CU Boulder study finds main cause of tree deaths in Colorado's subalpine forests is hot, dry summers

Bark beetles and wind kills far fewer trees in these forests, study found
Extreme heat, dry summers main cause of tree death in Colorado’s subalpine forests
Posted at 8:43 AM, Aug 04, 2021

BOULDER, Colo. — Trees in Colorado's subalpine forests are dying at increasing rates, and bark beetles and wildfires aren't the only factors to blame. Even in their absence, the trees are dying more and more due to warmer and drier summers, according to a study by the University of Colorado in Boulder.

The research was published earlier this year in the Journal of Ecology.

The study found that tree mortality in the state's subalpine forests has more than tripled since the 1980s — tree deaths increased from 0.26% per year during 1982 to 1993, to 0.82% per year during 2008 to 2019. And that was in areas not affected by bark beetles or wildfire in the last decade.

More than 10,000 square miles in Colorado is subalpine forest.

In Colorado, heat and drought alone are responsible for more than 70% of tree deaths in the 13 areas of subalpine forest researchers measured in the past 37 years, according to the study. Bark beetles are responsible for about 23% of tree deaths and wind causes 5%, according to the study.

Robert Andrus, lead author of the study and postdoctoral researcher at Washington State University, said that even in places where the forest looks healthy, mortality is increasing due to the hot and dry conditions alone.

“It is really challenging because it's not very visually obvious to the casual observer,” Andrus said. “But the thing to keep in mind is that while warmer, drier conditions are also causing more fire and bark beetle outbreaks, these slow and gradual changes are also important.”

The conditions are affecting both large trees and new trees trying to take root, especially in the southern Rocky Mountains of Colorado, southern Wyoming and parts of northern New Mexico.

“It was really surprising to see how strong the relationship is between climate and tree mortality, to see that there was a very obvious effect of recent warmer and drier conditions on our subalpine forests,” Andrus said. “The rate of increasing mortality is alarming.”

He said it's an early warning sign of climate change.

This is the longest-running study on tree mortality in Colorado. It was started by Tom Veblen, co-author of the study and professor emeritus of geography, when he arrived on campus in 1982. Every three years, graduate students, postdoctoral researchers and undergraduate field assistants have returned to 5,000 specific trees on the Niwot Ridge west of Boulder to record any changes to the trees' health.

The study expects the tree deaths to increase along with increased warming in the next few decades due to climate change. Temperatures in Colorado increased by about 2 degrees Fahrenheit since the 1980s.