Colorado's Subalpine Forests Are Dying from Extreme Heat

Tree mortality more than tripled in the forests over 37 years.

Colorado forest

Robert Andrus

Trees are often touted as a solution to the climate crisis, but the high heat and drought it brings with it are also harming forests’ ability to thrive. 

This is the case in the high-elevation woodlands of the Colorado Rockies, where warmer and drier conditions encourage bark-beetle outbreaks and more extreme wildfires. However, a new study published in the Journal of Ecology this year found that these warmer and drier conditions are killing trees even in forests that appear untouched by these obvious causes of death.

“It’s very clear that we need to take climate change seriously,” study lead author Robert Andrus of the University of Colorado (UC) Boulder tells Treehugger in an email. “It’s already affecting our forests. It’s not something that’s happening in the future.”

Alarm Bell

The study focused on more than 5,000 trees in the Niwot Ridge section of the southern Colorado Rockies. These trees are what is known as “subalpine forest,” the highest possible elevation of forest dominated by Engelmann spruce, lodgepole pine, subalpine fir, and limber pine. These are the trees familiar to anyone who hikes or skis in the Colorado Rockies, or simply drives over a mountain pass. 

Researchers checked every tree in the study area every three years from 1982 to 2019, and were, therefore, able to reach the following main conclusions about the impacts of climate change: 

  1. Tree mortality more than tripled in the forests over 37 years, even though they did not experience mass bark-beetle outbreaks or wildfires. 
  2. Rates of tree mortality were higher during years with warmer and drier summers.
  3. Larger and older trees died at higher rates than smaller and younger ones. 

The researchers were able to attribute 71.2% of tree mortality in the study area directly to climate stress and 23.3% of the trees died from bark beetle activity, but this was not the result of an outbreak. Instead, Andrus says, bark beetles are always present in Colorado’s subalpine forests, and trees that are already stressed by other factors are more likely to succumb. Only 5.3% of trees died from wind damage and only 0.2% from other wildlife impacts. 

A tagged subalpine fir tree, one of more than 5,000 marked trees monitored as part of this 37-year-long study in the Colorado subalpine forest on Niwot Ridge, west of Boulder.
A tagged subalpine fir tree, one of more than 5,000 marked trees monitored as part of this 37-year-long study in the Colorado subalpine forest on Niwot Ridge, west of Boulder. Robert Andrus

Andrus notes that the rate of tree death, while on the rise, is not currently very high: It increased from 0.26% per year between 1982 to 1993 to 0.82% per year between 2008 and 2019. However, it is significant first because it covers such a wide area and second because of what it promises for the future if nothing is done to halt the effects of climate change. 

“We’re expecting to see warmer and drier conditions in the future and that should increase rates of tree mortality,” Andrus says.

More tree death could seriously alter these subalpine forests. For one thing, study co-author Tom Veblen, also of UC Boulder, notes heat and drought could prevent the forests from regenerating. That is because new seedlings only establish in cooler years with above-average moisture levels. 

“[U]nder a warming climate we will continue to see a reduction in the abundance of large trees and probably of forest cover,” he tells Treehugger in an email. 

And the loss of larger, older trees could in turn hamper the forests’ ability to help us mitigate climate change. Subalpine forests have acted as a carbon sink from 1999 to the present, but it is precisely the bigger and older trees that store the most carbon, meaning this could shift if current trends persist. 

 “[T]his is sort of the alarm bell going off saying, ‘hey, we need to be aware of these potential changes to the ecosystem,’” Andrus says. 

Dead trees in subalpine Colorado forest on Niwot Ridge, west of Boulder.
Dead trees in subalpine Colorado forest on Niwot Ridge, west of Boulder. Robert Andrus

Change Over Time

The study only covers 13 plots of trees in Colorado’s Front Range, though Andrus says the study area is representative of similar forests throughout the southern Rockies. While it might have been ideal to monitor trees throughout the state, a study like this one requires the ability to return to the same trees over an extended period of time. And no one put in the work forty years ago to facilitate a statewide study. 

“This is the longest running tree mortality study in the state of Colorado,” Andrus says, “so at this point this is the best available evidence that we have.”

That even this evidence exists is thanks to the foresight of Veblen, who began observations in the early '80s and continued measurements with his students in the decades since. 

Before he established the study, Veblen had researched how forests changed based on climate variation over several decades to a century in New Zealand. 

“I understood how essential it was to establish long-term monitoring plots to assess trends in tree populations,” he says. 

That understanding meant he was in a position to observe as prediction became reality along Niwot Ridge.

“In the early 1980s forest ecologists recognized the likelihood of climate change caused by greenhouse gases but obvious changes in forests related to warming were not evident at that time,” he says. “In our dataset they started to become evident in the 1990s.” 

Now that those changes are evident, both Andrus and Veblen say that lowering emissions is the only way to prevent them from accelerating. 

Andrus points out that it is not really feasible to try and save single trees, by watering them or taking steps to fend off bark beetles. 

“It takes a lot of resources to protect individual trees, whereas we need to protect an entire landscape, and the way to protect the landscape is to stop emitting so much carbon,” he says.

View Article Sources
  1. Andrus, Robert A., et al. "Increasing Rates of Subalpine Tree Mortality Linked to Warmer and Drier Summers." Journal of Ecology, vol. 109, no. 5, 2021, pp. 2203-2218., doi:10.1111/1365-2745.13634