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The Secrets of Fire

Researchers are making a host of exciting new discoveries, from the fact that wildfires cause mercury pollution to the existence of "fire fingers."

By Susan J. Tweit

One night last July, I woke suddenly and sat bolt upright, my senses alert. My heart raced and I listened intently, braced to leap out of bed. My husband slept soundly next to me; the dog snored lightly from her bed. Still, it was a long time before I returned to sleep. The next morning, I saw and smelled the cause of my alarm. A murky yellow-orange haze of smoke filled the valley, obscuring the nearby mountains. Its familiar, slightly acrid smell had jerked me from sleep and back to my firefighting days.

Wildfire exerts a powerful hold on us, inhabiting our myths, our reality, and, after the fires of 2000, our politics. Yet our knowledge of how wildfire works and its effects on plants and animals is as patchy as a burn itself.

A wildfire is born when lightning strikes any flammable natural fuel, from a long-dead tree to a bunch of dry grasses. Up to a trillion watts of energy zap the contact point in multiple strokes, all in the space of a thousandth of a second. The energy is released as heat, generating temperatures as high as 3,000 degrees Fahrenheit and igniting whatever the lightning hits. What happens next depends on a host of factors, including the location of the lightning strike, the weather, the surrounding landscape, and the abundance, size, and flammability of nearby fuels. Like snowflakes, no two wildfires are the same. Change just one factor, and the fire's behavior may alter dramatically.

Take two lightning strikes from a summer thunderstorm. One strike zaps a long-dead snag, causing a curl of smoke to rise from its dry base. Eventually, small flames appear and begin creeping along the ground, burning the sparse cover of organic "litter"--dead leaves, twigs, branches, pinecones, and the other plant and animal detritus. This fire smolders for months, never rising off the ground. When the rain or snow of fall smothers its flames, the fire has burned an area of about two acres.

The other lightning strike hits a live tree, instantly superheating its sap. The sap expands and detonates the trunk, hurling firewood-size chunks of burning wood tens of feet to ignite tall clumps of dry grass. Fanned by a sharp downward gust of air from the thundercloud, the flames blow into a nearby stand of trees, leap to the crowns of the larger trees, and roar through the forest, charring some 20,000 acres.

 

The Nature of Wildfires

Grant Jerding

Similar lightning strikes can start very different fires. In the first illustration, lightning hits a downed snag in a forest with a sparsely vegetated understory. The dry log smolders in the duff on the forest floor and eventually--a few minutes to a few hours--ignites grasses and shrubs, starting a ground fire. This fire may creep around and burn for months, but it may never climb into the forest overstory. In the second illustration, lightning strikes a live tree in a more crowded forest and its sap explodes, throwing burning chunks of wood many feet away. Intense downdrafts from the thunderstorm fan the flames, pushing them from the grasses and shrubs on the ground into small trees, and from there into the forest overstory. Supercharged by the abundant fuel and oxygen in the tree crowns, the crown fire roars through the forest.

 

Crown fires, the kind that ignite whole trees like giant matches, are the most difficult to study and also the most dangerous. (The 1994 fire that killed 14 firefighters on Storm King Mountain in Colorado, for example, was a crown fire.) The leading edges of such fires can move in complex and chaotic ways that were until recently known only from firefighting legend. Now a team of scientists from the National Center for Atmospheric Research (NCAR) and the Forest Service's Fire Science Laboratory has "pictured" such extreme fire behavior.

Flying above wildfires in a C-130 transport plane retrofitted with computers and other scientific equipment, the team uses a "thermocam"--an infrared sensor that takes digital video images of heat patterns--and other instruments to document chaotic fire behaviors. In 1998 they brought back the first known pictures of deadly "fire fingers" (also called "hairpin vortices"), horizontal whirls of flame that shoot forward from the fire front like flamethrowers, advancing the flames hundreds of feet in seconds. One fire finger blasted out 500 feet in two seconds--moving about 170 miles per hour. These images show a striking difference between the behavior of crown fires and that of ground fires: Ground fires, whether in grasslands or forests, show no evidence of the dangerous forward-bursting behavior of crown fires. Researchers speculate that crown fires are supercharged by the rich layer of oxygen and volatile organic compounds "exhaled" by the forest canopy. "Fire spreads itself in much more dynamic ways [than previously described]," says Janice Coen, an NCAR researcher.

Ironically, the NCAR-Forest Service team couldn't take to the air to observe the fires of 2000: Their plane was being used to fight fires. However, says NCAR atmospheric scientist Lawrence Radke, they were able to sample wildfires in Canada that summer, collecting data on the large-scale atmospheric effects of wildfires. As the tremendous heat of a fire wilts vegetation and then incinerates it, chemicals from the plants are released into the smoke plume. Once airborne, these compounds can move long distances, both around the globe and into the upper atmosphere. Some, including methyl bromide and carbon dioxide, may accelerate global warming; others, such as mercury, are toxic. Radke and his colleagues originally assumed that all mercury emissions from wildfires stemmed from human-caused pollution that had settled on the vegetation and been rereleased into the atmosphere by the flames. But by speciating the mercury--checking its chemical pedigree--they found that forest fires also smelt and vaporize elemental mercury from the soil. Preliminary results from their research suggest that wildfire is a "huge" source of mercury pollution, Radke says.

The compounds in the smoke can also be beneficial. In southern California's fire-prone chaparral shrublands, some wildflowers rely on nitrogen oxides in the smoke to stimulate germination of their seeds. Without wildfire, says Jon Keeley, a research ecologist with the Biological Resources Division of the U.S. Geological Survey (USGS), these plants would disappear.

In the past few decades, we've learned that fire is a vital part of many North American ecosystems, from the saw-palmetto scrub of central Florida to the prairies of the Great Plains to the giant sequoia forests of the Sierra Nevadas. We know that certain species depend on certain kinds of wildfire. But we are only beginning to realize how complex the relationship between fire and wildlife is. Just as there is no typical fire, it is very difficult to generalize about wildfire's effects on wildlife. Wildfires burn in a patchy fashion--totally charring one area, leaving a mix of live and dead vegetation elsewhere, and barely burning other areas. Drastically different conditions can abut each other in the resultant post-fire mosaic, with bare mineral soil open to full sunlight, for instance, next to undisturbed vegetation. "Fire's effects are variable," says Craig D. Allen, research ecologist with the USGS Jemez Mountains Field Station in New Mexico, "just as fire is variable, just as landscapes are variable."

Post-fire species patterns can be surprisingly complex, as ecologist Natasha Kotliar of the USGS's Biological Resources Division discovered while studying burns in Colorado's Rockies. She set out to determine whether olive-sided flycatchers preferred severely burned forests--they do, in Rocky Mountain forests, at least--but she also discovered that the abundance of some two dozen other bird species, from hermit thrushes to hairy woodpeckers, was closely related to the pattern left by fire. Some species specialized, appearing only in particular post-burn habitats; others occurred more generally, but their abundance was directly tied to burn intensity; still others avoided burned areas altogether.

Fire is also critical to the survival of some plant species, whether the flames open sealed cones to release seeds or simply clear the ground and create the conditions necessary for germination. In northern and central Florida, where some 2,500 fires burned about a half-million acres in the early summer of 1998, native plant species gained ground overall in the burned areas, and weedy exotics lost. Populations of one native shrub, Rugel's pawpaw, exploded ten-fold after the fire, going from only 200 known plants to more than 2,000.

Some insect species actively search out fires, homing in on the chemical compounds in the smoke. These pyrophilic, or "fire-loving," insects include wasps, wood-boring beetles, and robber flies. Many were considered rare until recently, when a few entomologists began to follow fire crews right into burning forests and discovered pyrophiles by the hundreds. A case in point are black Melanophila beetles, which congregate at fires, arriving in time to lay their eggs in still-smoldering trees. The bean-size beetles apparently detect flames with a pair of infrared sensors on their thorax so sophisticated that the U.S. Air Force is funding research to study them.

Why follow fires? Smoke may signal widely dispersed insects to gather, increasing their chance of finding a mate--like nightclubs for bugs. The burned trees also provide a bounty of food for growing youngsters. In one wasp species, the mother lays her eggs under scorched bark, along with a wood-fiber-digesting fungus. The fungus, which would be expelled by a healthy tree, feeds the growing wasp larvae.

There's still a lot we don't know about wildfires, especially large fires like those that burned millions of acres of the West last summer. Though there is no doubt that large fires can be traumatic in the short term, their effects on wildlife may not be as catastrophic as once feared. "It's not that Smokey was all wrong," says Craig Allen, "but he sure as heck wasn't all right, either." Take the endemic Jemez Mountain salamander, a unique amphibian that inhabits the moist microclimates under logs and rocks in the forests charred by last spring's Cerro Grande fire, in northern New Mexico. When researchers sought the small, soil-dwelling salamanders in what Allen calls post-fire moonscapes, they were pleased to find them--alive--at all the sites the salamanders had inhabited before the fire. As it turns out, the heat of the fire did not sear the amphibians' subsoil refuges, even when it charred the surface of the soil.

After a decade studying the area's bats, USGS biologist Mike Bogan of the University of New Mexico wasn't sure what he'd find in the wake of the Cerro Grande fire. Yet when he and his colleagues mist-netted bats throughout the burn area within weeks of the fire, they found no significant changes in abundance, species composition, or reproduction. In fact, says Bogan, at a place called Pine Springs, right in the hottest part of the fire, bats were more abundant after the fire than before. The scarcity of open water for drinking might explain the high number of bats at Pine Springs, or perhaps a post-fire insect flush. "It's just a little slice of data," Bogan cautions. Both he and Allen point out that it's not clear whether bats and salamanders can survive in the greatly modified habitats.

Biologists Jim and Holly Akenson, managers of the University of Idaho's remote Taylor Ranch Field Station in the state's rugged Salmon River breaks country, were anticipating their third winter of research on cougar-wolf interactions when wildfires roared across their study area last August. Among the immediate post-fire changes: elk, deer, and bighorn sheep numbers are down, and the herds are concentrated in smaller areas than usual; cougars are ranging less widely; and wolves are "camped" near their prey. What the fires will mean for the carnivores in the long term is not clear yet. "We'll keep you posted," says Jim Akenson.

As Craig Allen puts it, "This is a story we don't know the answer to yet." What we do know is that wildfire is here to stay. Its flames are as much a fundamental part of this green planet as the oxygen we breathe.

Colorado writer Susan J. Tweit studied fire ecology in college, then worked for the National Park Service and the Forest Service, where she learned to use a Pulaski to fight fires and a drip torch to set them. She still chases wildfires whenever she can.


 
 

© 2001  NASI

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