Banner image: A forest in the San Juan range of the Rocky Mountains, with dead Engelmann spruce trees alongside live听aspen trees.听(Credit: Robert Andrus)
Bark beetle outbreaks and wildfire alone are not a death sentence for Colorado鈥檚 beloved forests鈥攂ut when combined, their toll may become more permanent, new CU Boulder research shows.
It finds that when wildfire follows a severe spruce beetle outbreak in the Rocky Mountains, Engelmann spruce trees are unable to recover and grow back, while aspen tree roots survive underground. The study, published last month in听, is one of the first to document the effects of bark beetle kill on high elevation forests鈥� recovery from wildfire.听
鈥淭he fact that Aspen is regenerating prolifically after wildfire is not a surprise,鈥� said Robert Andrus, who conducted this research while working on his PhD in physical听geography at CU Boulder. 鈥淭he surprising piece here is that after beetle kill and then wildfire, there aren't really any spruce regenerating.鈥�
Andrus鈥� previous research found that bark beetle outbreaks听are not a death sentence to Colorado forests鈥攅ven after overlapping outbreaks with different kinds of beetles鈥攁nd that听spruce bark beetle infestations do not affect fire severity.听
This new research, conducted in the San Juan range of the Rocky Mountains, shows that subalpine forests that have not been attacked by bark beetles will likely recover after wildfire. But for forests that suffer from a severe bark beetle outbreak followed by wildfire within about five years, conifers cannot mount a comeback. While these subalpine forests can often take a century to recover from fire, this research on short-term recovery is a good predictor of longer-term trends.
鈥淭his combination, the spruce beetle outbreak and the fire, can alter the trajectory of the forest to dominance by aspen,鈥� said Andrus, who is now a postdoctoral researcher at Washington State University.听
For those worried about the future of Rocky Mountain forests farther north, more research is needed on areas burned in the 2020 East Troublesome Fire to understand how the mountain pine beetle outbreak prior to that fire will affect forest recovery, according to Andrus.听
The next generation听
Each bark beetle species specializes in attacking鈥攁nd usually killing鈥攁 specific host tree species or closely related species. Several species of bark beetle are native to Colorado and usually exist at low abundances, killing only dying or weakened trees. But as the climate becomes hotter and drier, their populations can explode, causing outbreaks which kill large numbers of even the healthiest trees.听
These large, healthy Engelmann spruce and subalpine fir trees are the ones that produce the most seeds. When bark beetles kill these trees and then fire sweeps in, the researchers found there simply aren鈥檛 enough seeds being produced in the burned areas to regenerate the forest. 听
Aspens, however, regrow from their root systems. While all three of these higher elevation trees have thin bark and die when exposed to fire, with their regenerative roots underground, aspens can bounce back where conifers cannot.听
The researchers focused specifically on areas of forest affected by spruce bark beetle outbreaks, which attack Engelmann spruce, where fires such as Papoose, West Fork and Little Sands burned in 2012 and 2013 in Rio Grande National Forest. They found that for forests that suffer from a severe bark beetle outbreak followed by wildfire within about five years, Engelmann spruce and subalpine fir trees failed to recover in 74% of the 45 sites sampled.听
This information will help inform land managers and policy makers about the implications for high elevation forest recovery following a combination of stressors and events.听
And it鈥檚 more important information than ever. Not only do bark beetle outbreaks leave behind swaths of dead, dry trees鈥攁nd fewer trees to produce seeds鈥攂ut the climate is getting hotter and droughts are becoming more frequent, promoting larger fires.听
鈥淏ark beetle outbreaks have been killing lots and lots of trees throughout the western United States. And especially at higher elevation forests, what drives bark beetle outbreaks and what drives fire are similar conditions: generally warmer and drier conditions,鈥� said Andrus.听
But there is good news: The aspens that may come to dominate these southern Colorado forests can anchor their recovery, and keep forests from transitioning into grasslands.听
鈥淲here the aspen are regenerating, we expect to see a forest in those areas,鈥� said Andrus.听
Additional authors on this publication include Thomas Veblen at CU Boulder; and Sarah Hart and Niko Tutland of Colorado State University.听