New study helps quantify ignition time of beetle-killed trees
By Bob Berwyn
SUMMIT COUNTY — Chemical changes in pine attacked by bark beetles start as soon as two weeks after the bugs start to burrow under the bark and make the trees more prone to ignition.
Overall, beetle-killed trees in the early and mid-stages of infestation may pose a greater risk of fast-spreading crown fires, though other factors are also important, including the structure of the tree, the presence or absence of ground and ladder fuels and terrain and weather.
“Our results empirically demonstrate that the needles of attacked and red trees are more easily ignited than those of healthy trees … However, more work is needed to better understand the changes in fire behavior in beetle attacked stands over time in order to improve public and firefighter safety, according to a group of Forest Service researchers who recently published a study in the journal Forest Ecology and Management.
Other studies have suggested that some fires have slowed when they hit areas of long-dead standing trees that have lost most of their branches and needles.
The study focused on the fuel characteristics and ignition potential of lodgepole pines during the early stages of a pine beetle attack. The scientists analyzed samples from green and healthy pine needles and compared them to samples from trees at various stages of beetle attack, finding that red needles were much drier than green needles.
Finally, the researchers ignited the needles under controlled conditions, finding that red needles caught fire with just 11 seconds of exposure to a heat source; while it took nearly four times as long — 41 seconds — for green needles to catch fire.
The time to ignition is important because it gives a sense of whether surface fires burning at varying intensities can carry into tree crowns.
All this suggests what firefighters instinctively know: That less heat would be required to ignite the foliage of attacked trees.
The study also looked at other factors, including how the beetle-killed trees contribute to ground fuels with needle litter and the finer branches from tree crowns.
In any case, it’s critically important to remember that a lodgepole forest is a fire-adapted ecosystem, said Russell Parsons, a 47-year Forest Service veteran who worked on the study.
“The natural dynamic in lodgepole pines is that they get so uniform in size and shape … then you get that beetle activity cooking … mother nature abhors conformity, she’s going to do her best to disturb it,” Parsons said.
A preliminary look at the recent High Park Fire in Larimer County showed that the fire may have actually slowed down as it moved westward into higher terrain with stands of beetle-kill, possibly there were fewer ground and ladder fuels available to carry the fire to the canopies.
“It got up in and burned into the edge of the lodgepole, but not too far in,” Parsons said.
“The surface fuels are such a determinant of fire … 99.9 percent are surface-driven phenomena,” he said.