Warming Temps Not the Only Factor in Beetle Outbreaks


Thanks to Bob Berwyn for this interesting post! here is a link to it and below is an excerpt.

“This amount of warming could be the difference between pests surviving in areas that were historically unfavorable and could permit more severe and prolonged pest outbreaks in regions where historical outbreaks were halted by more frequent cold bouts,” said Aaron Weed, a former postdoctoral researcher at Dartmouth and now an ecologist with the National Park Service.

Despite the trend for warming winters across western U.S. forests, not all beetle populations between 1997 and 2010 have responded to winter warming. In the 11 coldest ecoregions, winter temperatures lethal to the mountain pine beetle have become less frequent since the 1980s and beetle-caused tree mortality has increased significantly in these regions.

But in the 12 warmer ecoregions, recent beetle epidemics cannot be attributed to warming winters because earlier winters were rarely cold enough to kill the beetles.

Although winter warming has been occurring across the western United States for decades, it has only permitted mountain pine beetle outbreaks in regions where winters historically killed more than 50 percent of the beetles — primarily in the Middle Rockies (Idaho, Montana and Wyoming), the Blue Mountains in eastern Oregon and northern Colorado. In these regions, winter temperatures during the 1980s were more likely than in recent years to drop below the lower lethal temperatures for mountain pine beetles.

But in coastal and southern regions, winters dating back to 1980 were never cold enough to cause substantial beetle mortality. However, these warmer regions are undergoing sustained and, in some forests, increasing impacts from beetles.

Forest managers need to consider other factors, including forestry practices that have influenced forest composition, the study concluded.

Here is the link to the study in Landscape Ecology.

Of course, may of us thought the situation (not exactly a forestry practice since our public western forests aren’t really managed forests) that leads to “lots of contiguous old lodgepole” could be a factor, at least with MPB.

5 thoughts on “Warming Temps Not the Only Factor in Beetle Outbreaks”

    • Jon, that study is one of many…that say the same thing.. but what they are refuting is an argument that really doesn’t exist, at least not among fire managers that I know.

      beetles kill trees, trees dry out and fall down, that changes fire behavior .. making them harder sometimes and less safe to fight if the trees are jackstrawed. I have yet to see a study that says that that is not the case.

      • There’s some logic to that theory, but if fires were harder to fight because of insect-killed trees, wouldn’t you expect that to show up in “the number of acres burned in recent severe wildfires?” So this study seems to be suggesting that is not the case (at least at a large aggregate scale).

        • No, because many severe wildfires are going to be severe without trees at all (e.g. southern Cal) and severity is a function of fuels (amount and condition) plus weather conditions at the time of the fire, plus fire suppression strategies and decisions.. Super bad conditions can lead to super small fires cause folks are so cautious.

          When you are trying to make a claim, scale choice can make a big difference. Back to my favorite example (back in the day of changes to the economy ) One claim was that there were no bad effects of reducing logging in the state of Washington because the state still looked good. But if you were sitting in Forks it did not look so good. Scale choice often has implicit values. Now there is a tendency for academics to try to generalize claims (rather than we studied three timber sales and …., we found that generally x causes y). I think this goes way back in the history of science to 19th century “physics envy.” Things look different at different scales but to understand why things are the way they are it seems to me you need to understand the mechanics of how they work in the real world.

          We used to say “you can’t infer causation from correlation” and I think that’s still true.


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