The City of Boulder recently completed forest thinning at the base of Mount Sanitas. Credit: Boulder Reporting Lab

Boulderites hiking Sanitas in the past few weeks may have noticed considerable tree thinning at its base. But what exactly is the city doing, and why now? 

Chris Wanner, the City of Boulder’s vegetation stewardship senior manager told Boulder Reporting Lab that though the thinning might look excessive, it’s part of a practice that Open Space and Mountain Parks (OSMP) has been undertaking in other areas near town for years, though perhaps nowhere quite so obvious.

“We’re just doing a combination of fire mitigation, forest health and non-native species removal,” Wanner said, “focused on removing the non-native tree cover and cleaning up the dead and downed material that could contribute to a fire.”

The non-native trees include green ash, crack willow and box elder. Wanner said some of the green ash had been impacted by the emerald ash borer — an invasive pest that’s done a number on Boulder’s ash population. But trees killed by the pest aren’t the only fire danger. “The crack willow, like its name, just cracks off and dies and leaves a bunch of material on the ground,” Wanner said. At the base of Sanitas, where there’s homes nearby, removing that potential fuel is paramount.

Why Sanitas is suddenly getting a haircut isn’t indicative of a new plan by the city, but rather limited resources that have kept OSMP busy in other areas in years past.

“We’re just making our way around the system,” Wanner said, “really focusing on the wildland-urban interface areas where we can reduce fire risk and fire intensity adjacent to the city.”

Other areas scheduled to receive a thinning treatment this summer are the Shanahan Ridge area, Sunshine Canyon and up Flagstaff Road to protect water infrastructure there.

Without wildfire, we’re on the hook to kill some trees to create “snags” for wildlife. Credit: Tim Drugan

Creating snags

As thinning projects move along the western border of town, Boulderites are sure to see trees with rings of bark chipped away 4 to 6 feet above the ground. Called girdling, the practice is done to effectively kill a tree but leave it standing. 

“We’re doing that to create wildlife trees, or snags — another name for standing dead trees.” “We’ll go in and girdle a certain amount of trees. The goal is to get them to die and rot out. Then you’ve got habitat for cavity nesting birds and owls, insects and small mammals who appreciate those rotted out, standing trees.”

This, like the thinning projects girdling accompanies, is intended to mimic wildfire in places where homes make prescribed burns too risky to undertake. Wildfires leave swaths of standing dead trees for the animals. So as we continue to suppress fire near our towns, we’re on the hook for making sure those animals have a place to live.

Tim Drugan is the climate and environment reporter for Boulder Reporting Lab, covering wildfires, water and other related topics. He is also the lead writer of BRL Today, our morning newsletter. Email:

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