Thousands of tall pine trees killed from the inside out. Large swaths of lush forest turned brown.
There's no doubt that the tiny mountain pine beetle has caused massive destruction along the Pacific Coast. But that doesn't mean that the bug has raised the forest fire risk, says a team of three researchers.
In fact, in some cases, forests destroyed by the beetle may actually be less likely to burn.
The idea that beetles killing off tress could actually be lowering the fire risk may sound counterintuitive. After all, dead trees are dry and should therefore be like a tinderbox primed for wildfire.
University of Wisconsin forest ecologists Monica Turner and Phil Townsend have been studying beetle-infected forests near Yellowstone National Park. Along with Yellowstone Vegetation Management Specialist Roy Renkin, the researchers have been using images taken by NASA's Landsat satellites to map the areas hardest hit by the beetle outbreak.
They've also hiked into the forest areas to confirm that the majority of the affected trees were indeed killed by beetles rather than by other causes, gathering evidence of the damage. Finally, they compared maps of beetle-killed forest with maps of recent fires.
After assembling the data, they've been able to build models of how much forest-fire fuel is in the damaged forest and how burnable it is.
And what they found suggests that large forest fires do not appear to occur more often or with greater severity in areas with beetle damage.
The researchers explain that while green pine needles might appear moister and harder to burn, they actually contain high levels of flammable volatile oils.
When those needles die, the flammable oils begin to break down. As a result, depending on the weather conditions, dead needles may be less likely to catch and sustain a fire than live needles.
Secondly, when beetles kill a lodgepole pine tree, the needles fall off and decompose on the forest floor relatively quickly. In a sense, the beetles thin the forest, so that the naked trees left behind are essentially akin to large fire logs.
However, just as you can't start a fire in a fireplace with just large logs and no kindling, wildfires are less likely to ignite and carry in a forest of dead tree trunks and low needle litter.
Forest ecologists noted this same phenomenon after the massive Yellowstone wildfires in 1988. After the large fires swept through and burned off all the tree needles, only the dead trunks remained. In the years that have followed, new wildfires have tended to slow and sometimes even burn out when they reach the standing dead forest; there simply hasn't been enough fuel to propel the fire.
The researchers note that are other factors involved in forest fire risk beyond pine beetle damage. Warmer temperatures, for example, cause all forests to become drier, and with climate change, many scientists predict fires to increase in number and size.
But Townsend notes that pest infestations and fires are natural parts of the ecosystem and have been since forests evolved.
"What we have right now is a widespread attack that we haven't seen before, but it is a natural part of the system," he said in a news release.
Renkin agrees.
"Disturbances like insect outbreaks and fire are recognized to be integral to the health of the forests," he said, "and it has taken ecologists most of this century to realize as much. Yet when these disturbances occur, our emotional psyche leads us to say the forests are 'unhealthy.'
"Bugs and fires are neither good nor bad, they just are."
