DENVER — The mountain pine beetle epidemic that ravaged Colorado’s lodgepole pines for two decades is over because most of the vulnerable trees are dead, but a second bug that attacks spruce trees is still spreading, forestry experts said Thursday.
The experts had good news about Colorado’s beloved aspen trees, which turn mountainsides bright yellow and orange every autumn: They’re generally faring well after suffering worrisome die-offs from drought in previous years.
The U.S. Forest Service and Colorado State Forest Service conduct an aerial survey of the state’s trees every year, and the 2015 survey was released Thursday.
It showed the mountain pine beetle has returned to pre-epidemic levels after attacking more than 5,300 square miles of forest since 1996, leaving large swaths of forest a dull reddish-brown.
The epidemic subsided because few vulnerable trees were left for the beetles to infect, the survey found. The beetle primarily attacks tall, slender lodgepole pines, but it also got into larger ponderosa pines.
Lodgepoles are resilient, and seedlings are growing amid the beetle-killed trees, said Bob Cain, an insect expert with the U.S. Forest Service. “Lodgepole regenerates very quickly when you get sunlight on the forest floor.”
A related insect, the spruce beetle, attacked another 285 square miles of spruce trees last year for a total of 2,500 square miles since 1996. That was a smaller increase than the year before, but he said it’s too early to tell whether the epidemic has peaked.
Spruce trees are generally found in wetter, higher-elevation areas than lodgepole pines, he said. The spruce beetle epidemic will not likely cover as big an area as the mountain pine beetle because spruce forests are more widely separated than lodgepole forests, he said.
Spruce forests may recover more slowly than lodgepoles because spruce prefer shade and don’t do as well in the direct sunlight of a dead forest, Cain said.
Persistent drought in the early 2000s contributed to both outbreaks by weakening the trees and making them more vulnerable, he said. Spruce beetles tend to start in trees blown down by high winds and then spread to standing trees, while mountain pine beetles start in standing trees, he said.