Mountain Pine Beetle (Dendroctonus Ponderosae) under a scanning electron microscope. [Credit: Leslie Manning/Canadian Forest Service]
** Editor’s Note: The staff of Scienceline is taking a short break to work on future stories. In the meantime, we will repost some of the site’s most popular articles from the last six months. We will return to publishing new content on May 19. This article originally appeared March 28.
Flying in a small airplane, high over the flat, austere landscape of central British Columbia, an endless vista of pine trees spreads out before you. Coursing through the landscape are huge swatches of dead trees, their needles turned the color of bright fall leaves.
To an untrained eye, it looks like fire might have seared the forest, but the trees have actually succumbed to a tiny black predator known as the Mountain Pine Beetle. Due to drier summers and warmer spring temperatures over the last 50 years, the beetles are thriving. And they have already destroyed tens of thousands of square miles of pine forest in Canada and the western United States.
In British Columbia, the affected area is now roughly the size of Indiana.
“Once the beetles are at the level they’re at in British Columbia, there’s nothing you can do — it’s like a rapidly spreading fire,” says Barbara Bentz, research entomologist with the U.S. Department of Agriculture Forest Service. If the beetle continues to devour trees at the current rate, 80 percent of British Columbia’s mature pines will be killed off by 2013, according to Natural Resources Canada, an arm of the Canadian government.
Global climate change, which is pushing temperatures higher, has altered the beetle’s natural life cycle. Now the insect threatens one of the world’s largest forest systems: Canada’s boreal forest, a 600-mile-wide band of pine woodlands that stretches from the Yukon in Alaska all the way to Newfoundland on the East Coast.
Some scientists think the damage inflicted by these voracious insects on the boreal forest might actually contribute to greenhouse gas levels and ultimately global climate change. When the beetle’s only food source, the pine tree, is destroyed, it ceases to trap carbon dioxide in the atmosphere and actually releases it back out.
The source of all this destruction is an insect not much bigger than a grain of rice. A native of North America, the pine beetle does its damage by burrowing beneath the bark and feeding on the living tissue of the tree called the phloem. This tissue is composed of long tubes that transport nutrients from root to limb, and once it is destroyed, the tree can no longer survive.
In the past, cold snaps — quick drops in temperature in the spring and fall — have kept beetle populations in check. Although the insects can survive temperatures as low as minus 35 degrees Fahrenheit in the winter, it takes time for their bodies to accumulate enough glycol, the same ingredient found in antifreeze, to survive such frigid temperatures.
According to researchers, including Bentz, warmer temperatures have reduced the number of cold snaps and bolstered the beetle populations. A 2004 study by the Canadian Forest Service shows that as temperatures in British Columbia have risen since the 1920s, so have the number of pine beetle outbreaks.
British Columbia’s forest management practices, specifically the suppression of forest fires, have added fuel to those outbreaks by creating large unbroken stands of mature pine, the only type that the beetle can inhabit.
“At the start of the current outbreak in British Columbia we had 3 to 3.5 times as much mature pine on the landscape as we would have had 100 years previously,” says Allan Carroll, a research scientist with the Canadian Forest Service.
The total area affected in British Columbia since outbreaks began in the mid-1990s stands at 35,000 square miles, according to a report released by the provincial government in 2007. The U.S. Forest Service estimates that in the same time frame the beetle has affected roughly 25,000 square miles of U.S. forest, mostly in the Rocky Mountain states.
In 2006, a windstorm in British Columbia created a “hail storm” of beetles that blew across the Continental Divide and into Western Alberta. Fortunately, a cold snap in October of that year killed off many of the beetles before they could establish themselves. Bentz thinks it is only a matter of time before they are back in force.
“As the climate has shifted, this insect’s range is shifting farther and farther north,” explains Bentz, “and it has also shifted east into Alberta.”
Carroll, the Canadian researcher, says that a corridor of climate suitable to the beetles will slowly open east across Canada in the next four to five decades. His projections are based on computer models, with increases in temperature and greenhouse gases set at conservative levels.
The climate corridor opens directly into Canada’s boreal forests, which are an important part of a worldwide northern forest system that pulls carbon dioxide out of the atmosphere and turns it into plant sugars and wood. These Canadian pine woodlands represent 20 percent of all the forests existing just below the Arctic Circle in Russia, Scandinavia and Canada, known collectively by the Russian word taiga.
When the taiga is growing at its maximum rate, during the spring and summer, worldwide levels of carbon dioxide fall and oxygen rises, according to a Web site created by Ulf Runesson, professor of forestry at Lakehead University in Ontario.
So could the beetles actually change Canada’s forests from a “sink” for carbon dioxide into a source?
It may already be happening, according to Jeff Hicke, professor of geography at the University of Idaho. He points to a 1999 study that shows that insect outbreaks, along with large fires in the 1970s and 1980s, changed the carbon dioxide balance in the Canadian forests.
“Immediately following a fire or insect outbreak the trees are dead and they’re decomposing and releasing carbon dioxide to the atmosphere. They’re not growing and photosynthesizing, which is a means of taking up carbon dioxide from the atmosphere,” says Hicke.
However, it is still not clear if the boreal forest will suffer as much damage as British Columbia. Carroll says it is less susceptible because there is greater variety in the type and age of the trees, unlike the large unbroken stands of trees in British Columbia. But he adds that beetles may indirectly kill trees they can’t eat by creating more fuel for forest fires, causing them to burn hotter and more aggressively.
“The dead trees themselves will be available to wildfires, and given the prediction that there will be more fires in the boreal forest with climate change, anything that adds fuel to those fires is going to exacerbate them even further,” explains Carroll.
Both Carroll and Bentz think that there is little that can be done to stop the increasing range of the insect. Since the beetle lives most of its life under the bark, it is difficult to kill with pesticides. Even if they could be treated in this way, the sheer size of the area they cover is too large for spraying to be practical.
At the beginning of an infestation forests can be thinned to prevent the beetles from spreading. In Alberta, there is a concerted effort underway to kill individual infested trees before the beetle can move further into the interior of the country.
There is hope that in some areas, the rising temperatures driving the beetle’s numbers may actually be their undoing. Regions in the northern United States and southern Canada may actually become too warm for the insect. However, Bentz says at that point it becomes an evolutionary race between the trees and the beetle to adapt to warmer weather. Given the beetle’s short life cycle, it might win out.
Related on Scienceline
Check out Kristin Elise Phillips’ story about another insect affected by climate change — butterflies.
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