The wildfires ripping through Southwest Colorado that consume downed and standing conifers have spared aspen in the same stands.
“Aspen don’t have the flammable characteristics of spruce and fir,” said Steve Hartvigsen, a forester in the U.S. Forest Service in the Pagosa Ranger District. “The chemicals in the conifer needles make them volatile.
“Conifer crowns are thicker, and conifer bark is not smooth like the aspen,” Hartvigsen said. “Conifer bark is ruffled so it can catch a spark easier.”
The ground cover under aspen, which have a more open canopy that allows sunlight to filter in, tends to be green – grass and forbs – that don’t burn like downed branches and trees in conifer forests do, Hartvigsen said.
“It’s not the live fuel, but the standing and downed dead fuel that is burning in the current (West Fork Complex) fire,” Hartvigsen said.
Firefighters often look at aspen stands to calm fires, Hartvigsen said.
“Conifers burn hot, but they cool down when they reach an aspen stand,” he said.
The 2002 Missionary Ridge Fire was an anomaly in regards to aspen, Hartvigsen said.
“It was so unusual,” he said.
An intense fire that started at lower elevation in pine and brush, pushed by strong wind under extremely dry conditions overwhelmed young aspen upslope, Hartvigsen said.
“The flames were able to transition to the crowns of the young aspen,” he said. “The small aspen were 10 to 12 feet tall instead of 60 to 80 feet.”
In some places, stands of mature aspen eventually cooled the advances of the fire, Hartvigsen said.
In the lightning-sparked Little Sand Fire that burned 25,000 acres northwest of Pagosa Springs last year and in the current West Fork Complex Fire, aspen have calmed flames, Hartvigsen said.