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Study: Extreme Heat Is Killing Subalpine Trees

A forest with green vegetation.
Jon Flobrant

Another heat wave is gripping much of the country this week, including areas in Idaho, Montana, and Wyoming. And research shows that's bad news for trees in high-elevation forests.

A study published earlier this year in the Journal of Ecology found that even without bark beetle outbreaks and wildfire, trees in Colorado's subalpine forests are dying at increasing rates from extreme summer conditions.

Researchers found that more than 70% of tree deaths were due to heat and drought. Bark beetles accounted for 23% of tree deaths, and 5% were due to wind damage.

The study focused on 13 areas of subalpine forest over a 37-year period.

Lead author Robbie Andrus said warmer and drier conditions draw more moisture out of trees and soil.

"Their physiological functions often decline or fail, and they can die as a result of that," said Andrus, who completed the research while at CU Boulder.

The study also found that it's the older and larger trees that are dying more rapidly. Andrus said, those trees store the most carbon, which could eventually lead subalpine forests to "function less as a carbon sink than they currently do."

Andrus said it's an early warning about climate change. "What we're sort of showing is this alarm bell going off, like, 'Hey, we need to be aware that things might be changing, or that things are changing.'"

Along with higher tree mortality, the study found that hotter and drier conditions are making it difficult for new trees to grow in high-elevation forests in Colorado, Wyoming, and New Mexico.

This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Nevada Public Radio, Boise State Public Radio in Idaho, KUNR in Nevada, the O'Connor Center for the Rocky Mountain West in Montana, KUNC in Colorado, KUNM in New Mexico, with support from affiliate stations across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting.

Maggie Mullen is Wyoming Public Radio's regional reporter with the Mountain West News Bureau. Her work has aired on NPR, Marketplace, Science Friday, and Here and Now. She was awarded a 2019 regional Edward R. Murrow Award for her story on the Black 14.
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