Beetles, Bark and Burns

Defying common assumption, new research from the northwestern U.S. suggests insect infestations might actually lessen the severity of wildfires, potentially alleviating fears that the spread of pine beetles may have a compounding effect when it comes to burns in the boreal forest.

A recent study led by Garrett Meigs examined the interaction between forest fires and outbreaks of mountain pine beetle and western spruce budworm in the forests of Washington and Oregon, and came to the conclusion that forests devastated by such defoliators tend to see a decrease in the severity of burns.

Mountain pine beetles are devastating the northwest, attacking more than 44 million acres of pine trees  an area the size of Missouri — over the past 15 years in B.C. and recently making their way past the border into the southern Northwest Territories.

“One of the things that’s exciting about the study is that, in terms of conventional wisdom, people expected more of a positive feedback where insects had changed forests, to make fire have a more severe impact,” Meigs told EDGE.

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It’s the first large-scale, advanced spatial analysis of the interplay between the two massive impacts on forests in North America, related to both climate change and land use issues. Looking at a large number of insect outbreaks and fires over 25 years, Meigs was able to conclude that there is a correlation between the two factors, and that insects tend to reduce the severity of wildfires.

The analysis used remote satellite imagery to determine the severity of burns over the study area of approximately 40 million hectares, based on the consumption of vegetation, or tree mortality.

For the most part, the study found insects had a neutral effect on a majority of burns; however, as time passed, the forests that had been killed by bug infestations showed a decline in fire severity.

Meigs believes the findings are related to the stages a forest goes through when it is devoured by insects. Within the first year, the needles die and change to a reddish colour. Then, after about three or four years, the needles become grey and fall. Finally, at around 10 to 15 years, the trees themselves begin to fall.

“As they fall, there’s less fuel,” Meigs says. “There’s a general thinning effect.”

Mountain pine beetles are devastating the northwest, attacking more than 44 million acres of pine trees an area the size of Missouri — over the past 15 years in B.C. and recently making their way past the border into the southern Northwest Territories.

While pine beetles were formerly unable to survive in the NWT due to the cold temperatures of the harsh northern winter, warming weather thought to be linked to climate change means the few bugs that have made their way into the territory are now surviving year-round.

Meigs believes his study holds important implications for those involved in fire management, adding insects to the list as another variable to consider among the growing number of climate-related factors influencing today’s complex fire regime.

“By dampening subsequent burn severity, native insects could buffer rather than exacerbate fire regime changes expected due to land use and climate change,” reads the paper. “In light of these findings, we recommend a precautionary approach when designing and implementing forest management policies intended to reduce wildfire hazard and increase resilience to global change.”

That said, Meigs recommends using caution when applying these findings to other areas, such as the NWT, which he acknowledges has its own local conditions and context.

“Insects are just another piece of the puzzle,” he says. “It’s already a complex management system, and insects add to that complexity in a way that increases the variability. But one of the implications could be that if insect outbreaks aren’t as extreme of a fire hazard as we expected, that might give fire managers more flexibility around prioritizing activities; that these areas are not necessarily the highest or only priority.”

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