Yellowstone burns yield clues to fire behavior

The Yellowstone conflagration of 1988 continues to teach fire managers what future fire seasons might hold

By Environmental News Network staff

The 1988 fires in Yellowstone National Park were the climactic act in the modern drama of wildfires.

While the fire season of 2000 is consuming nearly as much as much acreage as was burned in Yellowstone, 6.6 million acres so far in 2000 compared to 7.4 million acres in the park in 1988, the Yellowstone conflagration continues to teach fire managers what future fire seasons might hold.

Ray Renkin is a National Park Service biologist who has studied fires and fire ecology in the park for more than 20 years. He says that areas burned by fires in Yellowstone in 1988 are not promulgating new fires as rapidly as unburned areas.

In some cases the previously burned areas are slowing the spread of fires, or "knocking down" burns, in fire parlance, from adjacent unburned areas.

There have been 34 fires in Yellowstone during the 2000 fire season. Four of these were caused by humans; the remaining 30 were the work of lightning.

As the famous Yellowstone fires were only 12 years ago, the new burns are exhibiting much different behavior from the crown fires that can occur in mature stands. "We have had three lightning-caused starts this year in burn areas from 1988," Renkin said. "Two of them went out on their own within 24 hours."

The one fire in a previously burned area that did become active is the Boundary fire at an elevation of about 7,000 feet above sea level. The fire covers about 250 acres.

As Renkin flew over the fire in early September, he said, "The fire is still alive, but we've had cool weather and high humidity. When I flew over today, I could see six different little smoke plumes, but I couldn't see any flames.

In 1988, wildfires roared along the Lewis River in Yellowstone National Park

Because areas hit by the 1988 burn areas have such new growth, they are usually invaded by grasses and shrubs. These potential fuels are usually too green to burn well, and they carry a fire slowly.

There is plenty of fuel on the forest floor from downed trees from the earlier fires, but the grasses are needed to carry the fire and ignite the fuel.

The new Yellowstone fire initially moved under the influence of high winds. "After its initial run, the kind of behavior we've seen in much different than what we see in established forests," Renkin said. "It tends to creep along. It's being carried by very fine fuels."

As daytime temperatures cool down, the fire smolders in the downed timber. Then, as the day heats up by about 11 a.m., the fire also flares into flames about two feet high.

"It puts up a ton of smoke," he said. "Flying in from a distance, it looks like the fire is 500 acres and moving a mile a day based on the volume of smoke. "But when I get over it, I find the fire has increased in size by about five acres. It's a backing fire, burning hot, moving slowly, and consuming all the dead and down fuels."

The fire covers 250 acres, and has moved about the length of a football field. In contrast, the wind-driven crown fires of 1988 moved eight miles in a day.

The Moose Fire, on the north end of the Pitchstone Plateau at 8,200 feet, was started in an area unburned by the 1988 fires. Two lightning strikes about an eighth of a mile apart ignited the blaze. It had some active crowning and ran in a southwest-to-northeast direction, a typical behavior for many fires in the park.

Renkin said the Moose fire had two days of rapid crown fire runs, then bumped into a 1988 burn area. It threw out some spot fires, but none grew. "I'm attributing that to the fact that the high elevation grasses you find in those old burns have not cured out yet," he said.

Don Despain, another longtime Yellowstone fire researcher, said, "The fuels are drastically changed. The old growth forests are the ones most apt to burn, and they were taken out in (the 1988) fires. What you have left are the young trees. The young trees have more young needles and more water."

In young trees, Despain said, about half of the total weight of the tree is water. In older trees, only 30 percent of the weight is water. Since the older trees are much drier, they burn more readily.

"Older needs on old trees are about 75 percent of the biomass," Despain said.

In addition, he said, a lot of the rotten material on the floor of the forest that carries the fires into the crowns burned up in the 1988 burns.

"The trees aren't as tall and there's not as much fuel on the forest floor to generate the crown fires to produce the fire brands for long distance spotting," Despain said.

There are now grasses and herbaceous materials on the forest floor, he said, but their fire characteristics are much different from the fuel in an unburned area.

This pattern is confirmed by fire research in other parts of the world. Ecologist Robert Scholes of the South African research organization CSIR Environmentek, says that in southern Africa, more than 1 million square miles are expected to burn this fire season, an area twice as large as Montana, Wyoming, Idaho, and North and South Dakota combined.

The region's heaviest burning is concentrated in the moist subtropical belt that includes Angola, southern Congo, Zambia, northern Mozambique and southern Tanzania. The fires are driven by grasses that grow during the rainy season, and they do little damage to mature trees.

Copyright 2000, Environmental News Network, All Rights Reserved

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