By Katie Langford
The rate at which trees die in Colorado’s subalpine forests tripled over the past four decades and is linked to rising temperatures and less moisture, according to a study by University of Colorado Boulder researchers.
Researchers studied more than 5,000 trees in 13 plots along Niwot Ridge, near CU’s Mountain Research Station outside Nederland, focusing on areas of forest that were not impacted by fire or bark beetle outbreaks.
Between 1982 and 2019, tree mortality — or the rate at which trees die — increased from .26% per year to .82% per year.
“There is a very clear relationship between higher rates of tree mortality and warmer and drier conditions during the summer,” said lead author Robert Andrus, a postdoctoral researcher at Washington State University who conducted the research while completing his graduate degree at CU.
The study also found that larger and older trees are dying more rapidly than smaller and younger trees.
“Those are the ones that store the most carbon, and they’re dying more rapidly, so that has a potential effect on the forest’s ability to function as a carbon sink in the future,” Andrus said. Combined with past research that shows that tree seedlings struggle to establish themselves in warmer and drier conditions, the study is “an alarm bell going off,” said study co-author and retired CU Professor Tom Veblen.
Veblen established the 13 research plots in 1982 because he wanted to conduct long-term monitoring of how tree populations were impacted by climate variability, fire and insect outbreaks.
By the mid-’90s, Veblen said, the increase in tree mortality was clear.
“The general public needs to be aware that climate change, in the sense of warming climate and drier conditions, is not something we talk about being in the future. It’s been going on,” Veblen said.
Subalpine forests are typically very dense, but the increasing mortality rate and slowing of new growth could have impacts on carbon storage as well as water cycles, Andrus said.
“Climate change is having an important effect on the forest in the backyard of the Colorado Front Range,” Andrus said. “We need to take climate change seriously to protect the forest for future generations, and we’re seeing that effect outside of wildfires and bark beetles. There’s a wholesale change in our forests in Colorado.”