Arizona scientists dig into soil under wildfire burn scars for clues about ecosystem recovery

·10 min read
Boulders, logs and other debris sit in a a flood channel created after a 2021 storm over the Museum Fire burn scar near Flagstaff. The flow was able to move boulders approximately 1 meter in size.
Boulders, logs and other debris sit in a a flood channel created after a 2021 storm over the Museum Fire burn scar near Flagstaff. The flow was able to move boulders approximately 1 meter in size.

In the aftermath of a wildfire, what looks like an earthy creature sometimes seems to emerge out of the charred landscape.

When the rain washes through a burn scar up in the mountains, the creature raises its snout — a “coarse, boulder-y head,” as scientist Ann Youberg describes it — and begins to tumble down a channel. On its way down, its body deposits what Youberg calls “levees,” trails of sediment and rocks on either side that keep the creature confined on its path.

Eventually the weight of its own head stops it from moving forward, but its power doesn’t stop there. If it hasn’t already changed an ecosystem or wreaked havoc on a town or city, it still can. Floodwaters carry parts of the creature into streams and drainage systems, caking them with earth, sometimes carried far from the scarred area where a fire once raged.

The creature is called a debris flow, and Youberg has been chasing them for much of her career as a senior research scientist with the Arizona Geological Survey at the University of Arizona. She currently leads the geohazards group and focuses on improving predictions of where and when debris flows will likely form, how big they will become and who or what might be impacted by their tumultuous journeys down from the remote wilderness toward populated areas.

Youberg says it’s critical to study debris flows because they often create messes that are both harmful to human life and expensive to clean up. Post-fire floods can be deadly, but when a debris flow barges into the picture, it can leave behind enormous deposits of sediment. Youberg says a debris flow contributed to the over 100,000 pounds of sediment carried by floods into east Flagstaff two years after the 2019 Museum Fire. The flood damage cost over $1 million to clean up.

But Youberg isn’t interested in debris flows only because of the cost. She, like many other Arizona scientists, is interested in the geology of the land itself.

Youberg, like other scientists, can tell you about the immediate impacts of fire on the terrain it scorches. Fires turn the soil hydrophobic, for instance, meaning that it repels rather than absorbs water. That can contribute to erosion, or in worse cases, debris flows like the ones Youberg studies. It can also worsen floods.

But she and others also want to study the bigger picture, and what the larger, more intense fires of the period after the 2002 Rodeo-Chediski Fire mean for the future of the landscape.

Rodeo-Chediski scarred Arizona forests: How have they changed in 20 years?

“If you can divorce yourself from the fact that these ecosystems that we love are being threatened, this is the time that gets stored in the geologic record, because there's so much change,” she said. “All of this soil and these hills, slopes are vulnerable to erosion. And so you get big pulses of sediment in the geologic record. So if you can divert yourself from the fact that we're losing these really cool places, it's a cool time geologically.”

Soils tell the history of wildfires

University of Arizona graduate student Alexander Gorr, right, and Ann Youberg measure soil infiltration properties to assess runoff potential in an area burned by the 2021 Telegraph Fire near Globe.
University of Arizona graduate student Alexander Gorr, right, and Ann Youberg measure soil infiltration properties to assess runoff potential in an area burned by the 2021 Telegraph Fire near Globe.

Craig Rasmussen agrees. A professor of environmental science at the University of Arizona, Rasmussen studies soils and their relationship to fire. He described fires not as a single point in time but in the context of the geologic record, a history written on the ground.

“Deeper in the soil profile, there is a legacy of (any) wildfire,” he said. “Even down to a meter in the soil, we can see a legacy of this charred, organic matter stored for a long period of time. … It kind of spans from what's going to happen the week after the fire, and you get a monsoon storm, to a thousand years down the road.”

But that history is changing and now scientists who study geology, soil ecology, hydrology and other physical characteristics of the land after wildfires want to examine, in detail, a new era of megafires. It’s raising new questions, some of which are just beginning to be answered, and opening the door for new kinds of research and solutions. And it will only continue to grow as climate change exacerbates the scope of fires in Arizona.

“I've been digging holes in forests for over 20 years and I have yet to dig a hole in a forested system where we have not seen evidence of fire. … It's actually a critical component of that soil ecosystem itself,” Rasmussen said. “But I think one of the ongoing questions is, with more frequent, high-intensity fires in the same areas, do those systems ever get a chance to recover or do they get stuck heading off into a different direction?”

Don Falk, a professor at the UA’s School of Natural Resources and the Environment, is concerned that humans have actually been preventing the full process of ecosystem recovery after fires.

“Fire plays this critical recycling role, recycling nutrients,” he said. But human-caused climate change and fire suppression practices, he says, have changed the ways fire affects the landscape. “That said, the high severity component of the fires we're seeing today, especially during this extended severe drought and period of high temperatures, that's a toxic combination and that is absolutely producing soil and hydrologic effects that we don't think are characteristic of the system as it's operated for a very long time.”

Youberg thinks science has come further toward understanding those new soil and hydrologic effects than at the turn of the 21st century, but says there’s still a long way to go.

“We didn't know much (about post-wildfire debris flows) when Rodeo-Chediski happened,” she said. She also described how, since Rodeo-Chediski, multiple massive fires have started in the same location, significantly changing the foundation for debris flow research.

“The problem is that we're getting high-severity fires back to back to back," she said. "And that's not the way it has been historically."

What happens when an area burns again

In 2017, the Pinal Fire burned through Tonto National Forest. Luke McGuire, an assistant professor of geosciences at the University of Arizona, set up his monitoring equipment there.

He put in a rain gauge, to measure rainfall intensity, and pressure transducers — small cylindrical devices drilled into the bedrock that measure the amount of pressure that flooding or debris flows exhibit on the ground. The goal was to understand how much rain might lead to a flood or a debris flow, and how factors like the steepness of the terrain or burn severity play into the equation.

But in 2021, the Telegraph Fire burned over the study site. “Mostly we were wondering, ‘did our equipment survive?’ And luckily it did,” McGuire said.

Ann Youberg, right, and University of Arizona graduate students Olivia Hoch, center, and Alexander Gorr examine a debris flow deposit in an area burned by the 2021 Telegraph Fire near Globe.
Ann Youberg, right, and University of Arizona graduate students Olivia Hoch, center, and Alexander Gorr examine a debris flow deposit in an area burned by the 2021 Telegraph Fire near Globe.

That’s when his team got excited. The equipment was scorched, but intact. That meant they could look in detail for the first time at a reburn site.

“Fire has become frequent enough now that we are burning areas where we didn't plan for a fire experiment, but they burned anyway. And now you have a fire experiment, so you can pull samples from those same profiles you were monitoring before to play with background conditions and now get that post-fire response,” McGuire said.

McGuire thinks the results they find will be important because even if these geological changes don’t create impacts that do make it downstream, they’re still out there — and they will help scientists like him to create models that can help better predict future events. For now, generally speaking, the worst-case scenario is a high-severity fire in an area with steep terrain, McGuire says. But he wants to get much more detail than that.

“One thing that we've noticed from our research in Arizona is that we need to stop studying fire in terms of hazards in isolation. We can't just think anymore about ‘there was a fire, what's going to happen as a result of this one fire?’ There's a history of fire in all of these landscapes. And that history is important," he said. "And that's one thing that's come out of our research."

On McGuire’s website, interspersed with pictures of his team installing equipment or hiking through remote terrain, computer-generated images that look a bit like heat maps illustrate changes to the landscape in a language of math and computer science. It’s research McGuire hopes will improve with new instrumentation and technology and that he thinks must evolve to keep pace with more frequent, severe fires.

“A lot depends on the geologic setting of where the fire occurs, and it's something that's been understudied because there are so many factors that could influence things like burn severity, burn history, soil texture, parent material, regional climate, all these things," McGuire said. "And it's tough to tease out which are the most important."

Preventing erosion with moss

Northern Arizona University professor and biocrust researcher Matthew Bowker is studying the potential of biocrust mosses to restore wildfire-burned areas.
Northern Arizona University professor and biocrust researcher Matthew Bowker is studying the potential of biocrust mosses to restore wildfire-burned areas.

Matthew Bowker has been interested in moss for a long time.

Several years ago he was working on a post-fire survey and came across a tapestry of mosses and liverworts that had blanketed the ground in the year since the burn had occurred. All of a sudden, he had questions. When and why do mosses grow after a fire? And can humans coax that process to happen as a protective measure?

Since then, Bowker, an associate professor in the School of Forestry at Northern Arizona University, has been trying to figure out the best way to grow mosses on demand, in hopes of preventing soil erosion. His efforts are related to an ongoing category of research on protecting the biocrust, or the desert’s natural protective skin. But using mosses specifically after fires is an idea that he has not been deterred from, even in an age of megafires.

Bowker called the Rodeo-Chediski Fire a turning point, “because it was such a game changer in terms of scale,” he says. But even as he has watched bigger, more severe fires multiply in recent years, he thinks the value of protective moss measures remains the same.

Forest management: How thinning dense Arizona forests could prevent another megafire

“A lot of the techniques that we use now (after wildfires), they’re aimed at emergency stabilization,” Bowker said. “There's a whole battery of techniques called BAER, (or) burned area emergency response. These include treatments like mulching or there may be seedlings in some cases. And sometimes they're helpful and sometimes they're not. But they're definitely not perfect. It could be valuable to have some more tools in the toolbox, so to speak.”

Bowker’s mossy toolkit has so far involved small-scale trials to test out the tactics inherent in moss biology: They grow on the soil surface, extending tiny anchoring structures called rhizoids that weave through the surface of the soil. That should create what Bowker calls a “fuzzy green skin,” one that helps with moisture absorption and prevents soil from eroding away.

Northern Arizona University professor and biocrust researcher Matthew Bowker studies the potential of biocrust mosses to restore wildfire-burned areas.
Northern Arizona University professor and biocrust researcher Matthew Bowker studies the potential of biocrust mosses to restore wildfire-burned areas.

The project hasn’t been without its challenges. His team found out the hard way that mosses make a perfect post-fire snack for ants, and they’ve had to try out some anti-ant measures, such as planting the mosses in protective pellets, to keep them from being nibbled away. This fall, they’ll be perfecting those pellets in a small burned patch by their greenhouse.

Despite those roadblocks, Bowker hopes they’ll get to try the project out on a real fire site by 2023.

“We don't think of our method as something that's going to be the magic bullet that's going to solve all the erosion problems and all flooding problems immediately,” Bowker said. “But we think it could be potentially important for the long-term ecosystem recovery, to kind of get that help, get that soil stabilized and get something growing on the ground that's a native plant.”

Independent coverage of bioscience in Arizona is supported by a grant from the Flinn Foundation.

Melina Walling is a bioscience reporter who covers COVID-19, health, technology, agriculture and the environment. You can contact her via email at mwalling@gannett.com, or on Twitter @MelinaWalling.

This article originally appeared on Arizona Republic: What larger, more intense fires mean for the future of Arizona's land