Against the majestic backdrop of the Tetons, dead and dying whitebark pine defines a ridgeline in Wyoming’s Gros Ventre Range. Credit: George Steinmetz
In a little over a decade, the largest mountain pine beetle outbreak on record (by a factor of 10) has killed more than 70,000 square miles of Rocky Mountain forests -- an area the size of Washington State. From above, the infested pine trees seem color-coded: green is healthy, red is dead, and after three or four years, the dead red needles fall off, leaving behind a graveyard of bare gray bark -- or, if you’re worried about wildfires, what amounts to a field of 100-foot-tall matchsticks.
Colorado, already facing the most destructive wildfire season in state history, has 3.3 million acres of beetle-killed forests to worry about. No one doubts that dead and dying trees are a potential problem, but fears that the beetle infestation will fuel larger firestorms might be premature (at least in the short term). Across the West, some 40 scientific studies have failed to produce a clear picture of how millions of beetle-killed trees will burn.
One recent paper by researches at the U.S. Forest Service and University of Idaho predicts that during the “red phase” -- when trees are dead but still have rust-colored needles -- severe crown fires may burn through the treetops with greater speed and intensity than they would in healthy green forests. A study last year by ecologists with the U.S. Forest Service’s Rocky Mountain Research Station showed that in beetle-infested forests, the red, dead needles ignite three times faster than their living counterparts, largely because they have 10 times less moisture and different chemistries than living, green needles.
The intensity of the crown fires in red, beetle-killed forests, the researchers predict, could also launch embers farther, thus spreading the fire faster over a greater territory. Another model shows that lower fuel moisture in the canopies of red and gray forests and dead trees that fell to the ground during and after the gray phase increased the intensity of ground fires, which allowed crown fires to erupt with less wind than they usually require. Other studies show that gray forests, in which the needles have fallen from the trees, are likely to slow down crown fires. Trees in those forests, however, have a great risk of “torching” -- which means they burn individually with high, intense flames.
But other research contradicts the studies showing that beetle-killed forests are a cause for alarm. A report released last year by the Joint Fire Science Program at the National Interagency Fire Center in Boise, Idaho, holds that neither red nor gray forests are likely to burn more severely than a green forest, largely because the death of the trees would reduce the amount of fuel in the forest canopy. The paper maintains that climate and weather factors, rather than fuel, drive most wildfires.
Climate is also behind the severity of the beetle outbreak. Without prolonged sub-zero temperatures -- something most of the Rocky Mountains haven’t seen in a decade -- pine beetles thrive, while trees stressed by drought and heat waves grow more susceptible to pests. In a report earlier this year, Jeffry Mitton and Scott Ferrenberg at the University of Colorado documented that, due to the warming climate, mountain pine beetles in some Colorado forests have gone from reproducing once a year to twice a year, leading to an exponential increase in the number of insects. And they’ve got more places to spread: for most of the 20th century, overeager fire suppression made forests unnaturally dense. Now diseases and pests can spread through the woods like plague through an overcrowded slum.
In the end, the risk that many firefighters fear is not in today’s red or gray forests, but in the long-dead forests of the future. At least for now, most trees are still standing. But decades from now, when the beetle-killed trees fall to the forest floor and new pines grow above them, there’s bound to be trouble, as the one-two punch of dead timber on the ground and explosive canopy fires in the fresh trees above prove doubly difficult to fight.
When I was a seasonal wildland firefighter with the U.S. Forest Service a decade ago, the firefighters I worked alongside gave little thought to this tiny pest chewing through forests. Today, when they look at entire mountains covered with dead trees, the firefighters I’ve talked with can’t help but imagine what kind of hell they would face if all that timber were to erupt into flames at once. How would they survive it, much less stop it? That’s an answer perhaps no study can give us -- until it’s too late.
This is part of an occasional series on fires and climate change. Read related stories here.