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View of flat terrain with buildings in the distance, dark smoke clouds and orange haze fills the sky
The view from the Los Alamos municipal airport during the fire.


Last week, New Mexico’s famous Los Alamos National Laboratory, home of the atomic bomb, was shut down when a wildfire exploded from 2,000 acres to 49,000 acres over 24 hours, forcing the evacuation of the town of Los Alamos.

A wildfire that started May 29 in droughted Arizona scorched 538,000 acres – the largest in the state’s history.

Historically, wildfires have been usually battled as threats to life, limb and property. But scientists and land managers now see them as a part of nature that can be postponed but not denied.

UPDATED 6 OCTOBER 2014: Smokey turns 70.

This edition of The Why Files examines the ecology of fire in the forest.


Wooden fire danger sign with cartoon bear dressed as park ranger, sign cautions extreme danger

Photo: Ischa1
Smokey is a pro at preventing forest fires, but are his efforts a little over the top?

For a century, the highly successful Smokey the Bear ad campaign fueled fear and loathing of wildfires in the United States. Embezzlers have been more popular than wild fires, which scourged the landscape, burned the birds and rendered Bambi homeless. But in recent decades, ecologists have come to three startling conclusions about fire:

Wildfires are regular visitors to many ecosystems, including forests, prairies and rangeland.

Moderate fires cause little or no long-term harm to these ecosystems, and are often helpful.

Fires are inevitable: postponing them just makes the next fire bigger, harder to contain and more destructive.

Forests afire

One touchstone for the reconsideration of fire was the “catastrophic” conflagration in Yellowstone National Park in 1988 — which, despite the frightening photos, turned out to be a temporary setback for the ecosystem. Still, even ignoring the human toll for a moment, scientists have found that massive debris flows from denuded slopes can permanently alter the landscape.

More recently, discussion has shifted to reducing the intensity of wildfires, and to their interaction with a warming climate. How effective is controlled burning? Are global warming and the likely increase in drought already accelerating wildfires? Will more wildfires turn arid parts of Australia, the American West and Asia to desert?


Closed road gate with Group Camping sign next to it, forest in flames behind it

Photo: Jeff Henry;, U.S. National Park Service, 12144
The Yellowstone fire put a bit of a damper on camping in 1988.

An old debate


Traffic jam of two parallel lines of cars heading in one direction out of town clouded by smoke in the distance

Photo: June 27, 2011, Los Alamos National Laboratory
This is not rush hour traffic; it’s Los Alamos residents fleeing the fire.

Each fire is shaped by weather, geology, plant life, and topography, which makes them hard to study, let alone control. Beyond harming or killing plants and animals, fires force a broad range of changes in chemistry, pH, microbial activity, moisture, water flows, soil structure and erosion.

The debate over wildfire is old, according to Stephen Pyne, a fire historian at Arizona State University. Although it’s impossible to know for certain the prevalence of fire five centuries ago, for a 1998 Why Files, Pyne estimated that before Columbus, wildfires, often set to clear land for planting, burned five times as much area as today.

Pyne said the debate over wildfire in the United States when the first national parks opened a century ago “mirrored an earlier argument in Europe over the role of fire” in natural landscapes. The European emigrants to the New World associated fire with “primitive” agriculture, and the U.S. government sought to eradicate fire from its parks and forests. The policy of fighting pretty much all fires succeeded at first, Pyne said. “Absolute suppression will work for a number of years, even a few decades, but you are always going to have fires.”

In the long run, he contended, total suppression is futile or counterproductive, since it allows a buildup of fuel that makes future fires larger, fiercer and even harder — or impossible — to fight.

Controlled burns — a forest fire you can love!

In response to this fuel buildup, controlled (“prescribed”) burns have been used for decades to reduce the chance of a catastrophic fire and return forests to a condition adjudged to be more natural. Prescribed burns reduce the amount of fuel, try to remove the “ladder trees” that can carry a creeping ground fire into the treetops, and are the “primary management tool” in the Forest Service region that covers 18 national forests in California.

 Animation shows changes in the forest as new trees and shrubs move it.

USDA Forest Service, Rocky Mountain Research Station
Watch this piece of Montana’s Bitterroot National Forest grow denser as fire is excluded and trees are harvested. Before 1895, low-intensity fires burned through this forest every three to 30 years, until people began logging and suppressing fires. Click the link for a more complete explanation.

But prescribed burns are expensive, difficult to pull off (as they require a forest that is dry enough to burn, but not so dry that a raging fire will result), and studies of their efficacy conflict:

A 2008 study1 in the southern Sierra Nevadas in California showed that prescribed burning neither reduced fuels loads and ladder trees, nor helped restore the mix of tree species. The problem may relate to timing: Normally, these forests burn in late summer or early fall, but prescribed fires must occur during cooler weather, when they are easier to contain and onerous air pollution is less likely.

A 2011 study2 in the Sierra Nevadas found a 67 percent reduction in tree density eight years after a controlled burn. Fire was more deadly to younger trees, so the forest shifted in favor of older trees, but the burn had little effect on the ratio of tree species. The authors concluded that “long-term observations are needed to fully describe some measures of fire effects.”

To test whether prescribed burns reduce the intensity of subsequent wildfires, researchers need to chance upon a “natural” fire that follows a deliberate burn. In Washington State, a 2010 study3 found that 57 percent of trees survived a wildfire in an area that had previously been thinned and then burned deliberately; only 19 percent of trees survived the wildfire in an area had been thinned only, and just 14 percent survived in areas with neither thinning nor controlled burning.

In another measure of fire intensity, a 2009 study of the 2002 Biscuit fire in Oregon found that 30 percent less carbon and nitrogen was lost in a wildfire that followed purposeful burning.


Pine forest clouded by smoke, flames on ground, firefighter in the center walking

Photo: Brady Smith, Coconino National Forest
Prescribed burns, such as this in Arizona’s Coconino National Forest, are a management tool of choice for the U.S. Forest Service.

Do controlled burns damage trees?

Despite some successes from these deliberate burns, scientists have noted that they are sometimes followed by outbreaks of destructive bark beetles, or that fire in the heavy layer of organic matter left after a century of firefighting can kill tree roots – and trees. In a 2007 report, Sharon Hood of the U.S. Forest Service wrote that prescribed burning “is causing significant mortality of these high-value trees even with low intensity fires.”

In a 2005 test in Lassen National Forest and Lassen National Volcanic Park in California, Hood and colleagues looked at the effect of raking litter and duff away from ponderosa and Jeffrey pine trees before a controlled burn. Raking did not confer a survival advantage, perhaps because trees survived well in both the treatment and control groups, but raking did confer some advantage against beetle attack.

Bigger ecological picture

In the search to find out how fires affect forests, one theme stands out: The aftermath of fires is as varied as their weather conditions, biology and landscapes. In some cases, as we’ll see for Yellowstone, the ecosystem bounces back after a fire. But the results vary, even in one fire in one location. For example, the 2002 study of the Rodeo-Chediski Wildfire (which set an Arizona record at 189,000 hectares) found that about half the area was severely burned, and that many more years would be needed to restore the area despite efforts to replant vegetation and contain erosion. The mildly burned half section, however, had reverted to pre-fire conditions by 2009.

In the Arctic, the aftermath of a fire was much more serious: A report after the 1,000-square kilometer Anaktuvuk River fire in Alaska in 2007 documented a dramatic reduction in stored carbon. The researchers concluded that the growing frequency and intensity of fire would cause major changes in the ecosystem, climate and “the well-being of humans and other animals that inhabit Alaska’s North Slope.” After a severe burn, soil carbon, a key indicator of fertility, is “unlikely to recover to pre-fire levels over the next millennia.”


Satellite image of green mountains. Fires are large and small smoking, pink-orange patches.

Photo: Jesse Allen, based on data from Landsat 7 Science Team, NASA GSFC
These fires merged to create the Rodeo-Chediski fire of 2002; which held Arizona’s record — until 2011.

In general, animals get less consideration than plants in research on the aftermath of fires, but several studies of birds describe changes for better and for worse:

A study of birds following the Rodeo-Chediski fire found a reduction in the number and diversity of species on two watersheds, likely due to the size of the fire and a persistent drought. Curiously, bird numbers and biodiversity were similar in moderately burned areas as in severely charred locations.

Severe fires in Oregon4 produced a change in bird species, but, “Contrary to expectations, repeated high-severity fire did not reduce species richness, and bird densities were greater in repeat burns than in once-burned habitats.”

A 30-year study of a Minnesota fire found a radical change in bird numbers and species, as dead trees were replaced by shrubs and new trees: “Overall, bird species using the area after 30 years remained over 70 percent higher than in the mature forest before the fire.”

Open-air experiment in Yellowstone…

Much of what we know about the ecological impact of fire has come from Yellowstone National Park, where a giant blaze burned about 45 percent of the 1-million hectare park in 1988. Photos of towers of flame and exhausted firefighters became symbolic of nature run amok. Yet long-term studies of the aftermath produced surprising results, says Monica Turner, a landscape ecologist at the University of Wisconsin-Madison.

By 1998, 10 years after the blaze, Yellowstone was already on the rebound. Fish and mammals had survived the holocaust surprisingly well, and lodgepole pines—which dominated the park for 10,000 years — were poking through the shrubs and weeds, heralding a return of the park’s old ecosystem.

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Photos: 12120, Jeff Henry, U.S. National Park Service; 08161: RG Johnsson, , U.S. National Park Service; 15995: Jim Peaco, U.S. National Park Service

On cone-y island?

Why the quick rebound? Although the horrific photos from 1988 suggested that the vast sections of Yellowstone were uniformly charred, the severity varied from place to place. While intense crown fires killed all above-ground vegetation in some areas, trees and plants survived milder ground fires elsewhere, and the “mosaic” destruction allowed rapid, but patchy, regeneration. “In some places, very few trees are coming back, in other we see hundreds of thousands per hectare,” says Turner.

These extremes of tree density after a fire reflect that pattern of fire severity, Turner explains, and the biology of the dominant lodgepole pines. Many of these trees produce cones that, in a fire, open and release their seeds, which confront ideal growing conditions: Bare soil with little competition, plenty of sun, and the weather they are adapted to.

Forests can survive fires, but the fingerprints of global warming are now evident in western forests. Could “forest fire” have a whole new meaning in a warming world?

Other lodgepoles, however, release their seeds essentially on schedule, giving them less advantage after a fire. As the difference in tree density plays itself out over the decades, the fire’s imprint on the landscape can persist for more than 150 years, Turner says.

A flowering success

Because the soil was charred only to an average depth of 2 centimeters, and never more than 6 centimeters, some plants resprouted from roots or underground structures called rhizomes. By 1990, wildflowers were already abundant, Turner said. “Regeneration of these plants was very rapid, and it came from within the burned area. Even the really big fires leave a legacy of the plants that were there before the fire.”

In contrast, invasive species, did unexpectedly poorly after the fire, Turner said. “We had hypothesized that there might be an invasion by non-natives; the fires had created so much expansive, disturbed habitat, but the invasives have not appeared to spread, and are still where they used to be, along roads and trails.”

Burn and revive — or not

Over all, the fires had surprisingly little impact on wildlife, says Turner, who studied survival of elk and bison in Yellowstone, and the fire may even have given elk an advantage over the reintroduced wolf. “The young forest that is coming back after the ’88 fires provides quite a bit of cover for elk; the young pines are super-dense, it’s difficult to see your hand in front of your nose.” Furthermore, logs from the fallen trees killed by the fire can conceal elk and interfere with the wolf attempts to run down elk in open fields.

The summary word for Yellowstone is resilience, Turner says. The natural fire regime in the Yellowstone area includes a hot, crown fire “that replaces the whole forest and the cycle begins again about every 120 to 300 years. Big fires at the historic intervals are not detrimental to the system in any way.” Although these fires threaten homes and businesses, “from the perspective of plants and animals, fire is a normal event.”

Wildfires can carry other hazards, however. For example, a 2010 study of dry regions of Southeast Australia noted heavy erosion and debris flows after a big fire, mirroring what has been seen in the arid American Southwest. The debris flows were not seen in wetter forests, however.


 Forest hillside and path, trees are burned black, exposed soil and rocks on ground

Photo: 2009, Peter Campbell
The apocalyptic appearance of Victoria, Australia’s “Black Saturday” bushfires shows bare soil that can quickly erode after a fire.

Fire in a changing globe

Fire, obviously, removes stored carbon from the forest, making it a potential source of greenhouse warming. But the opposite is also true: global warming seems to cause more fires. According to experts on Western water and climate5 rapid climate change is underway in the American West, with:

“soaring temperatures, declining late-season snowpack, northward-shifted winter storm tracks, increasing precipitation intensity, the worst drought since measurements began, steep declines in Colorado River reservoir storage, widespread vegetation mortality, and sharp increases in the frequency of large wildfires.”

The “signature” of global warming is already appearing in western forests, agreed a 2006 study6 which identified a change starting in the mid-1980s toward “higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.”

In other words, the increase in large, intense forest fires was more likely due to global warming than to the increased fuel load left by a century of fire-fighting.

Acreage bottomed out at about 1 million in 1983, reached 10 million in 2005

In the United States, the area burned has gradually increased since 1983.

These changes are evident in Yellowstone, says Erica Smithwick, an assistant professor of geography and ecology who studies the aftermath of wildfires at Penn State. Historically, the “fire regime” — the average time needed to burn the entire area — is 120 to 300 years, but the lodgepole pines that dominate the plateau recover within a century, so the forest has survived regular large fires.

But Smithwick, Turner and colleagues came to an alarming conclusion when they compared projections for temperature and rainfall timing and intensity in 2050 to the history of fires when those conditions prevailed in the past.

Stumps, ash, and a few blackened trees on flat land in the sunlight.

Near Ryazan, Russia, 8 May 2011, mcsdwarken via Flickr
Record heat in Russia in 2010 led to a series of huge wildfires.

The interval between fires, they calculated, would be drastically shorter, and that is disturbing, Smithwick acknowledges. “If these projections are correct, there really might be a threshold in the vegetation where it would not be able to recover.”

Such a fire regime, she adds, is “more consistent with lower montane forests [with trees spaced far apart] or non-forests.”

What is the endgame of warmer, drier forests where fires are becoming more frequent? Could fires turn a forest to desert? Yes, according to a 2009 presentation by Daniel Neary of the Rocky Mountain Research Station in Flagstaff, Ariz. “Wildfire is now driving desertification in some of the forest lands in the western United States. The areas of wildfire in the Southwest U.S.A. have increased dramatically in the past two decades” from less than 10,000 hectares per year in the early 20th century to over 230,000 hectares today. “Individual wildfires are now larger and produce higher severity burns than in the past. A combination of natural drought, climate change, excessive fuel loads, and increased ignition sources have produced the perfect conditions for fire-induced desertification.”

It’s impossible to know the outcome in Yellowstone, a jewel of the U.S. national parks, Smithwick says. “I don’t think the ecosystem is doomed, but how do you manage a system like Yellowstone in that context? There should be some opportunity for the ecosystem to shift.” Eventually, grassland may replace forest, she notes. “Ecosystems are constantly shifting; that’s the kind of mindset we need to go forward. But this is a bit of a wakeup call. We are pushing the system, and we don’t know what is on the other side of the tipping point.”

– David Tenenbaum

Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


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    Harold S.J. Zalda et al, Forest Ecology and Management, 10 July 2008, Pages 168-179.
  2. Long-term effects of prescribed fire on mixed conifer forest structure in the Sierra Nevada, California
    Phillip J. van Mantgem et al, Forest Ecology and Management, Volume 261, Issue 6, 15 March 2011, Pages 989-994
  3. Fuel treatments reduce the severity of wildfire effects in dry mixed conifer forest, Washington, United States, Prichard, Susan J et al, Canadian Journal of Forest Research, Volume 40, Number 8, 1 August 2010 , pp. 1615-1626(12).
  4. Bird communities following high-severity fire: Response to single and repeat fires in a mixed-evergreen forest, Oregon, United States, Joseph B. Fontainea et al, Forest Ecology and Management, Volume 257, Issue 6, 10 March 2009.
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