CU Boulder scientist’s 40-year census research finds that climate change has tripled tree mortality and forestalled regeneration

Criticizing the Biden administration’s $3.5 trillion Inflation Reduction Act, a U.S. Senate candidate in Georgia singled out funding to plant and protect trees.

“They continue to try to fool you that they are helping you out. But they’re not. Because a lot of money, it’s going to trees,” GOP candidate Herschel Walker said while stumping at a fundraiser. “We got enough trees—don’t we have enough trees around here?”

A 2015 study in Nature estimated there are 3 trillion trees on the planet. Whether or not that’s “enough,” the survey also found that “the global number of trees has fallen by approximately 46% since the start of human civilization.”

Top of page: 1875 City of Boulder Reservoir, photographer J.B. Sturtevant (“Rocky Mountain Joe”), courtesy of the Carnegie Library for Local History, Boulder Public Library. Above: Tom Veblen, distinguished professor emeritus of geography, in approximately the same spot as the 1875 photo. Photo By Glenn Asakawa.

And according to a University of Colorado Boulder scientist who has been monitoring the health and number of trees in the Colorado high country for more than four decades, climate-driven changes in temperature and drought have not only tripled tree mortality rates in the past two decades, but also significantly undermined tree regeneration rates. 

And that matters.

“If we are losing forest cover, we are going to lose a variety of ecosystem services,” says Tom Veblen, Distinguished Professor emeritus of geography, who has been tracking changes in thousands of trees on Niwot Ridge west of Boulder since 1982. 

Declining tree cover results in damage to watersheds as debris flow and flooding increase, and in the loss of habitat for certain species. Perhaps most destructive, the loss of “above-ground biomass” removes a vital source of carbon storage, which further fuels climate change. 

“In most simulation models of ecosystem impacts of climate change . . . the trees grow back after fire. But we’re not seeing that as documented for montane forests in Colorado,” Veblen says. That results in “one of those nasty, somewhat unexpected positive-feedback loops that speeds up climate change because there is more carbon dioxide in the atmosphere. Even a politician in Georgia will potentially be affected by that.”

Veblen came to CU Boulder in 1981 after six years of research in Chile and New Zealand, which taught him the value of establishing plots where trees could be observed long-term. 

“I knew from my research experience in the Southern Hemisphere that I wanted to put in permanent forest plots, which are essential for understanding long-term changes in tree populations,” he says. “There is no substitute for that.”

With money from a short-lived program funded by the state of Colorado, he and his students established 40 “long-term monitoring plots,” marked 8,000 trees on Niwot Ridge and have been monitoring them ever since. 

“The proposal . . . was to assess the influence of climate variability on tree demography and population changes, mortality, and the establishment of new seedling recruitment (new trees),” Veblen says. A second goal was to study the effects of 19th-century fires on lower elevation ponderosa pine and Douglas fir forests.

One of the key findings from Veblen’s research: While tree mortality rates remained low and stable until 1994, they have tripled since then, even in higher elevation Englemann spruce and lodgepole pine forests. 

“That’s not at all surprising . . . given increasing temperatures and increasing drought,” Veblen says, noting that researchers have reached the same conclusions at locations across the western United States.

Meanwhile, new trees are not filling in the gaps.

Former CU Boulder graduate student Robert Andrus, now a postdoctoral researcher at Washington State University, harvested about 1,000 juvenile trees to determine their establishment dates and found that new trees grew in “pulses of single years, cooler, moister years, based on late spring and summer weather conditions,” Veblen says. 

But the occurrence of such years has plummeted by two-thirds in the latter half of the seven-decade record Andrus examined. 

“Without cool, moist years, we’re not getting establishment” of new seedlings, including after fires, Veblen says. “That’s an indicator of what is likely to continue with warming temperatures.”

Even lodgepole pines, famous for colonizing burned areas—the tree’s cones open after exposure to fire—are failing to regenerate in some places. In areas torched by severe fires in 2002 in the Routt and White River national forests that have been repeatedly sampled over a 15-year period, there are only sparse and patchy seedlings of this fire-adapted species, which usually take root within a year or two. 

 

If we want to have forests after fires, we need to not rely on natural regeneration. We need to invest heavily in artificial regeneration."

Those trends have convinced Veblen and other researchers and forest managers that Western forests need a helping hand from humanity, particularly after destructive wildfires. 

“If we want to have forests after fires, we need to not rely on natural regeneration. We need to invest heavily in artificial regeneration,” the cultivation and planting of seedlings in strategic areas, Veblen says.

Andrus agrees. “We have bark beetle outbreaks and wildfires that cause very obvious mortality of trees in Colorado. But we’re showing that even in the areas that people go hiking in and where the forest looks healthy, mortality is increasing due to heat and dry conditions alone,” adding:

“It’s an early warning sign of climate change.”

Veblen and the fire management community broadly agree that “living with fire” is increasingly challenging, as shown by modeling projections that say, “Exceptional fire seasons like 2020 will become more likely, and wildfire activity under future extremes is predicted to exceed anything yet witnessed.”  

In Wildland Urban Interface areas, so-called “red zones” that are abundant throughout the West, Veblen has recommendations: Property owners must still establish “defensible spaces.” Building codes should be used to require less-flammable building materials. “Fuels reduction” through a combination of tree cutting and prescribed fires should be prioritized near settled areas to give firefighters a foothold. 

However, Veblen says, in more remote areas, mechanical thinning alone is not effective and not practical. Instead, he says, managers are increasingly emphasizing the value of letting wildfires do the work of reducing fuels and buffering against future fire potential.  

“Agencies previously tended to strongly emphasize mechanical thinning to reduce fuels, but under the kind of extreme weather conditions that promoted the 2020 East Troublesome fire, no practical amount of fuel management can fully protect homes and communities,” he says.

Instead, he’d like to see resources currently dedicated to remote-area fuels reduction be redirected into seedling cultivation and planting in selected, suitable areas.

“We are not going to be able to prevent large, severe fires, so we need to be much more strategic in investing our resources to avoid or delay some of the worst outcomes of climate change,” he says. 

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Scientists explain why wildfires are happening, why we can't put out every fire and how to move forward in an ever-changing climate​

It can be tempting to think that the recent wildfire disasters in communities across the West were unlucky, one-off events, but evidence is accumulating that points to a trend.

In a , we found a 246% increase in the number of homes and structures destroyed by wildfires in the contiguous Western U.S. between the past two decades, 1999-2009 and 2010-2020.

This trend is strongly influenced by major fires in 2017, 2018 and 2020, including destructive fires in Paradise and Santa Rosa, California, and in Colorado, Oregon and Washington. In fact, in nearly every Western state, more homes and buildings were destroyed by wildfire over the past decade than the decade before, revealing increasing vulnerability to wildfire disasters.

What explains the increasing home and structure loss?

Surprisingly, it’s not just the trend of , or simply . While those trends play a role, increasing home and structure loss is outpacing both.

As fire scientists, we have spent decades studying the and , in both and . It’s clear that the current in the Western U.S. has human fingerprints all over it. In our view, now more than ever, humanity needs to understand its role.

Wildfires are becoming more destructive

From 1999 to 2009, an average of 1.3 structures were destroyed for every 4 square miles burned (1,000 hectares, or 10 square kilometers). This average more than doubled to 3.4 during the following decade, 2010-2020.

Nearly every Western state lost more structures for every square mile burned, with the exception of New Mexico and Arizona.

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Humans increasingly cause destructive wildfires

Given the damage from the wildfires you hear about on the news, you may be surprised to learn that . This is, in part, because the majority of area burned (65%) is still due to lightning-ignited wildfires, often in remote areas.

But among wildfires that do burn homes or other structures, humans play a disproportionate role – 76% over the past two decades were started by unplanned human-related ignitions, including backyard burning, downed power lines and campfires. The area burned from human-related ignitions rose 51% between 1999-2009 and 2010-2020.

This is important because wildfires started by human activities or infrastructure have and characteristics that can make them more destructive.

Unplanned human ignitions typically and that dry out easily and burn quickly. And people have built more homes and buildings in areas surrounded by flammable vegetation, with the number of structures , with every state contributing to the trend.

Human-caused wildfires also beyond the summer months when lightning is most common, and they are particularly destructive during late summer and fall when they .

As a result, of all the wildfires that destroy structures in the West, human-caused events typically structures for every square mile burned, compared to lighting-caused events.

,

The December 2021 Marshall Fire that destroyed more than 1,000 homes and buildings in the suburbs near Boulder, Colorado, . Powerful winds racing through neighborhoods and vegetation that was unusually dry for late December.

As human-caused leaves vegetation more flammable later into each year, the consequences of accidental ignitions are magnified.

Putting out all fires isn’t the answer

This might make it easy to think that if we just put out all fires, we would be safer. Yet a focus on is, in part, what . Fire risks just accumulate for the future.

The amount of flammable vegetation has increased in many regions because of an absence of burning due to emphasizing fire suppression, preventing and a fear of fire in any context, well exemplified by . Putting out every fire quickly removes the positive, in Western ecosystems, including clearing away hazardous fuels so future fires burn less intensely.

How to reduce risk of destructive wildfires

The good news is that people have the ability to affect change, now. Preventing wildfire disasters necessarily means minimizing unplanned human-related ignitions. And it requires more than message that “only you can prevent forest fires.” Infrastructure, like downed power lines, has caused some of the deadliest wildfires in recent years.

Reducing wildfire risks across communities, states and regions beyond individual actions. We need innovative approaches and for , provide power and , as well as mechanisms that ensure changes work .

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Actions to reduce risk will vary, since how people live and how wildfires burn vary widely across the West.

States with large tracts of land with little development, like Idaho and Nevada, can accommodate widespread burning, largely from lighting ignition, with little structure loss.

California and Colorado, for example, require different approaches and priorities. Growing communities can in flammable landscapes, support , and when wildfires do threaten communities.

remains the elephant in the room. Left unaddressed, warmer, drier conditions will exacerbate challenges of living with wildfires. And yet we can’t wait. Addressing climate change can be paired with in an increasingly flammable West.

, Professor of Fire Ecology, ; , Associate Professor of Geography and Director, Earth Lab, ; , Ph.D. Student, Dept. of Geography, , and , Director of the Earth Lab Analytics Hub,

This article is republished from under a Creative Commons license. Read the .

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Homes that survived the Marshall Fire harbored another disaster inside – here’s what we’ve learned about this insidious urban wildfire risk

On Dec. 30, 2021, on record in Colorado swept through neighborhoods just a few miles from our offices at the University of Colorado Boulder. The flames , yet when we drove through the affected neighborhoods, some houses were still completely intact right next to homes where nothing was left to burn.

Although the people who lived in these still-standing homes were spared the loss of everything they owned, when they returned after the fire, .

The aftermath of the Marshall Fire in Louisville, CO. 

Noxious smells and ash on their windowsills and doorways initially made their homes unlivable – and potentially hazardous to human health. Some of these residents were still reporting health problems from being in their homes months later, even after the homes had been cleaned.

We study wildfires and their , and we knew people who lost their homes in the Marshall Fire. We also knew we had to act fast to study the fire’s impact so lessons from the Marshall Fire could help homeowners elsewhere avoid similar hazards in the future.

Dangerous chemicals absorbed into homes

Early on, because of our expertise on air quality and health, members of our community reached out to us to ask how they could remediate their homes from the smells and hidden ash, and what health risks they should be concerned about.

But this fire was nothing like the wildfires that our research groups at the University of Colorado had previously studied. Most of what burned on that day was human-made rather than vegetation. When human-made materials like electronics, vehicles and home furnishings burn, they and may affect health differently compared to when vegetation burns.

The outdoor air pollution was less of an issue because the wildfire was short-lived – the quieted down and changed direction about 11 hours after the fire started, and the finally fell. This snowfall ended the fire and cleaned the outside air of pollution.

Wildfire ash and dust entered homes under doors and around windows. Courtesy of Joost de Gouw

The key concern was what chemicals lingered inside the undestroyed homes – soaked up into the fabrics of carpets, sofas, drywall, air vents and more – that would slowly release into the home for some time after the fire.

We hypothesized that there were lots of volatile organic compounds (VOCs) – toxic gases, which were emitted during the fire that had seeped into homes and become embedded in the fabrics and building materials. Of particular concern were aromatic compounds like , and (PAHs), which are emitted from wildfires and have known health effects. In addition, we were worried about metals in the ash and soot deposited in homes, and the potential for it to become suspended in the air again when people returned and heating systems came on.

Despite knowing that some of these gases were toxic, we did not know the levels inside the homes, or what remediation efforts to suggest to residents, because little scientific research had been published on . We realized that we needed to do some of that research to help our own community – and the next community affected by a wildland-urban interface fire.

Collecting evidence inside

Many community members volunteered their homes for study sites. When we toured these still-standing homes 10 days after the fire, we saw what a rapid evacuation looks like, with lunch in the process of being made, laundry being folded, toys in the middle of pretend play … and dust, lots and lots of dust resulting from the fire.

We collected dust samples in about a dozen homes and then analyzed the samples in our labs.

We looked for molecules that could help us think about the origin of the dust. Not surprisingly, the dust was a combination of windblown soil, ash from the fire and typical household dust. That ash was high in typical combustion byproducts that are known to be toxic, and there was lots of ash, so cleaning up all the dust was important to remediation.

The homes that had been exposed to heavy smoke also still smelled like a chemical fire. A colleague likened it to the smell of gunpowder.

A chart shows benzene levels in a smoke-infiltrated home decreased when an air cleaner with a carbon-activated filter was running, but then rose again when the air cleaner was turned off. Joost de Gouw

As quickly as we could, we moved a state-of-the-art mass spectrometer into one of the most heavily affected homes in Superior and made measurements of airborne pollutants for five weeks.

Shortly after the Marshall Fire, we found that many pollutants, including PAHs, were indeed at higher levels inside smoke-affected homes than we would expect, but in early February these pollutants had decreased to more normal levels.

We researched ways in which people could protect themselves and found through experiments that air filters with activated carbon could provide excellent temporary relief from the indoor pollutants.

We also observed the results of professional remediation efforts. We are still poring over the air pollution data to understand which materials that burned, such as plastics, car tires, furniture, carpet and roofing material, contributed the most to the air pollutants we observed in the homes.

Continuing health effects

In addition to the air pollution and ash concerns, people living in the neighborhoods that burned are concerned about their health.

In an initial survey, residents reported a variety of symptoms that they think may be due to the smoke or air quality concerns of the fire, with the most common being itchy or watery eyes, headaches, dry cough and sore throat. More than half of respondents also reported disrupted sleep due to the stress of the fire, and almost a quarter attributed headaches at least in part to the stress of the event.

The physical symptoms could be due to the exposure during the fire. However, of those who have moved back into smoke-damaged homes, they report the symptoms most often inside their homes.

Left: ; Middle: ; Right: .

This fall, more than nine months after the fire, some residents reported rashes and burning sensations despite having cleaned their homes of ash and the smell of VOCs having dissipated. Another round of surveys is now helping gather more information about lingering symptoms. In addition to physical health symptoms, we are also asking questions about mental health, which is a growing concern from so-called natural disasters.

While we know that the VOC concentrations inside the homes that we worked in have returned to normal levels, some individuals may be more sensitive than others. And while there has been research into the health effects of some VOCs, , nor have studies looked at the health impacts of combinations of VOCs.

As global temperatures rise and once-wild landscapes at the edges of cities, the . We hope that our work can help people deal with the air pollution aftermath of future blazes.

This article is part of a collaboration with , The Center for Environmental Journalism at the University of Colorado Boulder, public radio and to explore the impacts of the devastating Marshall Fire one year after the blaze. The series can be found at the .

This article is republished from under a Creative Commons license. Read the .

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