In British Columbia, a beetle upsets climate calculus

Nor is the challenge limited to North America's boreal forests, adds Olga Krankina, a forest ecologist at Oregon State University in Corvallis. Russia has had severe outbreaks of Siberian gypsy moths, which attack foliage.

Historically, the outbreaks have run in cycles, notes Allan Carroll, an entomologist with Natural Resources Canada who was part of the modeling project. The results appear in this week's issue of the journal Nature.

Western North America, where the mountain pine beetles occur naturally, has seen four or five outbursts in the past 100 years. But the current episode represents a perfect storm for British Columbia's forests. Wildfire suppression over the past 60 to 70 years has yielded a forest populated by large numbers of pines 80 to 160 years old – the beetles' habitat of choice. Milder winters and hot, drier summers have helped the beetles thrive.

The exploding bug population found "a real smorgasbord" to support it, Dr. Carroll says.

Spreading east

Those climate factors have also permitted the beetles to move east through mountain passes once too cold for them to clear. And in the summers, significant numbers of the flying beetles have gotten caught up in thunderstorms, which deposit them on the Rockies' eastern slopes.

This migration comes at a time when some 1,500 scientists from around the world are imploring the Canadian government to preserve large swaths of the country's boreal forests. Last May, they sent a letter to the Canadian government asking it to set aside at least half of the forest's 2.3-million-square-mile expanse and keep it free of industrial development, the balance to be managed sustainably.

The bugs' eastward march is likely to be slow, Carroll suggests. And it could further be slowed by the mix of trees east of the Rockies. Stands of jack pine, which the beetles can infest, exist in isolated patches in areas where they've crested the mountains. "The jack pines are not as contiguous as other species, so that would be a barrier," he says.

But control efforts would likely still be needed, he adds. That would mean flying crews into the infested area, cutting down dead and infested trees, and burning them on the spot. The alternative, he says, is watching a "disturbance agent move into a forest that hasn't seen it before ... which could increase the fire risk in the future."

And forest managers are coming around to the idea that it may not be wise to salvage harvest in every circumstance, Dr. Kurz adds. The forest's other benefits, from biodiversity to water and soil conservation, may be better retained from leaving dead trees where they are, especially if they stand along rivers and streams, deep in forests where building access roads would do more ecological harm than the deadfall, or along slopes, where removing trees would lead to serious erosion problems.

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