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The Ponderosa Pine: Wildfires and Controlled Burns

April 30, 2012

 

Every year, about 5 million acres burn in the United States as a result of wildfires (often caused by arson or sheer carelessness), causing millions of dollars in damage­.   As a fire begins, it spread quickly, at roughly 14.29 miles per hour.  It destroys nearly everything in its path.  The fires spread over brush and trees, like a frenzied monster, grasping at everything near it just to keep itself alive.  The fire can even spark additional fires, miles away, from embers drifting in the wind (Bosner, 2012).
 
Wood has a  flash point (the point at which it will burst into flames) of 572 degrees Fahre­nheit.  As wood is heated to reach this temperature, it releases hydrocarbon gases that mix with oxygen in the air, combust and create fire (Bosner, 2012).
 
 A fire requires fuel ­to burn, air to supply oxygen, and a heat source to bring the fuel up to ignition temperature. Heat, oxygen and fuel form the fire triangle. Fire­fighters often use the fire triangle in their strategic plans when extinguishing a wildfire.  The general idea is that if they can remove a leg of the triangle, they can gain some control and ultimately extinguish the fire (Bosner, 2012).

Three factors that determine how a fire spreads are fuel, weather and topography. Wildfires can quickly fizzle or turn into a raging blaze that scorches thousands of acres, all dependent upon the immediate conditions.  Historically, nature has used wildfires to benefit the forests by thinning the underbrush and seedlings, but with the encroachment of man, these wildfires tend to do more harm than good. 

Ponderosa pine forests are especially susceptible to fires due to the extreme flammability of the foliage surrounding them on the forest floor (Cromack, 2000).  The Ponderosa pine itself,  is considered to be fire resistant, and is only damaged when the fire “crowns” (burning at canopy height) and 60% or more of the tree is destroyed (nps.gov).  Dry pine needles of the Ponderosa add to the leaf litter and are extremely flammable.

The Ponderosa pine is prolific and its seedlings will grow even in full shade.  As great as this sounds, it is actually a problem.  As the seeds fall from the pine cones and land near to the mature tree, they germinate and quickly begin to grow.  This concentration lends itself to thick stands of seedlings that begin to compete for nutrients.  This competition keeps the seedlings from reaching maturity.

Controlled burns are the preferred method of thinning the Ponderosa pine forests.  The underbrush and thick stands of smaller seedlings can be burnt away, leaving the larger Ponderosa pines virtually unharmed.  Not only does this improve the likelihood of these trees reaching maturity, it also opens up the forest floor for grasses to grow.

So, what happens if a burn does not occur periodically?  The underbrush and seedlings choke each other out, the seedlings don’t reach maturity, and as mature trees die off, there are none to replace them.

Image 2.  Damage caused by the Mountain Pine Beetle in Rocky Mountain National Park (Bchernicoff, 2012).

Another reason that the burns are beneficial to the long term survival of the forest is that when thick stands of seedlings form, they are found irresistible by the Mountain Pine Beetle.  This particular beetle carries with it a fungus, the Blue Stain Fungus, in its saliva.  This fungus kills the Ponderosa pine.  Researchers have found that so long as the thick stands of seedlings are removed, the Mountain Pine Beetle isn’t drawn to the mature trees (nps.gov).

Through a study done on a Ponderosa pine forest in the Black Hills of South Dakota, researchers have found that the thinning of over-story trees to reduce continuity of canopy fuels and the removal of understory trees to limit ladder fuels (material between low-level vegetation and tree canopies) reduce the likelihood of a severe fire (quantified as a fire burning in excess of 10 ha2).  This form of thinning, coupled with controlled burns and/or mechanical treatments (such as selective logging) reduce surface fuel accumulations (Battaglia, 2008).   These treatments are applied with the expectation that they will change the potential behavior of wildfires from active crown fires to slow moving surface fires to aid in suppression efforts.  Retention of a low-density mature over-story coupled with ground disturbance can create ideal conditions for prolific regeneration (Battaglia, 2008).  Combined treatments (thinning and burning) tended to have the greatest effect on reducing surface fuels and stand density, and raising modeled crowning and the torching index (transition from surface to crown fire), as compared to burning or thinning alone (Fulé, 2012).

A recent study by Northern Arizona University found through fire behavior simulation studies, that treatments had consistent and significant effects in reducing forest vulnerability to crown fire by increasing wind speeds necessary to support active crown fire and Torching Index (Fulé, 2012).  To determine this, they had looked at the relationships between forest structure, fuel variables, and fire behavior.

In order to protect the longevity of our Ponderosa pine forests, it is advantageous to conduct controlled burns.  This ensures that the soil of the forest floor is rejuvenated, that new grasses grow, and that the underbrush is minimized.  All of these factors directly impact the success of the Ponderosa pine and the immediate plant community surrounding it.

RESOURCES

  1. Battaglia, Mike A.  Can prescribed fire be used to maintain fuel treatment effectiveness over time in Black Hills Ponderosa Pine forests?  Forest Ecology and Management.  Vol. 256, no. 12, p. 2029-2038 (2008 Dec. 10).
  2. Bchernicoff. Wikimedia Commons. 2012. Mountain pine beetle damage in Rocky Mountain National Park. http://upload.wikimedia.org/wikipedia / commons/5/52/Mountain pine_beetle_damage_in_Rocky_Mountain_ National_Park.jpg
  3. Bonsor, Kevin.  “How Wildfires Work”  29 May 2001.  HowStuffWorks.com. <http://science.howstuffworks.com/nature/natural-disasters/wildfire.htm&gt;  (2012 May  07).
  4. Contreras, Kent. Wikimedia Commons. 2003. B&B Fire (Booth area), 21 Aug 03. http://upload.wikimedia.org/wikipedia/commons/f/fd/B%26B_Fire_%28Booth_area%29%2C_21_Aug_03.jpg
  5. Cromack, K. Jr.  Assessing the impacts of severe fire on forest ecosystem recovery.  Journal of Sustainable Forestry. Vol. 11 (1/2), p. 177-228 (2000).
  6. Fulé, Peter Z.; Crouse, Joseph E.; Roccaforte, John Paul; Kalies, Elizabeth L.  Do thinning and/or burning treatments in western USA ponderosa or Jeffrey pine-dominated forests help restore natural fire behavior?  Forest Ecology & Management, Vol. 269, p68-81, 14p (2012 Apr.).
  7. http://www.nps.gov/wica/naturescience/trees-ponderosa-pine.htm
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One Comment leave one →
  1. May 11, 2012 9:53 am

    The beetle thing is pretty cool – I had no idea that it was the fungus they carried rather than their own direct actions that had such an impact on ponderosa pines. I’m curious about what the focus of the next post will be!

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