Why Colorado's forests are bugging out with beetles
Beetle-killed trees west of the Eisenhower Tunnel.
Archimedes had his bathtub, Newton his apple. Scott Ferrenberg's Eureka moment required more fieldwork to confirm, but it began when a bug the size of a grain of rice landed on his sleeve as he walked through the woods.
In late May 2008, the graduate student in ecology and evolutionary biology at the University of Colorado was strolling among limber pines at 10,000 feet along Niwot Ridge west of Boulder, home of the university's Mountain Research Station, one of the highest and oldest climate-research facilities in the world. Ferrenberg had no trouble recognizing the visitor on his arm as Dendroctonus ponderosae — the mountain pine beetle, a common sight in lodgepole and ponderosa pine forests across Colorado.
An adult beetle flying around in May on Niwot Ridge, though, isn't so common. Ferrenberg, who was accustomed to studying beetles at lower elevations in California forests, didn't immediately grasp the implications of what he was seeing. "If I'd been told that it shouldn't be there, I might have second-guessed myself," he says now.
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But he did think the sighting was unusual enough to report it to Jeff Mitton, a CU evolutionary biology professor with whom Ferrenberg had just started working on a beetle genetics study. Just over a week later — June 8, 2008 — Mitton and Ferrenberg were back at the research site, preparing to dig beetle larvae out of trees for their study. Instead, they found mature beetles diligently assaulting the forest.
"It was a gorgeous day," Mitton recalls. "Shirtsleeve weather. Adults were flying in the air and landing on our shirts. They were hitting the trees and beginning to bore into them. I thought, 'Gee, this is early.'"
Mitton, who's been studying bark beetles since the 1970s, described the scene to an entomologist buddy in the U.S. Forest Service. His friend told him he must be mistaken. Among bug fighters, it was an article of faith that the beetles don't get busy in Colorado until late August. Then they carve galleries into the trees and lay their eggs, which develop from larvae into pupae, then pale yellow adolescents known as teneral — a life spent mostly beneath the bark, until they emerge as pigmented adults late the following summer. Adults flying in June? At 10,000 feet? No way.
Mitton came away from the conversation deeply annoyed. "He was so intransigent about this," he says, "that I realized I must have seen something that's decidedly different from the historic norm."
Over the next two years Mitton and Ferrenberg embarked on a carefully planned study to figure out what was going on. During the winter they identified pristine trees at two sites that hadn't yet been molested by the beetles. They set up lures loaded with pheromones to invite beetles to establish broods in those trees. They visited the sites regularly over the summers of 2009 and 2010 and documented beetles attacking the trees in June and laying eggs. Some of those eggs had developed into a second adult generation by August or September, emerging to seek out fresh hosts and start their own broods.
Early flights of mountain pine beetles have been observed elsewhere in recent years, but the research conducted by Mitton and Ferrenberg — and published in The American Naturalist earlier this year — represents the first peer-reviewed report of the insect achieving two generations in a single summer. The accelerated life cycle, the authors suggest, is a direct response to climate change.
Since the 1970s, warmer weather along the Front Range has allowed the beetle not only to operate in forests above 9,000 feet, where it's scarcely been seen before, but it's more than doubled its flight season, from 50 days to up to 120 days. And the effect on the beetle population is potentially exponential; instead of one female generating 60 offspring a year, those offspring could, in theory, generate an additional 3,600 pine-munching hordes in the same season.
The CU discovery comes in the sixteenth year of an epic mountain pine-beetle outbreak in the western United States and Canada, the worst infestation of its kind in recorded history. In Colorado, what began as a flare-up in lodgepole pine in isolated pockets on the Western Slope in 1996 has left large swaths of doomed, red-needled trees and denuded gray ones across 3.3 million acres, including thousands of ghost trees in popular tourist areas such as Vail, Dillon and Grand Lake. Although the pace of the epidemic has slowed — largely, experts say, because the beetles have already killed most of the large lodgepoles they prefer as hosts — aerial surveys indicate that the affected area expanded by 140,000 acres in 2011, mostly in ponderosa forests in the northern part of the state.
Southern Wyoming has also been hit hard. But then, so have pine forests from the San Bernardino Mountains to the Front Range, from New Mexico to Yellowstone to British Columbia. Western Canada has been particularly pummeled; by some estimates, the pine mortality there has become so extensive, releasing so many megatons of carbon dioxide from tree decomposition into the atmosphere, that it could transform the country's forests from a carbon sink to a carbon source.
Mitton believes the beetle's adaptation to warmer temperatures helps to explain why the damage is so widespread. "Bark beetles have been going through epidemics since they arose 34 million years ago," he says, "but we've never seen one like this. It's much more synchronous, much hotter than any before. In Colorado, they used to go up to 9,000 feet; now it's 11,000 feet. And they're invading forests that haven't seen bark beetles ever, not since the glaciers receded. They're now in naive forests that are not well defended."
The Mitton-Ferrenberg paper has been greeted with skepticism from some beetle experts — and with shrugs by others. In internal e-mails, some Forest Service employees have questioned the pair's research methods and the limited scope of the data. Mitton, who has now expanded the study to four sites, calls the carping "horseshit."
"Most of what the Forest Service puts out is only reviewed by the people down the hallway," he says. "If I used their methods, I would never have seen any of this."
Others point out that adaptation to climate change, including an accelerated reproduction cycle, has already been detected in numerous other species, including the spruce beetle. In light of that record, Ferrenberg says, he was surprised by the "blowback" the paper has received in some quarters. "There are hundreds, if not thousands, of insects that have been documented to change their reproductive times or locations in response to a warming climate," he says, "yet there's just some barrier for professional foresters to admit that this is possible."
"The jury's still out from our entomologists," says Cal Wettstein, the incident commander for Rocky Mountain bark-beetle issues at the Forest Service, which is battling the epidemic in much the same way as it responds to a large fire. "Time will tell, on the climate change question. But it's an interesting aspect, and it may explain why the beetles have moved so fast."
Wettstein and other forest agency officials say that the epidemic's severity can be traced to several factors — some natural, some having to do with decades of forest-management decisions that are now being sharply criticized.
"This may be the largest epidemic we've experienced, but it's far from our first," says Sky Stephens, entomologist for the Colorado State Forest Service. "We had a lot of forests of uniform age, very limited forest-management activities, and we've spent a lot of time suppressing fire. All of this allowed a very large percentage of Colorado's pine forests to enter their most susceptible life phase at the same time. And then we had a significant period of drought."
Beetles thrive in drought years. Pines respond to beetle attacks by oozing resin, "pitching out" the invaders, but a lack of water weakens that defense process. Well-watered trees have been known to drive out thousands of beetles; a drought-plagued tree can succumb to a handful. If the beetles manage to set up house in the tree bark, they infect the tree with a blue-stain fungus that serves to nourish their young while strangling the tree's hydrology and eventually killing it.
But while there seems to be a general consensus among agencies about the causes of the epidemic — a perfect storm of drought, mild weather, impassive forestry policies, millions of acres of mature lodgepole stands — there is no similar agreement about what should be done to prevent future kills on such a massive scale. The Forest Service is devoting much of its resources to what amounts to a tree-removal service, spending hundreds of millions of dollars to clear dead stands that might topple on roads, trails, power lines or campers. Conservation groups are undertaking heroic efforts to save imperiled, high-altitude pines that serve a critical role in sustaining other mountain wildlife, trees that have survived for centuries in harsh conditions — but could be wiped out by the beetle in a matter of months.
The staggering toll of the infestation has forestry associations and timber interests clamoring for relief and denouncing the federal government's "failed policies." Local and state officials are nervously eyeing vast stands of dead, dry wood and worrying not just about the inevitable wildfires, but the threats to watersheds, air quality and public health those fires might bring. At the same time, some scientists are openly skeptical of the most alarming claims concerning the epidemic's impact and contend that a broad-based program to remove trees will only make things worse.
If nothing else, Mitton muses, the astonishing voracity of the latest beetle outbreak is confronting global-warming naysayers with a dramatic example of the phenomenon at work. "This beetle is just one of many species that have responded to climate change," he says. "A butterfly coming out early or moving north is not something people notice. Dead trees, they notice."
One of the most basic challenges of the beetle epidemic is figuring out just how many dead trees it's left in its wake, and what that means to the future health of western forests. Given the vast area involved — and the contentious politics of natural-resource management — the answers aren't as simple as they might appear.
Two years ago, Forest Service incident commander Wettstein did some "back-of-the-envelope math" in an effort to determine "where we are in the temporal continuum of the epidemic." Calculating from aerial survey data for northern Colorado and southern Wyoming that indicated 3.6 million acres had been affected at that time, and using a mortality rate of 100 trees per acre, he came up with a ballpark figure of 360,000,000 dead trees.
Wettstein estimated that the trees would begin to fall three to five years after the beetles were done with them, and that 90 percent would be on the ground after fourteen years. Since Colorado was already a decade into the epidemic, that meant trees were probably falling over the place. Dividing 360,000,000 trees over ten years (3,650 days), he worked out an average fall rate of almost 100,000 trees per day.
Wettstein never intended his rough estimate to be used in shaping management decisions. But the figure has acquired a life of its own.
"That thing will never die," he says. "I came up with that number, and I've regretted it ever since. The regional forester picked up on it, and it started making its own weather."
Indeed. Western Bark Beetle Strategy, a Forest Service report released last year, declares that "up to 100,000 dead trees killed by beetles fall to the ground every day in southern Wyoming and northern Colorado.... In the Interior West alone, there are approximately 14,000 miles of roads, trails and right-of-ways that could be adversely affected by falling trees, as well as approximately 1,400 recreation sites." The 100,000-trees-a-day mantra has also shown up in wide-eyed news reports and has been repeatedly cited in congressional testimony, most recently during field hearings in Montrose last month on the beetle infestation sought by Colorado representatives Scott Tipton and Mike Coffman.
Many scientists tracking the current epidemic have a hard time digesting that number. They say it's based on several debatable assumptions, starting with the size of the affected acreage. Stating that four million acres have been "affected" by beetles doesn't translate into four million acres of dead trees, as is sometimes reported. An affected acre may have only a few trees hit by beetles in a given year, enough to be observed by the aerial surveys.
State forest service entomologist Stephens, who flies frequently over the kill sites, says the survey data is complex and intended to track trends, not provide precise figures. "It's a lot of speculation rolled into some basic algebra," she explains. "We have to make assumptions about how many trees per acre there are in parts of the forest, then account for what percentage the host species represents on those acres.... The biggest problem with all of this is that we don't have good figures for trees per acre for most of our forests, so to get to a figure like the number of trees impacted in 2011, we're likely to end up with the kind of number that is so far stretched with assumptions that it loses value."
Although in some areas beetle mortality could approach 100 percent, the more likely scenario is quite different. The Forest Service's own National Insect and Disease Risk Map, which uses sophisticated computer modeling to predict how tree species react to various threats, projects no more than a 25 percent mortality rate from western bark beetles over 20 million acres of forests at risk across the region. To fit the NIDRM model, Wettstein's estimate would require a density of 400 trees per acre, with a fourth of them killed. CU's Mitton points out that means trees spaced about ten feet apart — a figure that seems too high for the large lodgepole trees targeted by the beetles. If the lodgepole are intermixed with aspen or spruce trees, he notes, the actual average per acre could be far less.
There's also the matter of how quickly a dead tree falls. "They're assuming that all the trees that are killed will fall within ten years, and that's in direct contradiction to numerous studies that have been done on the longevity of snags," says Chad Hanson, director and staff ecologist of the California-based John Muir Project. "Most trees stand for considerably longer. They can stand for fifty or a hundred years."
Wettstein insists the 100,000-trees-a-day figure is a conservative one, since it doesn't take into account the green trees that, once surrounding trees are killed and fall, end up being exposed to the wind and could also be toppled. Hanson says he isn't surprised that such an off-the-cuff estimate has been so widely disseminated by the Forest Service, which he believes has a vested interest in exaggerating the scope of the problem.
"The Forest Service is very good at pointing out the portions of the landscape where the beetle mortality is highest, but it's actually a relatively small portion of the landscape," he says. "What they're saying is wildly inaccurate."
Whether the actual daily death rate is slightly higher or much lower than the hundred-thousand mark, falling trees have become a top priority for the Forest Service, which has embarked on an ambitious program to protect infrastructure and increase public safety by removing thousands of dead trees in high-use areas. Crews are thinning stands around campgrounds, clear-cutting trees that threaten power lines and roads, laboring to remove tons of dry, red-needled fire hazards in areas close to mountain subdivisions and municipal watersheds. In 2011 the agency spent close to $85 million on such efforts to "treat" 187,800 acres, including kill zones sprawled across three national forests in Colorado.
Senator Mark Udall, one of Washington's staunchest boosters of the mitigation effort, recently announced that it could take twenty years for the Forest Service to complete its removal goals at current funding levels. The 2012 Farm Bill currently moving through Congress designates $100 million for beetle mitigation programs; Udall and other western senators are pushing for double that amount.
Wettstein notes that the removal projects are just the first phase of a plan to ultimately bring the forests back to robust health by introducing diversity in age class and tree species, using prescribed burns, thinning and other techniques to make the pines more resistant to large-scale infestation. "We know the best thing we can do for these landscapes is to introduce diversity," he says. "We're looking at building recovery and resiliency, and we're looking long-term."
In the short term, the sprawling hills of dead wood might seem to present a range of entrepreneurial opportunities for loggers, biomass and wood-pellet firms, beetle-kill furniture manufacturers and others. But supply has far outstripped demand; even with the Forest Service entering into "stewardship contracts" with private companies and paying them to haul felled trees away, the agency is still faced with tons of surplus pine that will have to be simply slashed and burned.
"The timing was unfortunate, given lumber markets and the meltdown of the housing industry," says Wettstein. "Lodgepole isn't the most valued construction material, anyway. You're dealing with a marginally valued product to start with, and that exacerbates the difficulty in moving wood."
Some industry observers say the government hasn't done enough to prop up ailing saw mills and cut through the paperwork of stewardship arrangements. At times the recent "field hearings" hosted by Representative Tipton played like a referendum on opening up the national forests to the private sector, with Tipton himself decrying "the devastating impact that decades of federal obstruction to effective management have had on Colorado's once-healthy forests. Dead timber, lost jobs, contaminated water, and landscapes eviscerated by catastrophic wildfire are hallmarks of a bureaucratic process that places politics and inaction ahead of common sense and conservation."
But Wettstein says that more active management would have had little effect on the current epidemic: "When you look at what's mechanically treatable across the impacted area, it's only 25 or 30 percent. You've got wilderness areas, roadless areas, areas that are too steep or inaccessible to do active timber management. If the Forest Service had been aggressively treating lodgepole stands, regenerating mature stands to prevent them from becoming beetle fodder, we still would have had this epidemic. It might have been tempered in 30 percent of the area, but we'd still have it."
Mitton agrees. "Between here and the Yukon Territory, there have been various management styles — including no management at all," he says. "Yet the bark beetle is hitting all of them. This is a consequence of increases in temperature. The Forest Service didn't cause that. Climate change caused that."
The vast majority of the dead trees included in Wettstein's calculations are in the backcountry, where the risk to human safety is negligible. The Forest Service is concentrating its efforts on more accessible hazards. But if 100,000 trees fall in the woods and nobody's around, what should be done about it?
As little as possible, Chad Hanson insists. The John Muir Project director has written extensively on the value of large dead trees and fire-ravaged areas in shaping new wildlife habitat and eventually regenerating forests. The newly opened canopy allows flowering shrubs and aspen to thrive. Colorful songbirds arrive and take up residence in nest holes in dead trees, holes colonized by woodpeckers that come to feast on bark beetles. Small mammals, including hares and woodrats, create dens in downed logs, followed by their predators.
"A dead tree supports a lot more life than a live tree of the same size," Hanson declares. "Snag forest habitat — where most or all of the trees have been recently killed by natural processes — is some of the most ecologically important and biodiverse forest habitat we have in the West."
Hanson doesn't take issue with clearing campgrounds or removing dead trees and other fire hazards in the immediate vicinity of mountain homes. But his own research and another scientific study indicate that, contrary to popular myth and Forest Service gospel, beetle-killed trees aren't more susceptible to fire than dry green trees. "A lot of the results in wildfire studies seem counterintuitive until you understand them," he says. "There are some volatile oils in live needles that enhance combustibility. Once the tree dies and the needles start to fall, there's not as much foliar fuel."
From his viewpoint, much of the government's response to the beetle outbreak is unnecessary and possibly counterproductive. "What they're really arguing for is opening these remote areas for more logging," he says, "ostensibly to protect them from beetles and fires. But the projects they're proposing would kill far more trees with chainsaws than their own science indicates would be killed by beetles and fires."
On a June morning, while the plume of the High Park fire looms over Fort Collins and the smell of woodsmoke wafts all the way to Denver, Jeff Mitton wanders through towering stands of lodgepole and limber pine along Niwot Ridge beneath an untroubled blue sky. He checks traps for beetles and occasionally uses a small hatchet to pry off a piece of bark, in search of new arrivals.
The trees at the research site appear, for the most part, stunningly healthy. Brush up against the bright-green needles and you'll release a dense cloud of pine pollen. A beetle-infested tree will continue to look green and healthy until the following growing season, when its needles turn into brittle, rust-colored strands and begin to drop — but most of these trees haven't been touched at all. It's an anomaly that Mitton and Ferrenberg managed to document the beetle's accelerated life cycle at a place that's seen little damage so far from the insect — particularly in comparison to the waves of red and gray trees on slopes just a few miles west, on the other side of the Continental Divide.
Still, there are casualties scattered conspicuously across the landscape: trees turning orange, bare-limbed skeleton trees riddled with beetle exit holes. Most of them are limber pines, which the beetle seems to favor here over the lodgepoles. Mitton pauses to examine signs of a recent invasion in a gnarled, eminent limber. There are telltale plugs of fresh resin oozing from several spots near the base of the tree, like wounds clotted with blood.
Mitton chips under the bark with his hatchet but fails to find any beetles. "There's no blue stain in there yet," he says. "I'm not certain if the beetle has been pitched out or if I just crushed her."
For now the limber seems to be holding its own. But the beetles will be back in force as the summer heats up. And the current epidemic threatens to have a far more long-lasting impact on some tree species than others — particularly the limber pine and other varieties of five-needle white pines that play a keystone role in high-altitude ecosystems.
The so-called High Five pines, including limber, whitebark, Rocky Mountain and Great Basin bristlecones, are stress-tolerant species that fare well in poor, rocky soils and harsh conditions, often at the edge of treeline on steep slopes. Generations of wind and cold can twist their limbs into exotic shapes, and they've been known to live for hundreds of years. (Bristlecones are among the oldest living organisms on the planet; one survivor in California has been core-dated as 4,842 years old.) For years, Mitton made regular pilgrimages to a favorite limber pine in Rocky Mountain National Park, more than a meter in diameter, that he estimated to be more than a thousand years old.
"I liked to take pictures of it," he says. "Two years ago, I went back and it was dead. There were beetle attacks all over it."
Despite or perhaps because of their venerable age, the slow-growing High Five pines have proven to be less resistant to beetle attacks than more common species; milder temperatures at high elevations have also allowed the beetles more freedom to operate. At the same time, the High Fives have had to contend with the creeping progress of white-pine blister rust, a fungal disease from Asia that kills trees slowly over years but for which there's no known treatment.
"This has been a one-two punch that's sending the Forest Service into high-gear problem-solving," says Diana Tomback, a professor of integrative biology at the University of Colorado Denver. "These trees are very important for high-elevation ecology."
Tomback started out as a student of avian ecology. Her research on the Clark's nutcracker, though, soon focused on its "co-evolved relationship" with the whitebark pine: The bird plunders the whitebark's cones for seeds and then caches them in thousands of sites, intending to retrieve them when food is scarce — but effectively planting new trees across the landscape. The whitebark helps protect watersheds and provides high-energy seeds for squirrels, bears and other animals; government studies indicate that grizzlies in Yellowstone end up in more deadly encounters with humans in years when the whitebark seeds are in short supply, as they have been in recent years.
Although limber and bristlecone pines in Colorado have also taken a hit, the fate of the whitebark, which grows farther north, is of particular concern to Tomback and other ecologists. Between the blister rust and the relentless march of the beetles, some areas in the northern Rockies are reporting a mortality rate among whitebark pines of 80 percent or more. Last year U.S. Fish and Wildlife Service officials ruled that the whitebark is in danger of extinction and warrants endangered-species protection but said the agency doesn't have the resources to list the tree. Canada has already accorded the whitebark endangered status.
Tomback, who is also the volunteer director of the Whitebark Pine Ecosystem Foundation, says blister rust has already wreaked havoc in much of the whitebark's range, and the beetles could finish off much of what's left. "Those few trees that are potentially resistant to this disease can be killed in one season by the pine beetle, which doesn't discriminate," she notes. "It's a bad problem that we don't think nature can overcome."
In Yellowstone and other hot spots, state and federal officials are scrambling to combat the possibility of extirpation — local extinction — of the whitebark pine. They're collecting seeds to try to develop blister-rust-resistant trees, then pondering how to protect those trees from beetles. Annual spraying can protect individual trees but is too costly, financially and environmentally, for widespread use. Another limited protective measure involves placing packets of chemicals on certain trees that mimic an anti-aggregation pheromone the beetles issue when they've taken up residence and want to keep other beetles from invading the host — a kind of "no vacancy" sign.
"We know what we need to do to restore these species," says Tomback, whose foundation is partnering with larger environmental groups to raise awareness of the threat. "It's painstaking, and it may take more than one generation to do. But without them, the forests become a lot more homogenous. It will impoverish us, from a biodiversity perspective. And very little work has been done on the impact on wildlife of this widespread pine-beetle kill."
Although a great deal of energy and planning is now going into efforts to restore habitat and revitalize forests, officials acknowledge that their ability to manage the epidemic is limited. Human engineering — what Tomback calls our ability to "alter the natural dynamics" — certainly contributed to the high toll of the beetle outbreak, from fire-suppression policies to the central role of increased greenhouse gas emissions in driving climate change. But our ability to engineer our way out of the problem is less clear.
Some experts believe that the epidemic, like a long period of drought and fire, essentially has to run its course and burn out. Even referring to the outbreak as an "epidemic" or an "invasion" is misleading, they argue, since the beetle is a native species that's been munching away at pine trees since the dawn of the forests; one fossilized specimen of a precursor of the mountain pine beetle found in Boulder County dates back to between 15 and 30 million years. "The bark beetle never really goes away," says Mitton, who's continued to find muted but persistent beetle activity in ponderosa pines on Sugarloaf Mountain outside Boulder, 35 years after the last big beetle kill swept through the area.
Yet forest agency officials are determined to avoid repeating the same devastating trajectory in the future. It's unlikely that the High Five pines will rebound within a generation or two. But lodgepoles are much more resilient; the younger trees spared by the beetles are expected to fare well. And that presents the prospect of the same cycle starting all over again, with dense and uniform stands of trees, all roughly the same age, vulnerable to massive beetle attacks.
"The most important factor in this epidemic is the condition our forests are in," says entomologist Stephens. "We might want to re-evaluate where we stand on things like timber harvest, fire suppression and forest management — and recognize that multiple-use policies on public lands are probably the best."
There's no "blanket approach" that will work everywhere, Stephens adds. For example, ponderosa pines can benefit from regular thinning procedures, but lodgepoles have shallower root systems — too much thinning can leave the remaining trees vulnerable to being knocked over in windstorms.
Ferrenberg believes that future forest management should focus more on identifying trees that have higher resin flow (and more resistance to the beetles) and encouraging their propagation. "If you come up with a management plan that involves cutting trees or prescribed burns, you have to figure out how to get it into some of the most rugged places in the world — and you're going to have to accept unexpected outcomes," he says, referring to the threat of prescribed burns going wild. "We need to start managing early. It's going to be more time-intensive management, but it's something we need to do."
Mitton sees Colorado's forests adapting to changing conditions, just like the beetles. The severely thinned lodgepole stands present an opportunity for aspen, which have extensive root systems and tend to colonize avalanche, burn and beetle sites. Aspen appear to be on the rebound across the state, after prolonged drought and a condition known as Sudden Aspen Decline that claimed large groves a few years ago, and the tree may well be on the verge of a decades-long proliferation in the state not seen in centuries.
"The forests are going to look more like what they were when the European settlers came to Colorado," Mitton says. "That's not an awful thing. Once the aspen are doing well, the lodgepole will come back and shade them out — and there we go again."
Aspen forests are rich in biodiversity. They let in more light, generating a lusher understory than conifers, and are important to a range of wildlife, including beaver and birds of prey. And as Mitton and other ardent nature photographers know, they offer something the evergreen and ever-beleaguered conifers can't provide.
"Is there a silver lining to this epidemic?" Mitton asks. "My thought is, aspen are pretty in the fall."
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