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Climate: Bark beetles invading high-elevation forests

Whitebark pines are in imminent danger of extinction, and global warming is one of the most significant threats to the species. Photo courtesy U.S. Forest Service.

Whitebark pines are in imminent danger of extinction, and global warming is one of the most significant threats to the species. Photo courtesy U.S. Forest Service.

Researchers see threat to whitebark pines

By Summit Voice

FRISCO —Bark beetles have already killed millions of acres of mid-elevation forests across the West, and warming temperatures are enabling the tree-killing bugs to invade higher elevations, where they are attacking trees that haven’t evolved with strong defenses to repel them.

Global warming is essentially giving the insects a huge advantage, as the trees, with their long lifespans, have no chance to develop biological resistance, according to researchers from the the University of Wisconsin-Madison, who report a rising threat to the whitebark pine forests of the northern Rocky Mountains.

The whitebark pine forest sat treeline provide critical habitat for iconic species such as the grizzly bear and play an important role in governing the hydrology of the mountain west by shading snow and regulating the flow of meltwater.

Federal biologists have acknowledged threats to whitebark pines, declaring the trees as “warranted but precluded” from listing under the Endangered Species Act. And Forest Service experts are studying ways to preserve and restore forests around the country.

“Warming temperatures have allowed tree-killing beetles to thrive in areas that were historically too cold for them most years,” said Ken Raffa, a UW-Madison professor of entomology and a senior author of the new report. “The tree species at these high elevations never evolved strong defenses.”

A warming world has made it easier for mountain pine beetles to invade new and defenseless ecosystems. With winters growing shorter and more mild, the bugs aren’t succumbing to the bitter cold snaps that once kept their populations in check.

“A subject of much concern in the scientific community is the potential for cascading effects of whitebark pine loss on mountain ecosystems,” said Phil Townsend, a UW-Madison professor of forest ecology and also a senior author of the study.

The mountain pine beetle’s historic host is the lodgepole pine, a tree common at lower elevations. Typically, the insects play a key role in regulating the health of a forest by attacking old or weakened trees and fostering the development of a younger forest.

However, recent years have been characterized by unusually hot and dry summers and mild winters, which have allowed insect populations to boom. This has led to an infestation of mountain pine beetle described as possibly the most significant insect blight ever seen in North America.

Because lodgepole pine co-evolved with the bark beetle, it has devised stronger chemical countermeasures, volatile compounds toxic to the beetle and other agents that disrupt the pine bark beetle’s chemical communication system.

Despite its robust defense system, the lodgepole pine is still the preferred menu item for the mountain pine beetle, suggesting that the beetle has not yet adjusted its host preference to whitebark pine. “Nevertheless, at elevations consisting of pure whitebark pine, the mountain pine beetle readily attacks it,” Townsend said.

The good news, he adds, is that in mixed stands, the beetle’s strongest attraction is to the lodgepole pine, suggesting that, at least in the short term, whitebark pine may persist in those environments.

The study was conducted in the Greater Yellowstone Ecosystem, one of the last nearly intact ecosystems in the Earth’s northern temperate regions. It showed that the insects that prey on or compete with the mountain pine beetle are staying in their preferred lodgepole pine habitat.

That is a concern because the tree-killing bark beetles “will encounter fewer of these enemies in fragile, high-elevation stands, Townsend said.

Whitebark pine trees are an important food source for wildlife, including black and grizzly bears, and birds such as the Clark’s nutcracker, named after the famed explorer and which is essential to whitebark pine forest ecology as the bird’s seed caches help regenerate the forests.

With their broad crowns, the high-elevation whitebark pines also act as snow fences, helping to slowly release water into mountain streams and extending stream flow into mountain valleys well into the summer.

“Loss of the canopy will lead to greater desiccation during the winter and faster melting in the summer due to loss of tree canopies for shade,” according to Townsend. “This is possibly a short-term effect of the loss of whitebark pine,” he explained “If – and it is a big if – other tree species replace it, eventually this service may be replaced.”

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