Tom's Blog

Wednesday, March 22, 2017

What to do with dead trees during oak savanna burns?

Scattered dead trees are characteristic of oak savannas. Some of these trees are fire-sensitive species such as birch and cherry, whereas others are oaks that die as part of the standard mortality process that occurs in forests.

Standing dead trees, often called “snags”, are an important part of the savanna ecosystem and are left for wildlife. Dozens of animal species depend on snags, including numerous cavity-nesting birds, some mammals, and a vast array of insects and other invertebrates.

In addition to snags, logs and other woody debris that fall to the ground, generally called “coarse woody debris”, play additional important roles in the savanna system.

However, the dead wood in a savanna becomes a special problem during savanna burns. As part of the rotting process, dead wood becomes fire sensitive, and can cause problems during the burn mop-up process.

Dead wood is subject to decay by wood-rotting fungi, which break down the lignin and cellulose that confer rigidity on the tree. Decay results in formation of wood that is soft and spongy, generally called “punky”.

Punky wood catches on fire unusually well. (Another name for punky wood is “tinder”.) A few sparks from a strike by flint and steel is often all that is needed to start a small fire. The radiant heat of a fire passing near a tree with punky wood may all it takes to start a small fire. Although these fires generally go out quickly, there is always the possibility of a larger scale conflagration.

Why is punky wood more flammable than intact wood? In intact wood, heat is conducted into the interior and dissipated. Punky wood has an open structure surrounded by air, so heat is retained, and the oxygen necessary for ignition encourages the flame. Everyone knows that if you want to start a fire in the woods to cook your dinner or stay warm, you first find some tinder (punky wood) and use it at the base of the campfire. According to U.S. Forest Service data, punky wood will ignite and create a flame at a lower temperature than grass (~230 C instead of > 300 C).

At Pleasant Valley Conservancy we work hard to protect snags from fire by clearing a fuel-free zone around the base of each snag. This generally works well, but occasionally we get fire in a snag and have to put it out. The photo here shows how we deal with this.

A dead white oak in the East Basin savanna. Although it had been cleared around (see base of tree), some of the punky wood still caught fire. Kathie is using the high-pressure water from the pumper unit in the Kawasaki Mule. It took only a few minutes to put out the fire.
If a snag smoker is well within the burn unit we generally leave it, as the fire usually goes out, and getting rid of the punky wood is actually beneficial. However, the tree shown in this photo is fairly near the edge of the burn unit, and there is about 40 acres of grassland nearby that belongs to someone else. Given those conditions, we put the fire out.

Sometimes the fire is not near the base, like this, but high in the tree. Putting out such a fire is nearly impossible, so the tree is cut down and broken up into pieces. (A chain saw is standard equipment for a savanna burn!)

The photo below shows a large fallen log that fell, next to a live bur oak. We did not bother to put this fire out, because the tree is well inside the burn unit so that there is no danger of a spot fire. Letting logs like this burn helps recycle nutrients into the ecosystem.

This log is a good example of large coarse woody debris. It is in Unit 19D, well inside the burn unit.


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