Annual Colorado forest health report shows the toll of climate change, pests

More than 200,000 acres of forest are affected by the western spruce budworm. When the budworm feeds on a tree, the tree becomes more susceptible to attack from bark beetles, making life even more difficult for drought-stressed fir and spruce trees.

West said observing forest health from a plane is cost effective compared to using satellite data, which is time consuming to process. The U.S. Forest Service and CSFS work together to fly over much of Colorado’s 24 million acres of forests. 

Along with defoliated fir and spruce trees, Coloradans are likely to see clumps of dead subalpine fir at higher elevations due to the western balsam bark beetle. The pest affected 27,000 acres of forest around the state, from the Grand Mesa to the West Elk Mountains, to Rocky Mountain National Park.

According to West, information from the health reports is needed to help decide where to focus forest management resources, which are often limited. Resources may be even more limited now that President Trump’s administration has made dramatic cuts to the U.S. Forest Service’s staffing and funding.

“Forest fires aren’t going to take four years off just because of who’s in the White House,” Colorado Gov. Jared Polis, a Democrat, said earlier this year.

The year-to-year changes of forest acreage affected by pests are sometimes small, but the accumulation of data show that mountain pine beetles have affected 81% of pine trees since 1996.

“Forty-one percent of our high elevation Engelmann spruce fir has been affected since 2004. When you really start to piece things together over time is when you start to see the impact of these insects in forest disturbance really starting to affect our state,” West said.

Aspen trees have different pests to contend with than fir and pine trees, but aren’t a large topic of the health report this year. The western forest tent caterpillar eats the leaves on aspen trees, and defoliated pockets around the state last year. Much of the damage was localized and not threatening to the overall health of aspen forests, according to the report. 

Different types of fungi can damage aspens, but severe drought conditions from 2020 to 2022, and below average rainfall in 2024, mean fungal issues are almost non-existent at the moment. 

CSFS uses all that information to decide where to fight the bug infestations, sometimes with simple tools like stapling chemical packets to individual trees. This is an example of an anti-aggregation treatment, and the method uses the way beetles communicate with each other against them. The bugs signal each other with pheromone plumes, like putting up a “vacancy” or “no vacancy” sign on each tree.

“​​The bark beetles first have to call in all their brothers and sisters and say, ‘hey, this is a good tree that we should attack because its defenses are weakened,’ and then once they do that, you've got to shut that call off and turn on the no vacancy sign,” West said. “Otherwise, all of the incoming beetles will just keep coming in and there won't be any food.” 

The treatment sends the “no vacancy” signal to the beetles so they leave it alone while they’re flying around during the summer looking for trees to attack.

This summer, West and other forest service workers are using the anti-aggregation treatment on 18,000 acres of trees around the state. After that work is complete — in the 90-day window the adult beetles are scanning for new trees to attack — it will be time for West to get back in the plane, and start collecting data for next year’s report.