Will Bark Beetles Spell the End of Forests As We Know Them?
Icefields Parkway winds its way through the Canadian Rockies between Banff and Jasper national parks. It’s obvious why many count the Parkway among the most scenic drives on the planet: majestic glaciers and epic mountain vistas lurk around every turn.
But despite the awesome scenery, there’s something wrong here: the forests are dying. The telltale signs of the epidemic appear trivial at first–just a few reddish trees dotting the carpet of living pines on the outskirts of Banff.
Nearing Jasper, though, the problem becomes hard to ignore. In some areas, vast swaths of dead snags march away to every horizon, an endless brown ocean with almost no living trees in sight.
The region has fallen victim to the mountain pine beetle—an invasive species in Alberta, even though it’s native to nearby British Columbia and most of the western United States. Yet even in the beetles’ native range, trees are becoming infected and dying at an alarming rate.
The pine beetle is just one of some 600 native North American bark beetle species. The pests feed on the pine, spruce, and fir (among other) species that dominate many of North America’s high elevation forests.
Mountain pine beetles, spruce beetles, douglas-fir beetles, and many more are wreaking havoc throughout western North America. (Further afield, beetles are also devastating forests in Europe—but that’s a matter for another article).
Bark beetles burrow into mature trees to feed and lay eggs in the phloem—the living, vascular tissue of trees just beneath the bark.
They kill trees by disrupting the phloem’s ability to transport water, nutrients, and sugars between the roots and leaves. Individual bark beetles are tiny—each roughly the size of a grain of rice—but if enough of them attack a large tree, they can overwhelm its defenses.
The pests generally target larger trees that are unhealthy or drought-stressed. “As trees get bigger, their phloem layer gets bigger. And so they can hold more beetles, ” explains Dr. Dan West, a forest entomologist with the Colorado State Forest Service.
Healthy trees resist infestation by using their own resins to effectively flush beetles out. Weakened, drought-stressed trees can’t produce enough resin to defend themselves and are thus easier targets for beetles.
They’re not invasive—so what’s the problem?
Bark beetles are, for the most part, native species with critical ecological roles. They thin the forest, culling weak specimens and keeping the population healthy.
“We don’t want to get rid of the bark beetles by any means. We would be run over with dead trees if we didn’t have bark beetles,” says West. Instead, the goal is to keep their populations in check so they can’t overrun whole forests.
Unfortunately, West and other forest managers are fighting a losing battle. Historic fire suppression tactics, combined with the worsening effects of climate change, have stoked beetle populations to epidemic levels since the 1990s.
The problem with fire suppression.
Throughout the early 20th century, land managers took a problematic approach to wildland fire that involved suppressing any and all burns, rather than letting them run their natural course.
“Back in the 1930s, the Forest Service installed a policy that’s called the 10 a.m. Rule,” explains West. The policy was that “should a fire start in our public lands, we want to try to put that fire out by 10 a.m. the next day, and that’s it.”
Researchers now understand that fires keep ecosystems healthy by thinning trees, opening up the canopy so new seedlings have a chance to flourish, and burning off excess fuel. “Fire is the catalyst that allows other species to move in,” explains West, resulting in forests that are diverse in both age and species composition.
In contrast, suppressing small fires causes forests to grow denser and older. Tree health can suffer from overcrowding, as more organisms compete for the same resources. And without regular fire disturbance to shake things up, one tree species can grow to dominate a wide territory.
“That basically means that should a bark beetle event start… it’s just a buffet. One tree after the other after the other for miles, if not hundreds of miles, that are all in the same age categories and pretty much all of the same species,” says West. “That just sets the stage for large-scale landscape disturbance from bark beetles.”
What about climate change?
The aftermath of the 10 A.M. Rule may have left huge swaths of forest primed and ready to burn, but historic fire suppression tactics are only part of the problem. The climate crisis also plays a major role.
In short, climate change is drying out much of the West. Warmer winters and lower snowpack have increased the frequency and intensity of drought in the West. And experts expect the trend to accelerate.
In recent times, “you might get adequate or above-average precipitation and snowpack, but then the next year, it doesn’t hold,” says West. “We’re in this period of variability right now… previous generations never had that.”
More drought means, among other things, more trees susceptible to beetle infestation. (Again, trees need plenty of water to produce the resin necessary to flush the pests out).
“It really is closely tied to our drought regimes and to our precipitation levels,” West explains. “When you think back on when we started, in the early 2000s, having these… consecutive years of below-average precipitation, that’s really where we started to see the demise of many of our forest ecosystems.”
So, why should we care?
Living forests are valuable carbon sinks. When trees die, they no longer trap carbon in their biomass and eventually emit much of it back to the atmosphere.
Beetle kill has cascading effects throughout ecosystems. Potential impacts are wide-ranging and complex but include increased erosion, habitat loss for native species, and reduced water quality and quantity in ecosystems. Outbreaks also make forests more vulnerable to fire.
According to West, regions are most at risk in the two to three years following beetle kill. That’s because dead trees don’t drop their needles right away. “There’s a lot of dead and dry fuel up in the tops of the trees… so fire can easily move from tree to tree” in what’s known as an active crown fire.
“That is where we see large amounts of acreage consumed by wildland fire in a pretty short amount of time,” like with last year’s Creek Fire in California. Experts believe high levels of beetle kill helped stoke that blaze to a 380,000-acre inferno that became the fifth-largest in California history.
Unsurprisingly, beetles are also bad news for the logging industry. Bloomberg recently reported that the epidemic in British Columbia has wiped out a decade’s worth of lumber harvests and will reduce the province’s allowable production by a crushing 40%.
Some logging companies salvage beetle-killed trees for sale. The practice removes fuel from the environment, and pine beetles, in particular, produce distinctive blue-stained lumber that builders covet. However, salvaging usable lumber from dead stands can be challenging—and markets for beetle-killed lumber can become saturated as outbreaks balloon in scale.
Outdoor recreation is also likely to suffer. “No one wants to see a forest for the dead trees,” West points out. He predicts the beetles will affect everything from backpacking to “the mega-industries of hunting and skiing… not to mention the intrinsic value of having forests and knowing that our forests are there for future generations.”
The problem is widespread.
This isn’t the first time bark beetle populations have gotten out of control and ravaged forests. But although North American forests are no strangers to beetle epidemics, the problem is getting worse.
According to a 2021 report in the Forest Service’s Fire Management Today journal, “several recent outbreaks are considered the most severe in history. Since 2000, for example, about 25.5 million acres (10.3 million ha) in the Western United States have been affected by mountain pine beetle.”
One 2016 study found that more trees in the West had been killed by bark beetles than wildfires over the previous 30 years.
Alaska is currently battling a massive spruce beetle outbreak on the Kenai Peninsula. Meanwhile, beetle-killed forests in California’s Sierra Nevada moutains are going up in flames as much of the state burns.
Most of the Rocky Mountains, from New Mexico to Canada, are similarly afflicted. In Colorado alone, some 40% of mature Engelmann spruce in the state have succumbed to beetles. Meanwhile, statewide mortality rates in lodgepole pine are approaching 80%.
What kills bark beetles?
Some insect and bird species, including several types of woodpeckers, eat bark beetles. But predation alone isn’t enough to control beetle populations.
Temperature fluctuations in fall and spring can lead to modest beetle mortality too, according to West. However, neither of these factors makes a big enough dent in beetle populations to stop major outbreaks.
Extremely cold winters can kill larvae before they can spread to new trees in spring, stopping outbreaks in their tracks. For instance, two back-to-back cold winters in Alberta have killed off 99% of mountain pine beetles in Jasper National Park since 2019.
But such cold winters are becoming less common, even in the Canadian Rockies, due to climate change. Many mountain regions further south haven’t seen winters that cold in decades.
In Colorado, West says he no longer factors cold snaps into his formula for the end of an outbreak. “The last time that we’ve seen (a winter that cold)… was in 1985 in the Fraser Valley.”
“I see no barriers for the bark beetle to continue to build their populations and continue to move,” says West.”We are seeing new acres that are affected, and we are seeing it intensify in certain areas” of Colorado.
So what could stop current outbreaks?
West says food availability is one of the biggest determiners of the length of an outbreak. Bark beetles prefer large-diameter trees and typically won’t attack anything smaller than four inches in diameter.
That’s because they don’t provide enough phloem for beetles to feed on and reproduce. So eventually, “they kind of eat themselves out of house and home,” West explains.
“The other thing that occurs,” he adds, “is that we get these periods of above-average precipitation, and so healthy trees can build back their defenses and basically defend themselves against bark beetle attack.”
Unfortunately, trees require several consecutive years of decent precipitation to recover. Trees start to shut down their growth to conserve water during droughts, and it takes time to reestablish. “I only wish it were as easy as if we had one good year of above-average precipitation or above-average snowpack, and then the bark beetles would be done. But that’s just not how it works.”
Building Forest Resiliency
In the mid-20th century, foresters treated bark beetle outbreaks by spraying trees with diesel and insecticides. These days, foresters manage stands to produce wide spacing between trees and a diversity of species and ages. Selective thinning of large, highly-susceptible trees is key.
“Thinning trees to a wide spacing is the best long-term solution to increase tree health and vigor and reduce the likelihood of bark beetle attacks,” reads a Forest Service brochure aimed at private land managers in California. “Thinning can also hamper the bark beetle pheromone communication system that facilitates mass attacks.”
Once a tree has been attacked, nothing (besides its own immune system) can save it. Sometimes, infested trees can be cut down and chipped or burned to hamper the pests’ invasion.
Foresters can also use preventative treatments, like insecticides or semiochemicals, to protect high-value trees. Semiochemicals mimic bark beetles’ own chemical signals, tricking them into thinking the tree is already full of beetles. “It’s the No Vacancy sign, basically,” explains West.
Such treatments typically work best in targetted applications, such as protecting trees near towns or ski slopes or stands with special historic or ecological value.
Easier said than done.
The goal is not to eradicate the beetles, but to improve forest health and resiliency. That way native beetle populations won’t get out of control and devastate huge swaths of forest.
“We’ve put all of our resources into just putting fires out, and now we have so much of our ecosystem in this highly susceptible category,” says West. “What I want to see happen, for our generation and for future generations, is to be able to add in some resiliency.”
That’s easier said than done. Beetles have ravaged millions of acres in Colorado alone. To fight infestations at that scale would be a massive undertaking. “It comes down to scale and it comes down to size,” said West. “How big an acreage are we able to treat?”
Difficult terrain and property lines make it even harder. Forest Service personnel can’t simply go in and do as they please in private lands, Wilderness areas, or national parks.
“There’s a lot of designations that are at play with trying to… string together a treatment plan that fits for a bark beetle that doesn’t care about boundaries.”
Even on public lands with relatively few restrictions, steep slope gradients and lack of roads can make management activities challenging.
What can we do?
The West is no stranger to beetle kill, but the scale of recent outbreaks is unprecedented. Forest managers face a daunting uphill battle: thanks to climate change and fire suppression, few natural barriers remain to stop the pests from spreading.
West suggests that successful bark beetle management will require massive buy-in from the public and from government officials. It’s more crucial now than ever before because the beetle problem isn’t going anywhere.
“As a society, we just haven’t placed a high premium on our forest resources, and until we really start to contact our state and elected officials and say, ‘Hey, I want to make this a priority,’ … I think we’re only going to see greater and more widespread disturbance events moving forward.”
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