Bark beetle invaded tens of thousands of untouched acres of Colorado forests last year as trees suffered under record-breaking heat and extreme drought, says a new report by the Colorado State Forest Service and the U.S. Forest Service.

Spruce beetle attacked 178,000 acres of Engelmann spruce. Although that’s fewer than the 260,000 acres in 2017 and the 250,000 in 2016, about one-third of the acres hit last year were in previously unaffected areas.

The roundheaded pine beetle in southwest Colorado more than doubled its presence to 27,000 acres, up from 11,000 acres in 2017, mostly in Dolores County.

Last year’s drought, the second worst in 124 years, and record-breaking heat were largely to blame, said Dan West, an entomologist with the Colorado State Forest Service.

“That has wide-reaching impacts as the trees are less defended and their susceptibility to invasion goes up,” West said.

A tree’s two chief defenses against bark beetle both hinge on precipitation, West said. Without enough water, a tree cannot build up stores of resin to “push out” bark beetles.

“Think of the resin ducts on the outside of a tree like a garden hose, but filled with resin instead of water,” he said. “For a beetle to bore into the tree and access the tree’s carbohydrates, it has to go through the resin ducts. Once the ducts are filled with enough beetles, the resin pushes them out of the tree like water would burst out of a hose if it was punctured.”

Certain compounds in tree resin also are toxic to bark beetle.

Acres of forest affected by bark beetle in 2018 by county
Areas that were notably affected by bark beetle included the San Juan Mountains and areas of Rocky Mountain National Park.

  • Hinsdale: 19,000
  • Larimer: 18,000
  • La Plata: 13,000
  • Grand: 17,000
  • Rio Grande: 9,900
  • Park: 8,700
  • San Juan: 6,000
  • Fremont: 4,800
  • Fremont: 4,800
  • Teller: 1,600
  • Mineral: 1,400
  • Boulder: 1,400
  • Dolores: 1,300
  • El Paso: 120

But extreme or prolonged drought hinders resin production. The trees’ “fine feeder roots,” which absorb water and micronutrients, die. To grow back, the forest would need multiple years of high snowpack, adequate precipitation and milder weather.

As of Tuesday, Colorado was at 93 percent of its normal snowpack and at 156 percent of last year’s, reports the U.S. Department of Agriculture.

Nonetheless, about 85 percent of the state is in some level of drought. About 10 percent, in southwest Colorado, is in exceptional drought.

The Fourth National Climate Report, released by the federal government in November, predicted an 8.6 degree increase in the average annual temperature in the Southwest by 2100 with high greenhouse gas emissions. The hotter weather will increase the possibilities of mega-droughts lasting decades.

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