Published online 23 April 2008 | Nature | doi:10.1038/news.2008.771

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Beetle tree kill releases more carbon than fires

Canada’s beetle infestation is turning some forests into carbon sources.

Beetle infestations have turned large chunks of British Columbia's forests red.Natural Resources Canada

An infestation of beetles could convert large swathes of Canadian forests to carbon sources rather than sinks, a new study suggests.

Fuelled by warmer winter weather, mountain pine beetles have swept through the conifers of British Columbia, killing trees in an estimated 74,000 to 94,000 square kilometres of forest.

“Those are very big numbers brought on by a very small insect,” says Werner Kurz of Natural Resources Canada in Victoria, British Columbia, who led the study. The infestation has had a huge impact on the logging industry and ecology of the region, and has turned vast areas of forest from green to reddish brown.

To gauge the effect of the spreading blight on the carbon cycle of the region, Kurz and his colleagues modelled the carbon ‘budget’ for 374,000 square kilometres of pine forest between the Canadian Rockies and the coastal ranges. They estimate that 270 megatonnes of carbon will vanish from the region between 2000 and 2020 as beetles kill the trees, they report in Nature1.

Some of this carbon disappears from the forest but stays locked up in timber, as trees are logged in an attempt to salvage the value of the wood. But much more is lost to the atmosphere as climate-warming carbon dioxide as the dead trees rot.

It has previously been proposed that increased carbon dioxide levels, warmer weather, and nitrogen pollution could increase the amount of carbon forests store by boosting growth, but Kurz says the negative impacts must be considered in climate models as well. "We shouldn't be accounting for the beneficial impacts and ignoring the detrimental impacts that are occurring as a result of similar processes."

Worse than forest fires

In the team’s model, a pine forest untouched by beetles but with a normal amount of logging is a slight carbon sink, sucking up more carbon (as carbon dioxide) than it loses (either as carbon dioxide or as timber). The only exception to this is when forest fires convert the forest to a net source, as they did in 2003. The beetles have an even bigger effect — in their worst year releasing 50% more carbon than the 2003 fires — and act over longer time scales, with additional logging making things even worse.

The planting and natural regrowth of new trees means that carbon loss abates as the infestation wanes. But in the 20 years of the model there wasn't sufficient regrowth of the forest to make up the loss.

"It certainly is a good initial sketch of what we might expect," says Art Fredeen, who studies the effects of forest management and carbon balance at the University of Northern British Columbia in Prince George, but was not involved in this study.

Salvage situation

Loggers have rushed to salvage wood from devastated forests as part of the province’s policy to try to mitigate the cost of losing this valuable resource. But not all the wood is in perfect condition for timber: the beetles carry a blue fungus, which discolours the wood, lowering its value. Kurz and others have suggested the dying trees be burned for energy, which would help retain the wood's value but contribute more to carbon dioxide emissions.

Fredeen thinks that salvage logging might also contribute to increased carbon dioxide emissions, as it can disturb the forest-floor plant life. “You have all of the shrubs, the moss, lichen; you have a lot of photosynthetic surface that's unperturbed by the mountain pine beetle,” he says. “When you clear cut, of course, all of that is removed.” Carbon dioxide monitoring stations above infested areas have, in their first year, shown a six-fold increase in carbon released above logged plots compared to infested areas left to decay, Fredeen says.

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Although the beetles are native to the region, their range has spread northward and to higher elevations with milder winters. Temperatures below -40 °C several nights in a row will kill the larvae, but such cold snaps have become rare in recent years.

Even if climate change brings further warm winters to the region, however, experts think this infestation has probably peaked. Mountain pine beetles can only reproduce in the largest trees, which were abundant thanks to a growth spurt after wildfires raged across western North America 80 to 140 years ago. Soon 80 to 90% of those large trees will be gone, Kurz says. “The beetle will eat itself out of house and home, and the population will eventually collapse.” 

  • References

    1. Kurz, W. A. et al. Nature, 452, 987-990 (2008). | Article | ChemPort |
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