Burning of the Alaskan tundra can release massive amounts of carbon dioxide, the major greenhouse gas, according to a study published in Nature this week. The Arctic is warming faster than the rest of the planet, causing scientists to wonder what will happen to the carbon that plants have stored in Arctic soils and plant matter, both living and dead.

The new study looked at the aftermath of the Anaktuvuk River wildfire, which burned more than 1,000 square kilometers of tundra on Alaska’s North Slope in 2007. Anaktuvuk burned for almost three months, and by itself, accounted for two-thirds of the total area burned in Alaskan tundra since 1950.

The immediate cause was lightning, but weather played a major role. Between July and September, 2007, the North Slope had the hottest weather in a 129-year record. When the fire was really roaring, daily highs were 5°C to 10°C above average. The Slope also received less than 20 percent of the average rainfall that summer, leaving the tundra abnormally arid.

In 2008, Michelle Mack, an associate professor of biology at the University of Florida and her colleagues visited the area and took samples from 1-square-meter quadrants both inside and outside the fire zone. Mack was in the field in Alaska, alas, and did not answer our emails, but her group calculated that the fire oxidized more than 2 million tons of carbon, which entered the atmosphere as carbon dioxide.

Accounting for carbon

The movement of carbon through soils, ecosystems, waters and the atmosphere is critical to the issue of global warming. Releasing carbon to the atmosphere as carbon dioxide speeds warming; and storing carbon compounds can slow or potentially reverse warming.

The moist acidic tundra under study covers as much as one-third of a billion square kilometers of the global Arctic – making it a major “sink” for carbon dioxide. The 2 million-ton release of carbon was equal to at least 50 percent of the amount of carbon stored annually in the Alaskan tundra, meaning this one fire almost cancelled the anti-warming benefit of photosynthesis in the region.

Chilling news about a burning issue

The link between global warming and fire also appeared in a new analysis of Yellowstone National Park. “Large, severe fires are normal for this ecosystem,” said Monica Turner, a Yellowstone expert and professor of ecology at the University of Wisconsin-Madison. Historically, the entire Yellowstone landscape has burned every 100 to 300 years, but Turner and company calculated that current trends toward hotter, drier summers, mean fires could consume the entire area every 30 years by 2050.

Wildfires are also becoming more common in the normally fire-resistant tundra of Alaska, and for reasons related to permafrost, reflectivity and feedback, the consequences could be dire:

And feedback moves us from the additive realm to the multiplicative one. In the Arctic, feedback also plays a central role related to the release of methane, which has even more warming potential than carbon dioxide. Many warming Arctic habitats have started releasing larger amounts of methane, which could warm the planet, feed back, and stimulate the release of yet more methane.

This feedback, like the one that may be affecting burning tundra, paints a darker picture of what could happen if we ignore the atmosphere and blithely assume that the future will be just like the present.

– David J. Tenenbaum

Southwest fires still ablaze

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.

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

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:

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?

An old debate

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.

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:

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.”

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:

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.

• While it looked catastrophic, Yellowstone’s infamous 1988 fire turned out to be a regular stage of ecological change.
  Photo: Jeff Henry, U.S. National Park Service, 12120
  
• Before: A stand of lodgepole pines tower above spruce and fir in Yellowstone 1965.
  Photo: RG Johnsson, , U.S. National Park Service, 08161
  
• 10 years after: The forest restored itself, as lodgepole pines sprout between dead ones in 1998.
  Photo: Jim Peaco, U.S. National Park Service, 15995
  

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.

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.

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:

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.

Data: National Interagency Fire Center

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.

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

All in all, the period since the ice age abated about 15,000 years ago has been pretty interesting. Melting ice raised the oceans, flooding the Bering Strait land bridge across which the Americas were populated. Temperatures rose around the globe, leading to the invention of cities, armies, writing and bacon.

Here’s an enduring question. Why were the giant mammals that made the Americas more zoologically diverse than Africa all exterminated within a few thousand years after the big melt-down? Bye-bye beavers as big as black bears, giant sloths, saber-toothed cats, and the elephant-like mastodon.

[svgallery name="mastadon"]

As Australian paleontologist Christopher Johnson wrote in Science this week, all 10 species of mammals weighing more than a ton had gone extinct in North America by 10,000 years ago.

Why?

Many theories are proposed for the sudden disappearance: An impact of a comet or asteroid around 12,900 years ago. Rapid ecological changes that accompanied the warming. Widespread wildfires. And hunting – the “overkill” hypothesis. Although similar disappearances roughly coincided with the arrival of people in Europe, Eurasia and Australia, and hunger is certainly the ultimate motivation, did people actually lay waste to entire groups of large mammals?

The debate may seem academic, and it has been one of the most brutal and tenacious debates in academia.

Reading the dung calendar

Now we get some solid evidence that the extinction of the mastodon and other large herbivores closely followed the arrival of humans in North America, and that it preceded a pervasive change in type and prevalence of trees.

The new evidence, contained in research by Jacquelyn Gill and Jack Williams of the University of Wisconsin-Madison, and colleagues, was published in Science this week, and although it does not prove the overkill hypothesis, it does usher a new type of evidence into the debate: spores of fungi that grow in herbivore dung.

Between 14,800 and approximately 13,700 years ago, fungal spores of the genus Sporormiella declined by up to 98 percent in sediments found in lakes in Indiana and New York State.

Mastodons eat black ash trees as the last ice age begins to abate. Image courtesy Barry Roal Carlsen, University of Wisconsin-Madison.

For decades, students of ancient ecology have been poking through pollen in sediments to see what plants were alive when the sediment was deposited. Pollen are durable structures, but it turns out that Sporormeilla spores are equally tough, and if you have the patience (Why Filers immediately excuse ourselves at this point!) counting spores provides a good gauge of the number of herbivores.

Because the same sample also contains pollen and charcoal, it’s also possible to document the co-existing plant community, and get an idea of the extent of wildfires.

Fungi are a new addition to the paleoecologist’s toolkit, says Gill, first author of the paper, and a graduate student in Williams’s lab. “Only recently have fungal spores been getting any attention; we used to basically ignore them if we counted them at all, but now we realize they are a good source of information about early conditions.”

Being skeptics, we asked whether the decline could simply represent a change in conditions that was less conducive to preservation, but Gill says not. “If so, you would expect other proxies to show similar transitions. Since the same sediment that contains the spores also contains pollen, we’d expect to see pollen disappear, but we don’t.”

The dating game

Having a firm date for the decline of mastodons and other large herbivores is mainly helpful for eliminating some possible explanations, says Gill. The decline started almost 2,000 years before the putative impact of a comet or asteroid. And a change in climate apparently did not cause a broad habitat loss, Gill adds. “The extinction started before the habitat changed; the vegetation is relatively stable until after the extinctions began. We do have evidence of warming taking place, but if climate change is causing the extinctions, it’s not through a loss of food.”

A major ecological change did follow the elimination of large mammals, however, as documented by pollen representing a new assembly of trees, including ash and ironwood, which had probably been held in check by hungry herbivores, growing along with less nutritious conifers like spruce and larch. Once the grazers left, these trees began to dominate the landscape — and then became fuel for wildfires that burdened younger sediment with charcoal.

Graduate student Jacquelyn Gill holds a sediment jar with a scrap of charcoal being prepared for carbon dating. Photo: The Why Files

Although the sexy “overhunting” hypothesis is sure to get a boost from the Science paper, Gill says one study hardly proves the case. And as Johnson notes in his commentary in Science, the Clovis people who spread across much of North America arrived more than 1,000 years after the decline began. Evidence for earlier North American populations is sketchy and scarce, but it is arising, Johnson added.

A second focus of the Gill paper may be equally important: the effect, rather than the cause, of the extinctions. “What happens when half of the species larger than a German shepherd go extinct in North America?” Gill asks. “Elephants eat 300 pounds of food a day, and when animals like the mastodon are rapidly taken out, you would think the landscape would notice, but that has been absent from the discussion. People were underestimating the power of these fungal spores to tell about the local presence of animals and vegetation.”

– David J. Tenenbaum