The goal of prescribed burning is to reduce the amount and density of surface fuels in a controlled manner. Prescribed burns also scorch and kill the lower branches of trees, preventing laddering (Fitzgerald 2005). This technique lifts the canopy off the surface, lowering the ability of the fire to climb to the high-density crown. Prescribed burns are typically carried out in regular intervals, much like the natural low-intensity fires of the past. One of the key difficulties in prescribed burns is that some preparation may be necessary in order to reduce the amount of fuels. Otherwise, the controlled burn could easily become an uncontrollable raging forest fire. Pruning and thinning of tree stands may be necessary in order to reduce the available fuel before the prescribed burn (Fitzgerald 2005). Mowing and grading of heavily mulched areas may also be needed. In order to mow or remove the surface fuel, it may be necessary to thin the tree stand so that large equipment can get to where it is needed.

Mowing, Pruning, and Thinning

Mowing, pruning, and thinning are other ways to reduce available fuels. The technique of mowing is done with a brush hog or other large mowing implement. The goal of mowing is to reduce the amount of brush, reverting the areas between trees to grass (Fitzgerald 2005). Grass provides much less fuel than underbrush and scrub. Mowing also helps to reduce surface fuels, such as needles to smaller particle sizes, which can decay more rapidly (Fitzgerald 2005). Mowing must be performed frequently and there are some places that may not be accessible for this type of treatment. For instance, this treatment may not be practical on steep slopes, rocky terrain, or in remote areas. Mowing will not be possible in all areas.

Pruning means the removal of the lower branches of trees to artificially lift the crowns. This creates a physical distance between the ground and crown of tree. This technique is more appropriate for younger tree stands where the branches are still low to the ground (Fitzgerald 2005). Care must be taken so that prunings do not become fuel themselves. One might recall, that older Ponderosa pines shed their lower branches as a result of light deficiency. Pruning mimics this natural process.

Thinning of the stand means increasing the space between individual trees by removal of the entire tree. Thinning removes intermediate and smaller trees to reduce the possibility of ladder fires (Fitzgerald 2005). Thinning is often done in stages, so that the forest has a chance to adapt to changing light and wind conditions. The goal of thinning is to leave the oldest, most fire-resistant trees, enhancing the stand's natural fire resistance. As with mowing and pruning, after thinning has taken place, debris must be piled and burned in order to reduce the amount of surface fuels available.

It might be noted that there are two types of thinning. Thinning from above means taking out the tallest trees. This is not preferable in a fire Ponderosa pine forest, as this reduces the number of older more fire-resistant trees. This thinning method actually increases the fire vulnerability of the stand. The only trees that are left are those that are young, have extra foliage and that have not developed the thick bark of the older trees. Thinning from above is a technique that is associated with timber production and sustainable forestry in hardwood forests. However, it is not consistent with fire prevention strategies in the Ponderosa pine forest.

Thinning from below is the preferred fire prevention strategy. This means removing the younger, least fire-resistant specimens of the stand. This type of thinning removes surface and intermediate fuel, but does not encourage continued renewal of the old-growth forest. It preserves the existing stand, but does not facilitate future replacement of that stand. The type of thinning selected depends on the ultimate goals for the stand. If it is long-term timber production, then thinning from above may be the better method, but this must be done with consideration to the increased fire potential. If the old trees are gone, all that is left is the younger, more vulnerable stand. The crown is closer to the ground and the thick bark is not present. Thinning from above means leaving a stand that could easily be wiped out entirely by fire.

In order to determine which method or combination of methods is appropriate, the U.S. Forest Service has developed a simulator that mimics the effects of Forest Vegetation. The Forest Vegetation Simulator (FVS) is the most widely accepted application for determining the best fire mitigation strategy (Hollerstein n.d.). Use of the FVS represents state of the art technology in fire prevention.
The Healthy Forests Restoration Act considers fire to be a major risk to old growth forests. One of the key goals of the program is large tree retention. The protection of endangered species must weigh the costs and benefits of habitat disturbance with the risk of fire. In some cases, such as the red-cockaded woodpecker, fire is necessary for their survival (U.S. Department of the Interior 2004). However, other species are adversely affected by fire due to habitat destruction. The costs and benefits of prescribed burning must take both the plant and animal ecology of the area into consideration.

The overwhelming consensus of expert opinion, supported by academic study, is that pre-treatment of Ponderosa pine forests in the western United States results in fewer high-intensity, wide area fires. In a majority of the cases, pre-treating to reduce ladder fires and preserve large fire-resistant trees can significantly reduce the impact of wildfires (Brown, Agee, and Franklin 2004). Current practices mimic the natural defenses of the forest and significantly reduce the impact of fire, should it occur.

Cited References

1. Agee, J.K. 2002. Fire behavior and fire-resilient forests. In Fitzgerald, S.A., editor. Fire in Oregon's forests: risks, effects and treatment options. A synthesis of current issues and scientific literature. Special Report prepared for the Oregon Forest Resources Institute, Portland, or; 119-126. In Fitzgerald, Stephen. 2005. Fire Ecology of Ponderosa Pine and the Rebuilding of Fire-Resilient Ponderosa Pine Ecosystems. USDA Forest Service Gen. Tech. Rep. PSW-GTR-198. [Internet]. [Cited 2009 February 19]; Available from:

197-225. http://www.fs.fed.us/psw/publications/documents/psw_gtr198/psw_gtr198_n.pdf

2. Brown, Richard, Agee, James and Franklin, Jerry. 2004. Forest Restoration and Fire: Principles in the Context of Place. Conservation Biology. [Internet]. [Cited 2009 February 19]; 18 (4): 903-912. Available at http://www3.interscience.wiley.com/journal/118784304/abstract?CRETRY=1&SRETRY=0

3. Fitzgerald, Stephen. 2005. Fire Ecology of Ponderosa Pine and the Rebuilding of Fire-Resilient Ponderosa Pine Ecosystems. USDA Forest Service Gen. Tech. Rep. PSW-GTR-198. [Internet]. [Cited 2009 February 19]; Available at http://www.fs.fed.us/psw/publications/documents/psw_gtr198/psw_gtr198_n.pdf

4. Hollerstein, Kurt. n.d. Simulating Fuel Treatment Thinnings and Biomass Flow from Western Forests. Oak Ridge National Laboratory, Bioenergy Feedstock Development Program. [Internet]. [Cited 2009 February 19]; Available at http://bioenergy.ornl.gov/papers/western2/methodology.html

5. Skinner, Carl and Ritchie, Martin. 2008.The Cone Fire: A Chance Reckoning for Fuel Treatments. Fire Science Brief. [Internet]. [Cited 2009 February 19]; (4). Available at http://www.firescience.gov/projects/03-2-3-20/03-2-3-20_brief.pdf

6. U.S. Department of Agriculture 2007. An Assessment of Fuel Treatment Effects on Fire Behavior, Suppression Effectiveness, and Structure Ignition on the Angora Fire. [Internet]. [Cited 2009 February 19]; Available at http://www.fs.fed.us/r5/angorafuelsassessment/

7. U.S. Department of the Interior. The Healthy Forests Initiative and Healthy Forests Restoration Act. Interim Field Guide. FS-799. February 2004. [Internet]. [Cited 2009 February 19]; Available at http://www.fs.fed.us/projects/hfi/field-guide/web/page10.php#determining