The mountain pine beetle has severely affected pine forests across more than 25 million hectares (60 million acres) of western North America in the last decade. In the aftermath of this epidemic, is there a significant increase in wildfire threat?
For many given the task of managing wildfires the answer is a resounding yes. However, a recent research paper from a team of scientists based in Colorado suggests that is not the case and that forests killed by the mountain pine beetle do not pose an increased risk to wildfire (www.pnas.org/cgi/doi/10.1073/pnas.1424037112).
The paper relies on the recent area burned in wildfires, superimposed on the area recently infested by the beetle. Because the areas don’t overlap, the authors contend the risk posed by the mountain pine beetle is over-blown. Those in the wildland fire management and suppression fields who disagree base their disagreement on first-hand observations of extreme fire behavior in forests killed by the mountain pine beetle.
Who is correct in this debate? A paper published in 2014 in the on-line journal Fire Management Today (http://www.fs.fed.us/fire/fmt/fmt_pdfs/FMT72-4.pdf) sought to address this problem by looking at two critical factors: the history of beetle attacks in a forest, and the time since the infestation.
To many people it would seem 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. Critical to building and maintaining the intensity of a running crown fire (the most extreme fire behavior) is the presence of plenty of dead wood on the forest floor. Red-attacked trees plus dead wood on the forest floor is a recipe for extreme fire behavior. Red-attack trees and little or no dead wood on the forest floor is much less of a risk and, once the needles fall off, it is a very low risk 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). This is a weak point in studies looking at large areas and only crown fuels (surface fuel characteristics are difficult to appraise through a forest canopy).
Throughout much of British Columbia over the last several decades, mountain pine beetle outbreaks have been common. With each passing epidemic more trees are killed by the beetle, leading to the last major epidemic, beginning in about 2008, which resulted in the death of entire forests and landscapes.
Trees killed in an outbreak 20 years ago are mostly fallen and have accumulated as large quantities of dead wood on the forest floor. This situation has resulted in some very impressive fire behavior in the last decade and will continue to be a very high hazard for several decades to come.
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 dividing 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.
There is a fourth option: doing nothing. In a landscape in which the trees are exhibiting red crowns or the needles have all fallen and there is no large quantity of dead wood on the forest floor the risk is fairly low — as is shown by the study and a do-nothing-for-now approach could be warranted. However, once the dead trees start to fall and a new forest begins to grow through all the dead wood, the forest quickly changes to a very high risk and will stay there for quite some time until all that dead wood decomposes. You can’t take an entire forest, kill it, place it on the forest floor, and not have a high to extremely high fire risk. If this wasn’t the case, all our existing knowledge of fire behavior science would be wrong.