Tuesday, January 29, 2013

Forest Fires and Unintended Consequences

In the  Bitterroot low-altitude Ponderosa Pine forest in Montana the historical records, plus counts of annual tree rings and datable fire scars on tree stumps, demonstrate that a Ponderosa Pine forest experiences a lightning-lit fire about once a decade under natural conditions. A program of active fire suppression began around 1910 and became effective after 1945. Under this program small fires were aggressively attacked by fire fighters in an effort to eliminate or limit forest damage. It was remarkably successful; the effect was less so. 

The mature Ponderosa trees have bark two inches thick and are relatively resistant to fire. This allows the mature trees to resist fire while the smaller, more fire-sensitive Douglas Fir seedlings that have grown up since the last fire take the brunt of the damage.. After only a decade's growth until the next fire, those seedlings are still too low for fire to spread from them into the crowns of the mature trees. Hence the fire remains confined to the ground and lower brush. As a result, many natural Ponderosa Pine forests have a park-like appearance, with low fuel loads, big trees well spaced apart, and a relatively clear "understory".

The effect of aggressive fire suppression for decades let the understory fill up with Douglas Fir saplings that would in turn become valuable when full-grown. Tree densities increased from 30 to 200 trees per acre, the forest's fuel load increased by a factor of 6, and Congress repeatedly failed to appropriate money to thin out the saplings. Sheep grazing in national forests may also have played a role by reducing understory grasses that would otherwise have fueled frequent low-intensity fires. When a fire finally does start in a sapling-choked forest, whether due to lightning or human carelessness or (regrettably often) intentional arson, the dense tall saplings may become a ladder that allows the fire to jump into the mature trees' crowns. The outcome is sometimes an unstoppable inferno in which flames shoot 400 feet into the air, leap from crown to crown across wide gaps, reach temperatures of 2,000 degrees Fahrenheit, kill the tree seed bank in the soil, and may be followed by mudslides and mass erosion.

So the well intentioned--and successfully performed--fire suppression of early fires ironically results in bigger and more destructive fires and forest damage.

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