Nav: Home

Post-wildfire erosion can be major sculptor of forested western mountains

April 18, 2016

Erosion after severe wildfires can be the dominant force shaping forested mountainous landscapes of the U.S. Intermountain West, suggests a new research paper by two University of Arizona geoscientists.

The study is the first to assess the impact of wildfires on such landscapes by combining several different ways to measure short-term and long-term erosion rates, said study co-author Jon Pelletier, a UA professor of geosciences.

After the 2011 Las Conchas fire in New Mexico, soil and rock eroded from burned watersheds more than 1,000 times faster than from unburned watersheds nearby, the team found. Most of the erosion happened in the first year after the fire.

Caitlin Orem had been studying erosion in seven mountain watersheds near New Mexico's Valles Grande but wasn't focused on wildfire. When the Las Conchas fire burned two of her study areas, she seized the opportunity to compare the rates that watersheds were denuded of soil and rocks before and after a severe wildfire.

"We knew that wildfire increased the rate of erosion, but we didn't know how important it was over long time scales," said first author Orem. The research was part of her doctoral studies in the UA Department of Geosciences.

"It was a really huge opportunity to learn a lot about wildfires. There are very few times you can see that big of a change and can actually document it."

Orem and Pelletier calculated total erosion rates for their study area for time scales up to 1 million years ago. The scientists found more than 90 percent of the erosion happened in the geologically brief time intervals right after forest fires. Those post-fire intervals constituted just 3 percent of the total time.

The research is part of the UA Santa Catalina Mountains & Jemez River Basin Critical Zone Observatory, a project funded by the National Science Foundation.

Pelletier, co-director of the CZO, said, "I think we can generalize this to similar landscapes in the Intermountain West - landscapes that are forested, have very little bare ground, and have few areas with slopes steeper than about 25 degrees."

The paper, "The predominance of post-wildfire erosion in the long-term denudation of the Valles Caldera, New Mexico," by Orem, now a geologist for BP in Anchorage, Alaska, and Pelletier, has been accepted for publication in the Journal of Geophysical Research: Earth Surface, a publication of the American Geophysical Union.

In 2010, Orem and Pelletier began studying the role of erosion in sculpting the mountain watersheds that drain into the Valles Grande, including those on Redondo Peak and on Cerro del Medio.

Pelletier said, "The goal is to determine topographic change--the volume that has been removed or deposited."

Working with the National Center for Airborne Laser Mapping, the UA geoscientists used a technology called LIDAR (Light Detection and Ranging) to create a digital map, or digital elevation model, showing the area's surface relief at that time.

Although Redondo Peak, one of the team's study areas, now has steep-sided ridges and deep valleys, previous investigators showed that Redondo Peak formed 1.24 million years ago as a rounded volcanic dome.

To calculate long-term erosion rates for Redondo Peak, the team needed to figure out how much rock and soil had been stripped off the mountain and how long it took for that material to be removed.

Using the digital elevation model, known as a DEM, Orem calculated the volume of material that eroded from the original dome over time. Dividing that volume by the mountain's age gives the average long-term erosion rate.

Such a long-term rate incorporates many different events in the mountain's history. The researchers already knew events such as flood or wildfire could increase erosion rates.

The team corroborated their DEM-based calculation by measuring how much beryllium-10 had accumulated in the soil. Pelletier said beryllium-10 analyses provide an "erosion clock" over time scales of thousands of years.

To calculate the day-in, day-out background rate of erosion in the absence of disturbance, Orem, Pelletier and colleagues took regular samples of stream water from the Redondo Peak watersheds from 2008 to 2012. By measuring the amount of sediment suspended in the water, Orem calculated the background rate of erosion.

The researchers found the long-term erosion rate for Redondo Peak was 100 times greater than the background rate, indicating erosion rates on the mountain had been greater in the past. Redondo Peak had no wildfires during the time the team took stream samples.

A nearby mountain with similar terrain, Cerro del Medio, had a severe forest fire in 2011, giving the team the opportunity to measure post-wildfire erosion directly. Post-fire, the increase in erosion was obvious--boulders the size of office desks had rolled down the slopes and into the meadow below.

The team had already made a pre-fire digital elevation model, or DEM, of two Cerro del Medio watersheds. The team made new DEMs of the changing landscape right after the fire and again 10, 13 and 22 months later.

By comparing the pre-fire DEM to the series of post-fire DEMs, the scientists found the burned watershed lost 1,000 to 10,000 times more rocks and soil in the first year after the fire than did a similar but unburned watershed on Redondo Peak.

The researchers calculated that over a million years, if such post-wildfire erosion occurred for a year just once every 30 to 300 years, enough material would be removed to sculpt Redondo Peak's original dome into the steeply incised mountain it is today.

"Over millennia there's a gradual transfer of soil from high spots to low spots," Pelletier said. "Most of the post-fire erosion is in the streambed. In the time period between fires, soil is still moving, but it's moving to fill in the hole created by the flooding just after the fire."

The team's estimate of past wildfire frequency matches what other researchers found by studying the natural records of wildfires contained in the region's tree rings and lake sediments.

Orem said, "Other researchers have found that in the Western U.S., the area being burned and the severity of the burns are increasing. With that increase, we expect to see more wildfire-caused erosion."
The Geological Society of America also funded part of the research.

Researcher contacts:

Jon Pelletier
University of Arizona Dept. of Geosciences

Caitlin Orem

Media contact:

Mari N. Jensen
University of Arizona

Related Web sites:

Jon Pelletier

UA Santa Catalina Mountains & Jemez River Basin Critical Zone Observatory

University of Arizona

Related Wildfires Articles:

Study synthesizes what climate change means for Northwest wildfires
A synthesis study looks at how climate change will affect the risk of wildfires in Washington, Oregon, Idaho and western Montana.
Climate change increases the risk of wildfires confirms new review
Human-induced climate change promotes the conditions on which wildfires depend, increasing their likelihood -- according to a review of research on global climate change and wildfire risk published today.
Fire blankets can protect buildings from wildfires
Wrapping a building in a fire-protective blanket is a viable way of protecting it against wildfires, finds the first study to scientifically assess this method of defense.
Stanford researchers have developed a gel-like fluid to prevent wildfires
Scientists and engineers worked with state and local agencies to develop and test a long-lasting, environmentally benign fire-retarding material.
UCI team uses machine learning to help tell which wildfires will burn out of control
An interdisciplinary team of scientists at the University of California, Irvine has developed a new technique for predicting the final size of a wildfire from the moment of ignition.
New wildfire models to predict how wildfires will burn in next 20 minutes
While it's impossible to predict just where the next wildfire will start, new Department of Defense-sponsored research from BYU's Fire Research Lab is getting into the microscopic details of how fires initiate to provide more insight into how wildfires burn through wildland fuels.
Tiny airborne particles from wildfires have climate change implications
Wildfires are widespread across the globe. They occur in places wherever plants are abundant -- such as the raging fires currently burning in the Brazilian Amazon.
How California wildfires can impact water availability
A new study by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) uses a numerical model of an important watershed in California to shed light on how wildfires can affect large-scale hydrological processes, such as stream flow, groundwater levels, and snowpack and snowmelt.
Wildfires could permanently alter Alaska's forest composition
A team of researchers led by Lawrence Berkeley National Laboratory projected that the combination of climate change and increased wildfires will cause the iconic evergreen conifer trees of Alaska to get pushed out in favor of broadleaf deciduous trees, which shed their leaves seasonally.
How wildfires trap carbon for centuries to millennia
Charcoal produced by wildfires could trap carbon for hundreds of years and help mitigate climate change, according to new research.
More Wildfires News and Wildfires Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Teaching For Better Humans 2.0
More than test scores or good grades–what do kids need for the future? This hour, TED speakers explore how to help children grow into better humans, both during and after this time of crisis. Guests include educators Richard Culatta and Liz Kleinrock, psychologist Thomas Curran, and writer Jacqueline Woodson.
Now Playing: Science for the People

#556 The Power of Friendship
It's 2020 and times are tough. Maybe some of us are learning about social distancing the hard way. Maybe we just are all a little anxious. No matter what, we could probably use a friend. But what is a friend, exactly? And why do we need them so much? This week host Bethany Brookshire speaks with Lydia Denworth, author of the new book "Friendship: The Evolution, Biology, and Extraordinary Power of Life's Fundamental Bond". This episode is hosted by Bethany Brookshire, science writer from Science News.
Now Playing: Radiolab

Dispatch 3: Shared Immunity
More than a million people have caught Covid-19, and tens of thousands have died. But thousands more have survived and recovered. A week or so ago (aka, what feels like ten years in corona time) producer Molly Webster learned that many of those survivors possess a kind of superpower: antibodies trained to fight the virus. Not only that, they might be able to pass this power on to the people who are sick with corona, and still in the fight. Today we have the story of an experimental treatment that's popping up all over the country: convalescent plasma transfusion, a century-old procedure that some say may become one of our best weapons against this devastating, new disease.   If you have recovered from Covid-19 and want to donate plasma, national and local donation registries are gearing up to collect blood.  To sign up with the American Red Cross, a national organization that works in local communities, head here.  To find out more about the The National COVID-19 Convalescent Plasma Project, which we spoke about in our episode, including information on clinical trials or plasma donation projects in your community, go here.  And if you are in the greater New York City area, and want to donate convalescent plasma, head over to the New York Blood Center to sign up. Or, register with specific NYC hospitals here.   If you are sick with Covid-19, and are interested in participating in a clinical trial, or are looking for a plasma donor match, check in with your local hospital, university, or blood center for more; you can also find more information on trials at The National COVID-19 Convalescent Plasma Project. And lastly, Tatiana Prowell's tweet that tipped us off is here. This episode was reported by Molly Webster and produced by Pat Walters. Special thanks to Drs. Evan Bloch and Tim Byun, as well as the Albert Einstein College of Medicine.  Support Radiolab today at