Fire Behavior On the Ground

It's my job out on the fire line to predict what the fire will do next. I have to factor in weather, terrain, and the types of materials there are to burn. The new models help a lot, but I also think about fires I've seen before.

I'm very concerned about firefighter safety, so I always look at the worst case possible.

I always keep in mind the basic fire triangle:

Remove one part of the triangle, no fire. Adding extra to one part can overcome a lack of another.

Anything that can burn has an ignition temperature. The hotter the air is, the less energy it takes to reach that point. Moving air--wind--gives a steady stream of oxygen just like when you blow on your campfire to get it going. Finally, if the fuel is damp, it will take extra heat to dry out before it can get hot enough to ignite.

What I really like about my job is to watch nature doing its own thing. Our suppression attempts for the past 100 years have not overcome natural processes. Even if we wanted to stop every fire completely, there are a lot of things we can't control. These include lightning storms; dry, hot weather; and long-term droughts. Even though we can reduce fuel loads by thinning and prescribed burning, we don't have the resources to treat all the acreage that needs it.

A fire with the least human intervention and the most ecological benefit fire starts by lightning in early June and roams freely until fall snow puts it out.

You might think of the raging infernos in Yellowstone in 1988 as the typical wildland fire. But most fires creep through the trees, flaring up here and there. They create a patchwork quilt of habitat ranging from completely open space to snags and very lightly scorched spots.

Fire suppression has created a lot of what we call "ladder fuels" -- low branches and small trees that create a ladder for fire to burn from the ground up into the crown of the tree. Once this happens, it is very difficult to keep the fire from spreading. Firefighters then have to wait for the weather to change or until the ladder fuels burn up. The fire will eventually go back on the ground, making it safer and easier for firefighters to control.

Even without a crown fire, a lot of trees will be killed by the heat of an active ground fire.

It's easy to think that the whole tree has to torch to kill it. In fact, trees can be killed when the flames scorch it badly, even without setting the crown on fire. If the fire is hot enough on the ground, the roots can be damaged or killed by the heat.

Some stands will only have a crown fire under the most extreme conditions. But in a dense stand, lots of trees will die even with a less intense fire.

Risa
Lange-Navarro
fire behavior specialist