2015-04-19

Forests killed by the mountain pine beetle don't pose increased wildfire risk

In the past decade, the mountain pine beetle has severely impacted pine forests across more than 50 million acres of western North America. In the aftermath of this epidemic is there a significant increase in wildfire threat? For many tasked with managing wildfires the answer is a resounding "yes." However, a recent research paper from a team of scientists based in Colorado suggests that forests killed by the mountain pine beetle do not pose an increased risk to wildfire. The paper relies on the recent area burned in wildfire superimposed on the area recently infested by the beetle. Because the areas of recent wildfires and beetle activity don't overlap, the authors contend that the risk posed by the mountain pine beetle is overblown. Those in the wildland fire management and suppression fields who disagree base their disagreement on actual observations of extreme fire behavior in forests killed by the mountain pine beetle.

So who is correct in this debate? A paper published in 2014 in the online journal Fire Management Today sought to address this problem by looking at two critical factors: the history of beetle attacks in a forest and the time since beetle infestation. To many people, it would seem that the year after initial infestation when the tree crowns all turn red is the time of highest risk. It certainly can be, provided other elements of hazard are also present. Red-attacked trees, in addition to heavy accumulations of dead wood on the forest floor, are a recipe for extreme fire behavior. Red-attack trees and very little or no dead wood on the forest floor is much less of a risk and, once the needles fall off dead trees, fire risk can be low for quite some time.

The authors of the paper challenging wildfire risk do not describe the fuel structure in these forests - only the state of the tree crowns (red or grey). The lack of attention to surface fuels is a weak point in such studies. Throughout much of British Columbia, Canada, over the last several decades mountain pine beetle outbreaks have been fairly common. With each passing epidemic more and more trees in the forest are killed by beetles. After large outbreaks with high mortality, dead trees gradually fall and accumulate on the forest floor. Heavy accumulations of downed wood associated with past beetle outbreaks have contributed to some very impressive fire behavior. Since 2009, British Columbia has seen large areas of beetle-killed forest burn under very high intensity and severity with much of the area burned having experienced significant epidemics in the 1980s and mid-2000s. The fuel structure in these forests has included red and grey crowns plus large quantities of dead wood on the forest floor. Unfortunately, to mitigate the threat there are only a few options and none are very palatable to the public. An increase in logging of the attacked or soon to be attacked trees has been advocated for large areas in the West but it can often lead to a worse fuel hazard if not done diligently; large-scale prescribed burning has been done in some areas with the intent of breaking up a contiguous landscape of fuel; and, relying on natural ignitions to reduce the hazard can be effective provided the fire doesn't burn too severely.