that really can cause a lot of damage to people and infrastructure, but we’re going to have more fire so we can make some choices about it.”

ahannon@durangoherald.com