Tom's Blog

Thursday, October 30, 2008

Great burn on the north woods

Starting the burn of the north-facing woods. The first line of fire was started at the bottom of the ridge.

Getting controlled burns done in the fall is tricky, as the weather has to really cooperate. This week the weather was ideal and we spent the day doing some fairly tricky burns. We had six people from a contractor and five from Pleasant Valley Conservancy.

Leading up to today, we had four days of great weather, including some strong winds to dry things off, lots of sunshine, and low humidity. Today the temperature got into the upper 60s, the humidity was around 35%, and the dewpoint around 25 F.

Although we burned two planted prairies, our main goal was to burn the north-facing woods, which is fairly hard to burn. You need several freezes to kill back all the vegetation and bring down lots of oak leaves. Then you need the Indian summer weather we had today. These are the conditions that the Native Americans waited for when they burned their woods and savannas (which is why this warm October weather is called "Indian" summer).

The principal (really, only) fuel in a woods burn are oak leaves. They are quite flammable, burn hot, and are curled when they fall, so that the fire can jump from one leaf to another. Oak leaves "carry a fire", as they say. Leaves of other tree species do not burn very well at all.

One reason the woods burns better in the fall is that the leaves are newly fallen, and have not been compressed by snow. Also, snow remains long in the spring on the north-facing slope, meaning that the oak leaf fuel remains wet and will not burn. Conditions in the fall are quite different. With the weather favorable, we jumped at the chance to burn.

Because our woods are on a steep hill, we used four drip torches (and very little water). The technique used is called a "strip head fire". After putting in a blackline at the top of the ridge, next to our mowed fire break, the other three drip torches moved in parallel to the contours, the one at the bottom first, then the second, and then the third. The steep hill encourages the fire to move up, and the wind was strong enough to push the fire in front of it. By using three drip torches, we don't need a continuous fuel load across the whole unit, like you get in a prairie burn. This is important because downed logs and timber often block the movement of fire in a woods burn. Since each fire line can burn both up and down, coverage is potentially lots better.

The burn of the whole 25 acres was accomplished in less than an hour, after which we spent two hours mopping up. (We also spent two hours before the burn getting the fire breaks in final form.) Fortunately, mop up was not too much trouble, since the woods had been burned a couple of years ago (in the spring) and a lot of the punky downed timber that had been present was burned up then. The photo to the left shows mop up of a burning dead tree.

Below is an interior view of the woods, showing the completeness of the burn. All the oak leaves are gone.


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