by Michael Kodas, via On Earth
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.