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.