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Climate change, bark beetles harmful to western forests

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Pine Beetle Decline

Dead pine trees that fell victim to pine beetle infestation tower above the ground June 26, 2012 in the Shoshone National Forest near Meeteetse.

CODY — Predicting the future of western forests, which are under increasing assault from various forces, presents a challenge.

“Uncertainty may mean that not all news is good news, but we’re not certain about that,” said Daniel Tinker, associate botany professor at the University of Wyoming. He spoke about the forests’ fate during UW’s Saturday University on Oct. 28 at the Buffalo Bill Center of the West.

Wildfire and bark beetles, for example, are two critical impacts on forests, Tinker explained.

“Climate change complicates all this,” he added. “We know the climate is changing” and can exacerbate the repercussions from fires and beetles.

Also known is that lodgepole stands have burned in 100- to- 500-year intervals over the last 100,000 years, while beetle infestations have occurred in cycles. Still, he observed, fires and beetles are changing western forests in shorter intervals, during a human lifetime.

Some forests, like the lodgepole stands in Yellowstone National Park, may return to some semblance of their former condition. Certain trees of that species sport serotinous cones that contain millions of seeds and require combustion to open, so they provide “post-fire regeneration,” Tinker said.

“We’re seeing more and more big fire years,” he said.

The cause is climate change and the resulting warmer temperatures and earlier spring snowmelt, which combine to cause longer fire seasons, he said. Between 1987 and 2003, there have been four times more large fires and six times more area burned.

Another growing impact comes from bark beetles. Endemic to certain tree species, especially lodgepole, they erupted in population in the late 1990s.

“The current beetle epidemic in the western United States is unprecedented in extent, severity and duration for recorded epidemics,” said Tinker, citing 2016 statistics of 5 million acres in Wyoming and Colorado, 30 million in British Columbia, and a total of 150 million acres in North America. “This is truly a continental-scale epidemic.”

Their life cycle occurs under the bark, Tinker explained. When the adults invade a tree, they emit pheromones to attract other beetles so they’ll kill the host and stop its counterattack with resin. Once the tree dies, the beetles emit anti-aggregation pheromones to signal success.

The beetles swarm in the summer, preferring bigger, older trees, and lay eggs under the bark. The eggs enter the larval phase in the winter, emerge in the spring and feed on the phloem layer, which lies under the bark and conducts nutrients. They carry blue-stain fungus spores that infect the inner xylem layer, blocking the flow of nutrients and water and consequently killing the tree. In 45-60 days, infected trees transform from green to yellow to red. Eventually they turn gray, becoming what Tinker called “zombie trees.”

He identified the epidemic’s causes as stress from drought, the lack of sufficient freezing days during the shoulder seasons and an abundance of suitable trees. However, the loss of mature trees and their over story allows young trees to grow quickly.

“Studies show advanced regeneration” in certain areas, Tinker said. “Some of the forests are doing pretty darn well. We’re still going to have forests,” though perhaps with different makeups.

He noted that there’s no consensus about whether beetle-killed trees increase the fire threat, except when they’re in the red phase. The beetle infestation of lodgepole seems to be waning, but it’s increasing in spruce, while blister rust has been perhaps fatal on white bark pine. The options are few – management through mimicking wildfires for smaller burns, artificial selection of white bark pine resistant to rust, and forest restoration by planting and/or thinning.

“Forests are the source of our water in the West,” Tinker reminded the audience. And despite the assault on western trees, he offered some hope. “Keep the faith. Think about resilience. Think about forests in the future.”


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