Increased fire intensity in western Ponderosa pine forests

Fire Ecology in Ponderosa Pine Forests

The Ponderosa Pine forests of the western United States are a spectacular site to see. However, as one listens to the wind rustle through the needles high above and listens to the songbirds sing, it is easy to forget the intimate relationship of the pine forest with fire. Fire is a constant mechanism of change and renewal in the pine forest. However, pine forests are resilient and able to bounce back quickly after a fire. Recently, concerns have arisen that the fires that are occurring are becoming more intense and lethal. They consumer large expanses of the pine forest and threaten its ability to recover and thrive (Fitzgerald 2005). This research will explore the more recent research into the reasons for the growing intensity of these fires and will explore modern theory as to what can be done to help prevent them and fight them when they start.

Historical Changes in the Pine Forest

According to the UDSA Forest Service, one of the reasons for increasing intensity of fires is changes in the pine stands themselves. When the area was first settled the area, stand densities were from 20-50 trees per acre (Fitzgerald 2005). Now the average stand density is 1235-24070 trees per acre (Fitzgerald 2005). This increase in stand density provides an enormous amount of fuel for the fire once it gets started. These increases in stand density have also had an impact on the health of the trees and landscape health (Fitzgerald 2005). The soil and ecosystem are overtaxed. Unhealthy trees, with dying and dry branches are excellent kindling for fires.

Ponderosa Pine has several adaptations that allow it to survive fire (Fitzgerald 2005). Ponderosa pine is one of the most fire-resistant species in the west (Fitzgerald 2005). The tree develops a thick, cork-like bark that increases the tree's fire resistance as it ages (Fitzgerald, 2005). The bark of the pine sloughs off when it is on fire, minimizing damage to the inner cambial cells and transport system (Fitzgerald 2005). The bark of the pine sloughs off on a regular basis, collecting at the base of the tree. This creates an excellent fuel for fires on the forest floor. However, frequent low-intensity fires help to reduce the accumulated mulch.

The root system of the Ponderosa Pine is deep. If surface roots are killed by the heating of the soil, the deeper roots can still convey water and nutrients to aerial parts of the plant (Fitzgerald 2005). The open crown structure of the pine tree also helps heat to dissipate during a fire (Fitzgerald 2005). The pine needles have a high moisture content, which helps to increase the fire resistance of the tree. Low intensity fires helped to clear seedlings and saplings, leaving the larger more resilient trees intact. As one can see, the Ponderosa pine has developed many characteristics that help it to resist fire and increase its survival rate.

Historically, lightening was the primary source of fire in western Ponderosa pine forests (Fitzgerald 2005). Areas that tend to receive greater amounts of severe weather were the most likely to experience high frequencies of fires as well. Human activities have also played a significant role in fires. More heavily populated areas tend to experience higher frequencies of fire than more remote areas (Fitzgerald 2005). Annual periods of drought also increased the frequency of fires throughout history (Fitzgerald 2005). Livestock grazing also helped to reduce under story growth and fuel. Dendrochronology has provided many clues about pre-settlement and historical fire patterns. Old photos of the region also tell us what the Ponderosa pine forests were like in historical times.

One of the key concerns over recent fires in the Ponderosa pine forests is that they are becoming more intense and destroy larger area of the forest, often to the point where recovery is difficult. There are several historical factors that have led to the conditions the presently fuel intense fires. Ponderosa pine forests used to have a cycle of regular, low intensity fires (Fitzgerald 2005). These fires kept fuel from accumulating on the forest floor and thinned the stands by destroying the small saplings. Thinner stands meant that it was more difficult for fires to spread from tree to tree, wiping out large areas at once. During the late 1800s and early 1900s, heavy grazing altered the under story and reduced the number of low intensity fires (Fitzgerald 2005). As cattle do not eat pine saplings, they were allowed to grow and thrive, increasing stand densities.

In 1910, an active fire suppression regime was started due to increasing populations of human habitation in the area (Fitzgerald 2005). The fire suppression regime was meant to protect property. However, a lack of fires in the pine stands has led to an above normal accumulation of fuel on the forest floor (Fitzgerald 2005). Trees are now closer together, allowing the fire to spread across large areas. In addition, selective logging has removed the larger, more fire-resistant specimens (Fitzgerald 2005).

This removal of the old-growth pines has opened the canopy to allow for the intrusion of Douglas fir and Western larch (Fitzgerald 2005). These species are shorter than the Ponderosa pine and create a "ladder" that allows surface fires to climb into the crowns. These changes in land usage and management have produced conditions that are right for more intense fires that are more difficult to control once they get started.

Understanding Ponderosa Pine Forest Fires

Fire behavior is affected by the type and amount of fuel available, the weather, and topography of the land (Fitzgerald 2005). A change in any one of these factors will influence the fire's behavior (Fitzgerald 2005). Fires tend to travel faster uphill, as the flames are slanted towards new fuel sources (Fitzgerald 2005). Firefighters cannot control the weather or topography, but they can use them to their advantage when a situation presents itself.

The one factor that firefighters can control, to a certain degree, is the amount of fuel available. For instance, performing small, controlled burns, or digging fire lines will help to limit the fuel available to larger fires. Small controlled burns act like the natural fire suppression mechanism of the forest itself. One of the key goals of controlled burn techniques is preventing the fire from "torching" and moving up into the crown of the trees (Fitzgerald 2005). A crown fire is difficult to control, as it has a readily available, high-density fuel source. Winds higher up help the fire jump from tree to tree. As flaming debris from the crown fire fall, they ignite the forest floor. This creates a vicious circle that is difficult to control. Therefore, limiting the ability of the fire to "ladder" up into the crowns is an important step to controlling the fire.

The rate and intensity of the fire are determined by a number of factors. One of the key factors is the amount of moisture in the foliage. The amount of moisture changes as the season progresses. Moisture decreases due to drought, creating a fuel source that is more easily ignited (Fitzgerald 2005). Knowing the moisture content of foliage in the area is a key step in predicting the behavior of the fire. A slow moving fire can evaporate moisture in the foliage, drying out the foliage in front of it, leading to conditions that are difficult to control. When one adds wind into the picture, it can be even more difficult to control.

Preventing High-Intensity Fires

As we discussed earlier, humans have altered the natural mechanisms in Ponderosa Pine forests that helped to control the intensity of fires. Preventing all fires is unrealistic, as there are many factors that cannot be controlled, such as drought or lightning strikes. However, measures can be taken to limit the intensity of the fire and its ability to spread rapidly. Some of these measures mimic the natural processes that once controlled fire intensity in the forests.

Once a fire begins, its ability to spread is a function of the amount and density of fuel available (Fitzgerald 2005). Moisture content and wind also play a major role in the ability of the fire to spread. According to Agree (2002), the following methods will help to limit the intensity of fires. In order, these treatments are:

reducing surface fuels reducing ladder fuels leaving large, fire-resistant trees in place spacing tree crowns

These treatments help to restore the natural fire resistance of the Ponderosa pine forest.

In a study conducted in the Black Mountains Experimental Forest, an accidental comparison of treated and untreated forest occurred in September of 2002 (Skinner and Ritchie, 2008, 2004). In this study, an accidental fire started in a section of forest that contained treated and untreated stands. In the high-density untreated stand, fire survival was about 1%. In the treated stand that had been subjected to prescribed burns, pruning and thinning, survival rate of the trees was around 80% (Skinner and Ritchie 2004). Findings from a similar study in the Angora fire resulted in similar findings (USDA 2007).

Best Practices

One of the most difficult tasks in fire prevention and control in Ponderosa pine forests is choosing which methods are best suited for a particular site. 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