Some of the 1,000-year-old redwoods in Muir Woods National Monument are so tall their crowns disappear in the sky. Their broad trunks, standing side by side, nearly create walls, and their thigh-thick roots contour the forest floor. The recent history of this forest, however, is not written in the tops of the ancient redwoods, but in the understory. In 1995, Muir Woods, just across the Golden Gate Bridge from San Francisco, was one of the first places hit by a new forest pathogen – sudden oak death. The disease devastated the understory of tan oaks and caused a brief panic that the redwoods might be next.
Susan Frankel heads up the Sudden Oak Death Research Program and has been studying the pathogen since before it even had a name. On a recent walk through Muir Woods, she recalled the first outbreak. “It was like a river of dead trees,” she says. While focused on sudden oak death – which she says “will significantly change the coastal forest of California” – Frankel, a pathologist, is also concerned about the big picture of forest disease. “There will be winners and losers,” she says. “Some species will be favored and other species will fade out.”
Global climate change is making many trees sick — and in the process releasing more carbon into the atmosphere
Trees infected by sudden oak death are clearly the losers, and that makes them easy to spot. The leaves of the diseased oaks, which are normally evergreen, turn brown after the first year of infection. The entire tree falls about five years later. In the late 1990s, the interior of the Muir Woods understory was so infected by sudden oak death that the normally lush green forest was patchy with the brown of dead and dying trees. A decade later, the dead tan oaks have made way for sunlight to enter the forest, creating room for new growth of redwood, maple, and the next generation of oak. Although a new understory is emerging at Muir Woods, the concern is that sudden oak death still exists in the park and is spreading – just waiting for the right conditions for another big outbreak.
The story of Muir Woods is being retold across western North America. As the number of tree deaths surpasses historic records, and as new pathogens like sudden oak death become prevalent, forest diseases are no longer being looked at as “pests” or “nuisances,” but as threats to the very survival of forest ecosystems. Decades of strict forest management to maximize timber production and the suppression of wildfire across the West have created forests that are so uniform that they are especially susceptible to disease. But an even greater force is at play – global climate change.
It’s uncertain what exactly the forests of the future will look like if the atmosphere alters much further. The relationships between pathogens, the weather, and forests contain so many variables that it is difficult for forest researchers to connect all the dots between a localized disease epidemic and global climate change. What at least is clear is that increases in temperature and shifting precipitation patterns are already affecting forest health. And that is bad news, not only for the forests themselves, but also for the balanced climate they depend on.
One example of a climate change-related forest pathogen is the Swiss needle cast, a fungal foliar disease that is affecting Douglas fir in the Pacific Northwest. The disease provides “compelling evidence that climate change will exacerbate the problem,” says Terry Shaw, chief scientist at the Western Wildlands Environmental Threats Assessment Center in Oregon.
The fungus, like many other forest diseases, has always existed in the forests of the Pacific Northwest. For decades, it was considered an insignificant forest health problem. Then, in the mid-1980s, a mysterious disease began infecting the forest of Oregon’s Coast Range, from Coos Bay to Astoria. Following the dogma that Swiss needle cast was not a problem in North America, researchers began looking for other causes. “Swiss needle cast does not kill trees, it just slows growth,” says Jeff Stone, a plant pathologist and professor at Oregon State University. The conventional wisdom was wrong. Stone and his colleagues eventually concluded that the pathogen that had always been present at low levels in the Pacific Northwest had turned into a full-blown disease outbreak. The fungus, which slowly strangles trees, is, according to Stone, now present in half a million hectares of Oregon’s stands of Douglas fir, 160,000 hectares of which are severely affected.
What changed in the forests of Oregon’s Coast Range to have caused an increase in the severity of Swiss needle cast? The weather: Warmer winter temperatures, combined with more rain in the winter and spring, are favoring the pathogen. As Stone explains, “Changes in climate are going to result in changes in disease.” Another critical factor in the epidemic is that plantation stands of Douglas fir were being planted in ecological zones where Western hemlock is the dominant species.
Dothistroma, or red band needle blight, is another foliar fungus linked to climate change. When Alex Woods, a regional pathologist for the British Columbia Ministry of Forestry, started investigating stands of dying lodgepole pine in 1997, he “wasn’t looking for a climate change-related story.” At first, he didn’t see a link between infected forests and red band needle blight, which has now damaged nearly 50,000 hectares of ecologically and economically valuable lodgepole pine in the province. Like the Swiss needle cast, the blight had always existed in BC’s lodgepole stands, and occasionally would produce small outbreaks in isolated pockets. According to Woods, “Foliar diseases are not really thought of as being able to kill healthy trees.”
Then, in 2002, Woods took a flight over the Kisipox Valley and recalls being “quite shocked by the devastation.” After that, he began looking for a climate change connection. By analyzing monthly temperature and precipitation records from 76 weather stations across Canada and overlaying that data with recorded disease outbreaks, Woods established a relationship between weather changes and tree deaths. The evidence demonstrated “spikes in weather events that coincided with disease outbreaks,” Woods says. The problem wasn’t increased summer temperatures, but rather an increase in summer rain. Woods and other researchers were able to determine that more warm summer rain in the high-latitude forests of British Columbia could be a result of global climate change. The shift in climate, slight though it may be, has serious implications: “If that trend continues, then we can expect bigger changes in more places,” says Woods.
While the weather data collected to study the red band needle blight did not indicate a summer warming trend, it did demonstrate a winter warming trend. Winter warming is consistent along the Rocky Mountains. Combined with a lack of disturbances such as fire and decades of forest management, winter warming is believed responsible for the devastating mountain pine beetle outbreak. Considered the largest forest insect epidemic ever, by the end of 2007 the beetle had chewed through 3.9 million acres of the forests along the spine of the Rocky Mountains in the US and 22 million acres in Canada.
Across western North America, the shoulders of summer are becoming broader and winters are losing their insect-killing bite. This combination has set the stage for the unprecedented beetle surge, which doesn’t show any indications of slowing down. In Canada, the mountain pine beetle is expected to eat 80 percent of British Columbia’s “merchantable” pine by 2013, according to a BC Ministry of Forestry and Range study.
Several factors are at work. Warming temperatures speed up the metabolism and reproduction rates of the beetles. An increased metabolism translates into more timber consumption, and by taking advantage of a longer summer season, the pine beetle is able to produce two generations instead of one in some areas. The concern in British Columbia is the toll the beetle blight will have on the timber industry and tourism, staples of the province’s economy. In some areas in the northwest of BC, the pine beetle is eating ponderosa and lodgepole pine, while the red band needle blight is crippling what’s left. Unless there is a winter cold enough to slow beetle reproduction, the pest could launch into the jack pine of Canada’s boreal forest and start a devastating march to the Atlantic.
The scenario is the same in Colorado, where the mountain pine beetle is threatening to destroy the lodgepole pine population. The recorded damage beginning in 1996 through 2007 is about 1.5 million acres of mostly lodgepole pine, according to Jeff Witcosky, a Forest Service entomologist. The numbers are expected to increase once the aerial survey information from 2008 is compiled. Though large-scale pine beetle epidemics have happened in the American Rockies before, the fact that the beetles are moving to higher elevations and even jumping the continental divide are new trends. The biggest fear is that the pine beetle will switch species and begin devouring ponderosa pine in Colorado’s Front Range. The loss of trees – terrible in itself – would also likely increase the risk of fires in the Front Range, where most of Colorado’s population is centered.
Some forest ecologists say there may be a silver lining to the infestation. If the trees of the Colorado are the losers, the forests as a whole may be the winners as the tree deaths allow for more biodiversity in over-managed tracts. “The forest was ready for an event like this structurally,” says Sloan Shoemaker. Shoemaker, the executive director of the Wilderness Workshop, a public lands conservation organization, says Colorado’s forests, after years of management and fire suppression, are too homogenous. This is particularly true in Summit County, home to ski towns like Vail and Breckenridge. The forests there are dominated by 120-year-old lodgepole that have outcompeted younger spruce, fir, and aspen. The pine beetle epidemic is “a natural disturbance regime reestablishing forest health,” says Shoemaker. “What you’ll get is a diverse forest.”
In some cases, neither pests nor pathogens are responsible for tree deaths. Rather, the very changes in the weather can impact tree physiology, threatening a forest’s health. Yellow cedar decline, which has caused half a million acres of tree mortality in southeast Alaska, is one example.
Yellow cedar decline was first reported in 1909. By 1920, there were aerial photographs indicating large stands of the dead cedar. The trees, which can live for hundreds of years, have evolved to grow slowly and dedicate resources to a chemical defense that guards against pathogens at the cost of being able to reproduce rapidly. By the 1970s and 1980s, the mortality among yellow cedar was so high that pathologists thought some kind of disease was infecting the trees. “It took a while to figure out that wasn’t a promising avenue,” says Paul Hennon, a US Forest Service pathologist who has been studying cedar decline since 1981.
The decline, it turns out, is not caused by a disease, but by a lack of insulating spring snow pack. Snow, which used to linger longer into the spring, acts like a blanket on the ground, keeping the soil and fine cedar rootlets at a consistent temperature. When the regular snow pack disappeared, it left the cedar’s shallow roots vulnerable to late spring freezes. Because of its unique physiology, yellow cedar is particularly susceptible to late season freeze injuries. Once the fine rootlets of a tree are damaged, a chain reaction unfolds, ultimately causing the tree to die. “That’s what has really changed,” Hennon says. “The amount of snow.”
A subtle shift in weather is the theme that runs through the increased severity of forest diseases. As climatologist Kelly Redmond puts it: “Subtle does not mean unimportant.” Redmond, who is a deputy director for the Western Regional Climate Center, says, “It’s been known a long time that insects and forests are shaped by climate,” and that “lots of pathogens are held in check by climate.”
It’s unclear what the long-term effects of these small changes will be. When asked what the forests of the future will look like, Rona Sturrock, a forest pathologist with the Canadian Forest Service, says, “It’s like trying to hit a moving target.” Sturrock says there are conflicting conclusions in the scientific literature. One potential scenario is that some trees will thrive as increased greenhouse gases like nitrogen and carbon dioxide encourage them to grow faster; warmer temperatures and increased precipitation may favor some species. The other scenario is that as the climate changes, disease, drought, and fire may increase, pushing many species to the brink.
Developing accurate predictions of future forest health is so important because forests play such an essential role in regulating the planet’s atmosphere. Large, healthy stands of trees, like the boreal forest of Canada’s northern latitudes, store huge amounts of carbon, at the magnitude of megatons. When large swaths of trees die, they release the carbon back into the atmosphere.
Karen Wattenmaker, kwphoto.com
An article in the January 2008 issue of Nature measured the British Columbia forest affected by the mountain pine beetle to see if it was a carbon sink or carbon source. The authors concluded: “Insect outbreaks such as these represent an important mechanism by which climate change may undermine the ability of northern forests to take up and store atmospheric carbon.” Rather than store carbon, the affected forested area of British Columbia is predicted to emit 270 megatons of carbon from 2000 to 2020. This is the same amount of carbon that Canada pledged to reduce between 2008 and 2012 in order to comply with the Kyoto Protocol. Partly due to the loss of forests, the Canadian government now says it is unable to meet its promised carbon reductions. The forest’s ability to store carbon has many ramifications, both ecological and political.
Despite the high stakes involved, there is only a small team of forest pathologists who are collaborating on the issue of disease and climate change. “Isn’t that scary?” asks Susan Frankel. The discipline of forest pathology grew out of a need to understand disturbance in forests managed for timber production. During the last 15 years, the US Forest Service has moved away from a production-centric posture toward a more ecological one. Now, as foresters are watching entire management areas succumb to unprecedented disturbance, the view is slowly being adopted that forest management based on the way forests used to be is no longer adequate. With fresh attention focused on revamping expiring forest management plans, Frankel and her pathologist colleagues are trying to ensure that Forest Service policies take into account the increased role of pathogens. The goal is to “rebuild the historical record of what’s known,” Frankel says. “To predict, based on what we know, what are the risks to the forest.”
—Daniel McGlynn is a freelance journalist based in the San Francisco Bay Area.
Canada’s majestic boreal forests can seem dense and endless. Home to grizzly bears, moose, and eagles, the forests — covering a third of the country — are often referred to as the lungs of the Northern Hemisphere.
But the excavation of Alberta’s oil sands is wreaking havoc on this important ecosystem. And if the world’s hunger for oil continues on pace, large swaths of these lush and biodiverse woods could look more like an arid moonscape in a decade: the earth shaved off to reveal oily black layers, carved up by roads for monster-size machinery, and cleared for hardscrabble camps, temporary homes for tens of thousands of transient workers.
The proposed expansion of the oil sands industry will also exacerbate Canada’s greenhouse gas emissions, jeopardizing forests worldwide. About 188 pounds of carbon dioxide are released in the course of producing one barrel of oil from oil sands. This greenhouse gas, of course, is one of the major causes of global warming, which causes significant harm to forests. Climate change has been blamed for lethal bark beetle infestations, migration of non-native plants and trees that compete with indigenous trees, and other complicated effects that change and endanger the local forest ecology.
The oil sands (also called tar sands) industry is centered just south of the Athabasca delta, with the once sleepy town of Fort McMurray now a booming industrial hub. At least 180 square miles of boreal forest have already been cleared for oil sands excavation, according to the Canadian nonprofit Pembina Institute, and 1,150 square miles more are leased for this purpose.
About 30 percent of Alberta’s estimated 175 billion barrels of oil sands are accessible by surface strip mining, which involves clear-cutting and scraping off the earth’s surface. Oil sands that are too deep for surface mining can be extracted through wells by heating the gooey mixture to separate an oily mix from the sand and sucking it up as if through a giant straw. This leaves some of the surface forest intact, but huge swaths of land are nonetheless cleared for service roads, wells, and other machinery.
“This devastates the hydrology of the area; any groundwater sources and surface sources become contaminated,” says Indigenous Environmental Network oil sands campaigner Clayton Thomas-Muller. “Both forms of development are devastating to the ecology of the region, because both equal clear-cutting of pristine and diverse boreal forests.”
The boreal forests’ health depends on rivers, and these rivers, including the mighty Athabasca, could be sucked dry or contaminated by the well operations. It takes an estimated 2 to 4.5 barrels of water to produce one barrel of oil this way. Many also fear massive contamination from the toxic tailings ponds that store waste from the oil sands — waste that includes benzene, paraffin, and polycyclic aromatic hydrocarbons (PAHs).
First Nations communities around oil sands operations have already reported berries and wildlife being wiped out, and fish with mutations and oozing pus. That’s just from slow leakage; if one of the dams that contain the tailings ponds were to break from seismic activity or structural inadequacy, the results could be catastrophic.
As of 2006, at least 20 square miles of Albertan boreal forest had been covered with tailings ponds; that number is expected to increase to 85 square miles in the near future, according to the Pembina Institute.
With public awareness about the oil sands’ destructiveness growing, opposition is on the rise. So far, the industry has been able to proceed at a breakneck pace because the Alberta government has near-total control over land use, with almost no federal oversight. The Indigenous Environmental Network is arguing that oil sands extraction violates First Nations treaty rights; they are hoping to limit the industry on these grounds. But it will be hard to remedy the damage done to date.
“The horse is already gone from the barn,” says Simon Dyer, oil sands director for the Pembina Institute. “Whether it will be possible to put an effective land-use plan in place remains to be seen. We’ve seen nothing yet in terms of what’s coming if current projects go through.”
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