Reducing hazardous fuels on woodland property: thinning

Why thin?

Thinning is one of the most powerful forest management tools available to landowners for achieving a wide range of goals and objectives. Thinning influences:

• Trees’ growth rates and potential economic value.
• Which species of trees and other plants will be in the stand.
• Trees’ resistance to insects and disease.
• Quality of wildlife habitat.
• Forage production.
• The stand’s aesthetic appearance.

Another, very important effect is that thinning increases a forest’s ability to survive wildfire.

Thinning is:

• Removing trees that, if left in the stand, could increase fire risk and could lessen stand vigor (for example, through overcrowding).
• Retaining and managing trees that will best meet your long-term goals and objectives.

Aggressive fire suppression over the last 100 years means many forests are much more dense than they were before European settlement. As a result, in Oregon today more than 70% of forests are at dangerously high risk of severe wildfire. It’s not a question of whether a fire will occur but when and how severe it will be.

Ladder fuels

In many unthinned forests, branches on smaller trees extend to the ground and can ignite easily. Then, the small trees act as “ladder” fuels, carrying fire up into the crowns of big trees (Figure 1). This way, a low-intensity fire on the ground can develop quickly into an uncontrollable wildfire.

Low thinning

Also called “thinning from below,” low thinning leaves mostly the larger, dominant trees — which are usually the healthiest and most vigorous (Figure 2). Smaller, less vigorous trees and those with poor form are removed. Low thinning, combined with treating fuels on the ground, can greatly reduce fire hazard.

How does thinning affect fire behavior?

Low thinning (Figures 3 and 4) increases a forest’s ability to survive wildfire in three ways.

How is thinning done?

A wide variety of equipment is used in thinning. Crews equipped with chainsaws are highly versatile and can handle thick or sparse stands and flat or steep ground in all kinds of weather. Caterpillar tractors, skidders, farm tractors and even horses and all-terrain vehicles are used to skid timber and small trees that have been felled with a chainsaw.

Mechanical harvesting machines that fell trees, remove limbs and cut the stems into specified merchantable lengths sometimes are used instead of a worker with a chainsaw (Figure 6). In some mechanical operations, whole trees are skidded to central work areas, called landings, and limbed and bucked (cut into specific lengths). This helps remove much of the slash from the forest and creates a clean appearance. In other cases, trees are limbed and bucked in the woods. That leaves more nutrients on the site but also more slash and thus increases fire hazard.

New types of small-scale, “light touch” harvesting equipment have been developed that work well on small tracts and with small-diameter trees (Figures 7 and 8). Stand conditions, topography and landowner objectives help determine which equipment combination will work best.

Some thinning operations can break even or generate a profit if sawlogs are harvested (Figure 9). In most thinnings to reduce fire hazard, however, costs generally exceed revenues.