Fire Ecology in Ponderosa Pine

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).

Prescribed Burns

There are several methods for achieving these conditions within the forest. The first is prescribed burning. 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.

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.