Colorado forests are transformed by bark beetles and fire

Bark beetles and wildfires are taking a heavy toll on forests in Colorado. Researchers at CU Boulder report that while the forests can recover from one or the other, the collective impacts of bark beetles and fires could lead to permanent changes. 

The study is one of the first to document the effects of bark beetles on high elevation forests as they recover from wildfire.

The experts found that when wildfire follows a severe spruce beetle outbreak in the Rocky Mountains, Engelmann spruce trees are unable to recover and grow back. Meanwhile, aspen tree roots can survive underground.

“The fact that Aspen is regenerating prolifically after wildfire is not a surprise,” said study lead author Robert Andrus. “The surprising piece here is that after beetle kill and then wildfire, there aren’t really any spruce regenerating.”

In previous research, Andrus determined that Colorado forests can survive bark beetle outbreaks, including overlapping outbreaks with different kinds of beetles. He also found that spruce bark beetle infestations do not affect fire severity.

The current study, which was conducted in the San Juan range of the Rocky Mountains, shows that subalpine forests that have been spared from bark beetle invasions are more likely recover after wildfire. On the other hand, in affected forests that also experience fire within about five years, conifers cannot bounce back. 

“This combination, the spruce beetle outbreak and the fire, can alter the trajectory of the forest to dominance by aspen,” said Andrus.

According to the study, when bark beetles attack Engelmann spruce trees before a fire, there are not enough seeds being produced in the burned areas to regenerate the forest. On the other hand, aspens regrow from their root systems, which gives them the ability to bounce back.

Based on 45 study sites, 74 percent of forests that endured fire soon after a bark beetle invasion had Engelmann spruce and subalpine fir trees that failed to recover. 

“Bark 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,” said Andrus.

The good news, according to the researchers, is that the aspens that may come to dominate these forests can anchor their recovery, and keep forests from transitioning into grasslands. “Where the aspens are regenerating, we expect to see a forest in those areas.’

The study is published in the journal Ecosphere 

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By Chrissy Sexton, Earth.com Staff Writer