Most wildfires smoke or smolder and don’t amount to much. But some blowup, and then they are dangerous and destructive.
When we think of wildfire, we tend to imagine a raging crown fire, like the one in Figure 1, repeated from the last post. Such fires make for dramatic photos and stories in the media. But raging crown fires are not typical. Lightning ignites hundreds of wildfires every summer, and most go out naturally after burning less than half an acre. Others consume isolated or small groups of trees and eventually go out on their own. In Yellowstone National Park, 72% of fires consume less than 0.2 acres, and 84% consume less than 10 acres. Figure 2 shows the Buffalo Fire burning in Yellowstone National Park on 9/3/2016 – a far cry from the huge wall of fire that threatened Old Faithful in 1988!
Figure 3 shows the forest floor along the Avalanche Lake Trail in Glacier National Park. An old forest can get positively junky as downed wood accumulates on the forest floor. Fire won’t typically jump too far from log-to-log, it more easily spreads to something nearby. The more downed wood, the better the fire spreads.
Similarly, fire doesn’t like to burn in wet wood – anybody who has ever tried starting a fire with wet wood knows what I mean. In the national parks, fires only burn vigorously if fuel moisture levels drop to 13%. How dry is 13%? Well, kiln-dried lumber, like you buy at the lumber yard, is 12%, so, it is pretty dry. In an average year, the moisture content of downed wood in the national parks is 14-18%, too moist to burn ferociously. In really dry years, however, it can get as low as 5%, and then watch out!
Finally, as anybody who has ever tried to start a campfire knows, moving air makes a big difference. If you have a fire started, what do you do to make it grow? You blow on it. Same in the national parks. A raging fire requires wind.
Thus, three factors have to come together to make a raging crown fire: lots of downed fuel so the fire can spread easily, dry conditions so the wood burns readily, and strong winds to whip the fire into an inferno. Only when those three conditions come together does fire behavior become “extreme,” as they say. When these conditions do come together, however, wildfire becomes extremely dangerous, capable of moving fast, even of hurling fireballs up to a mile away. In 1988, Yellowstone experienced the worst wildfire season in its history. The fire started on June 14. By early August, 8 fires were burning. During the 2 weeks from 8/6 to 8/19, the fires in Yellowstone consumed an average of 11,607 acres per day. Now, that’s already pretty active fire behavior. But on 8/20, the fire exploded, consuming 152,959 acres – 13 times as much. And that wasn’t the worst of it. On 9/9 the fire ate 228,137 acres – in a single day!
So, most wildfires are small and go out on their own. But some blow up, and when they do, they become destructive and dangerous.
The worst fire blowup in United States history may have occurred in 1910 (The Big Blowup). Over 2 days in August that year, fire in Washington, Idaho, and Montana consumed 3 million acres and killed 87 people. The U.S. Forest Service was just 5 years old, and that experience was very influential in the development of the ethos that every wildfire must be suppressed as quickly and vigorously as possible, without exception. We know more nowadays, and the ethos has changed, but it held sway for a long time.
One final characteristic must be mentioned here: how you view a fire depends on where it is burning. A fire in your barbecue is one thing, a fire in your bedroom another. The American landscape is a patchwork of different kinds of land. Some is private, but even public lands belong to a variety of agencies: the National Park Service, the U.S. Forest Service, the Bureau of Land Management, state parks, state forests, wildlife refuges, etc. Differing policies regarding wildfire apply in each of these kinds of land.
Generally, the first, overriding policy is to protect life, whether it be that of firefighters or civilians. The second overriding policy is to protect significant assets, such as homes, mine buildings, ranch buildings, and visitor facilities. After that, the policies vary. In the national parks, fire is thought to be an important part of the ecosystem that is necessary for the health of the forest (more on this in the next post). In the national parks, fire is allowed to fulfill its natural role in the ecosystem without interference, unless it is threatening life or significant assets. The national forests, however, harvest their trees. Thus, the trees are themselves an important asset, and wildfire is much less likely to be allowed to burn without a suppression effort of some sort.
In the next post I’ll look at the role that wildfire plays in renewing the forest, and why it is an important part of the natural ecosystem. I had the chance to see this right before my eyes, and it was pretty impressive.
Rothermel, Richard, Roberta Hartford, and Carolyn Chase. 1994. Fire Growth Maps for the 1988 Greater Yellowstone Area Fires. General Technical Report INT-304. Intermountain Research Station, U.S. Forest Service. Downloaded 9/30/2016 from http://www.fs.fed.us/rm/pubs_int/int_gtr304.pdf.
Forest History Society. 2016. U.S. Forest Service Fire Suppression. Downloaded 9/30/2016 from http://www.foresthistory.org/ASPNET/Policy/Fire/Suppression/Suppression.aspx.
Yellowstone National Park. 2008. The Yellowstone Fires of 1988. Downloaded online 9/30/2016 from https://www.nps.gov/yell/learn/nature/upload/firesupplement.pdf.
Yellowstone National Park. 2016. Yellowstone Resources and Issues Handbook. National Park Service. Downloaded 9/28/2016 from https://www.nps.gov/yell/learn/resources-and-issues.htm.