Brian Howell has seen every tree in Colorado. As manager of the U.S. Forest Service’s aerial survey program, Howell spends hundreds of hours each year in the air, riding shotgun in a bush plane. His pilot loops around mountain ranges at dizzying proximity to granite cliffs. Howell and his co-surveyor munch ginger snacks to ward off nausea, while trying to map areas of our forests impacted by a complex range of pests and diseases.
“It’s spectacular,” he says. “But it’s not for the faint of heart.”
This year’s survey, released in January, adds another year of meticulous observations about the pests and diseases in Colorado’s forests. Monitoring is the foundation of forest management, and spotting diseased areas from a plane is the cheapest and most efficient way to lay this foundation. But, Faraway satellites produce low quality images, and computer programs have a hard time catching the tell-tale signs of one type of pest or disease over another.
And that’s why Howell sat in a propeller plane — named “Sparky” for the time it clipped a powerline and lived to tell the tale — for much of the 318 hours it took to map forests in Colorado, Wyoming and parts of South Dakota.
For much of that time in the air, Dan West sat on the opposite side of the cockpit, peering down into the forest below, helping to expertly diagnose tree diseases from the air. West is an entomologist with the Colorado State Forest Service. Like Howell, he is tall and good-humored. But he gets slightly more excited when talking about bugs — even if they have wrecked huge swaths of forest.
One of Howell and West’s major observations this year is bad news. Spruce beetle, an insect that wriggles beneath the bark of spruce trees to feed and raise its young, expanded its already massive territory across southern Colorado. Since 1996, the spruce beetle has affected more than 1.5 million acres of forest in the state, leaving countless dead trees in their wake. This year, spruce beetles invaded about 182,000 new acres of forest.
The invasion is bad for two reasons, West says. When beetles claim new territory, they land on the thickest and oldest trees in the forest. Spruce beetles, of course, mainly attack spruce trees. But after an infestation, thousands of acres of forest are littered with dead and dying trees. In a state that’s constantly on guard for wildfires, these dead stands can increase the risk that a fire would burn out of control, burning hotter and claiming more forest. In some places, the risk of more intense fires is already heightened, thanks to a century of putting out forest-cleansing fires in a landscape where naturally occurring wildfire is an important part of the ecosystem.
“We’ve allowed some of our forest stands to become overstocked, and so it’s kind of like a buffet for these insects to move through,” West says.
Spruce and other bark beetles, as well as other diseases, also impact Colorado’s tourism. Dead trees are dangerous to campers and hikers. Dead or defoliated trees, too, are unsightly. Every fall, tourists line up on the sides of Rocky Mountain highways to glimpse aspens erupting yellow. A leaf disease, called Marssonina leaf blight, might not kill trees, but after a particularly wet year the fungus can eat up entire stands of aspen leaves. This year, the survey detected aspen defolating diseases on about 81,000 acres, a slight increase from last year.
But West emphasizes all of the major tree diseases actually belong in Colorado’s forests. In fact, pests and diseases have an important function in small numbers.
“They serve the role of sanitizers of the forest,” West says. “That’s an important thing to remember, that they serve a purpose in our forests when they’re in background levels.”
This year, however, spruce beetles are emphatically not in the background. West explains that the outbreak has continued to spread in more recent years largely thanks to a drought that’s plagued the southern half of Colorado, while the north has seen a recent reprieve. Because trees have evolved along with these pests, West says, the trees have developed powerful defenses against them. It’s only when a prolonged drought creeps into an area that these defenses weaken.
Unless southern Colorado catches a break from the drought, West expects that spruce beetles will continue to claim more forest in the next year.
This year’s aerial survey did reveal some good news, however. The area of forest affected by mountain pine beetle, a bark beetle that typically infests pine trees, shrank again for the seventh year in a row. Since that epidemic began in 1996, the insects have affected about 3.4 million acres of forest in Colorado. This year, only about 5,000 acres were newly infested.
West says this decline happened for a few reasons. Along the northern Front Range, where most of the mountain pine beetle outbreaks occurred, the pests had steadily eaten the best trees. That impact leaves only smaller and less desirable trees each season. Scarce food combined with a few wet years, which hardened trees’ defenses against bark beetles, could explain the decline.
Today, West says, mountain pine beetle populations are back to typical levels. He expects them to stay that way at least for the next year. Instead, state and federal agencies will be focused on the spruce beetle, as well as the Douglas-fir beetle, which added 10,000 new acres to its 2,000-acre domain this year, and a variety of other diseases.
Taking a step back, it’s unclear if any trends in these diseases have emerged because of human-caused climate change. West says pests and diseases are always in a kind of dance with local weather — some spreading in dry years and others in wet years. It’s also not clear what those trends might look like as forests warm and precipitation patterns change.
In a 2014 report, U.S. Forest Service scientists wrote that because there are thousands of bark beetle species living in a range of environments, the impacts of climate change will likely be quite different across those populations. Some hints are emerging that warmer and drier weather could expand some species’ range, and make their host trees more susceptible to attack, among other impacts. To know for sure, however, Howell, West and their colleagues will have to keep a close watch.