Longer warm seasons explain severity of epidemics, not lack of cold snaps
WINTER PARK, Colo. – Rising temperatures have caused the mountain bark beetles that have plagued forests from British Columbia to New Mexico, right?
Well, not exactly, Jeffry Mitton, who has been studying beetles since 1976, was in Winter Park recently, near the epicenter of the epidemic that in places has killed 80 to 90 percent of lodgepole pine trees.
Mitton said it’s not that the winters haven’t been cold enough to pare beetle populations. Instead, he said that spring has arrived six to eight weeks early. As a result, the beetles produced two generations each summer.
“Two generations instead of one means that there’s an exponential increase in the number of beetles in the forest,” Mitton said, according to an account in the Sky-Hi News. “That means there’s an exponential increase in the number of trees being attacked.”
Beetle attacks on spruce trees of south-central Colorado have recently been expanding, creating scenes at Wolf Creek Pass seen a decade ago around Winter Park. And in the American South, another species of bark beetle has been expanding its range.
“Three species of bark beetles all doing similar things – it’s all related to temperatures,” Mitton said.
“I really think it’s summer temperatures that are hurting them more than the (absence of) winter cold,” he said.
Mitton observed that in the last 25 years the mountain bark beetles have climbed 2,000 feet above their previous range in Colorado, and they have traveled 400 miles farther north in Canada.
Epidemics in lodgepole forests typically occur on average every 60 years, he said, but not with precise regularity. He said it’s unclear how the warming climate could affect the cycles of bark beetle epidemics in the future.
But according to the Sky-Hi News, Mitton was not appalled by the forests’ decimation. Aspen can replace lodgepole stands killed by beetles.
“Yes, it’s awful that the trees died,” he said. “But hey, that’s not so bad.”