Beetle attack overturns forest carbon regime

The largest outbreak of mountain pine beetles on record is turning a forest in British Columbia from part of the solution into part of the problem in the fight against greenhouse gases.

Climate modelers typically count the great boreal forests that stretch across Canada and Russia as friendly assets, helping to take up and store a bit of the excess carbon dioxide that human activity releases into the atmosphere.

Not so anymore for a section of Canada’s forest in south-central British Columbia, says forest ecologist Werner Kurz of Natural Resources Canada’s Pacific Forestry Centre in Victoria. Records plus a computer model show that beetle damage will probably make the region a net source of carbon at least until 2020, he and his colleagues report in the April 24 Nature.

“What is unique and new is that we have been able to improve the model and the data so that we can run the model with, and without, the beetle,” Kurz says. This refinement marks the first time greenhouse-gas bookkeeping has weighed the effects of an insect outbreak, he says.

When infested trees die, the forest takes up less carbon dioxide. Also, the wood starts decaying and the dead trees themselves release the carbon they once stored. Carbon pal becomes carbon problem.

The computer model finds a big carbon footprint from the beetles, comparable to about a quarter of the emissions from Canada’s transportation sector per year. The researchers estimate that the insect infestation may lead to the release of a total of 270 million metric tons of carbon between 2000 and 2020.

Such upsets in forests need attention as scientists try to understand climate change, says forest ecosystem ecologist Tom Gower of the University of Wisconsin–Madison. Previous generations of climate models treated a forest as a “closed box with the climate acting on it,” he says. In the real world, though, wildfires blaze through and insects attack, sometimes with enough of a wallop to shift the bottom line of carbon accounting. “Don’t forget disturbance — it’s important,” he says.

Ironically, the beetle disturbance is itself fueled by climate change. The mountain pine beetles (Dendroctonus ponderosae), each about the size of a rice grain, are native North Americans. The beetle larvae spend winters under the bark of mature pines. Every once in a while, beetle populations boom until they “eat themselves out of house and home,” says Kurz. Warming temperatures allowed them to expand their range northward and higher up mountain slopes, and warm winters failed to blast them with larva-killing cold snaps of -40° Celsius. So starting in 2000, the beetle populations began rising explosively in British Columbia. They’ve infested an area about the size of Alabama. “We’ve never seen an outbreak like this before,” says Kurz.