Bugged Forests Bad for Climate

Trees savaged by pine beetles slow to recover function as greenhouse gas sponges

Northern pines are under siege from bark beetles, with some infested stands rapidly losing at least 50 to 80 percent of mature individuals. Over the next couple of decades, the decomposition of those trees is expected to release large amounts of carbon dioxide, a greenhouse gas, posing a potent risk to climate. A new analysis now identifies additional climate repercussions of the severe beetle outbreaks.

Chief among the problems: Even after a forest has ostensibly recovered from a beetle outbreak, it continues to suffer a long-term drop in the rate at which growing trees remove carbon dioxide from the air. That makes the forest less efficient at locking up the carbon dioxide emitted by fossil fuel burning, according to a new analysis by Eric Pfeifer, Jeffrey Hicke and Arjan Meddens at the University of Idaho in Moscow.

A forest may regain its previous biomass in seven to 25 years, but the rate at which it takes up carbon dioxide will remain diminished for much longer—in some cases well over a century, the researchers report in the January Global Change Biology.

All forested lands don’t suffer equally from beetle invasions. Those with the highest proportion of old pines, which are the most vulnerable to attack, initially take the biggest hit. However, young survivors suddenly get more light and have access to more of the available nutrients. This should propel a spurt in their growth—and carbon storage.

In fact, because trees grow more slowly as they age, those stands where beetle victims had been oldest and the survivors youngest will pose the most limited risk to climate, the study finds.

Pfeifer’s group measured and assayed the health of every tree more than 7 centimeters in diameter within a series of 400-square-meter plots. They worked in a central Idaho forest dominated by lodgepole pine that for a decade had suffered a moderate to severe outbreak of mountain pine beetles.

Data on each tree was fed into a computer program that projects tree growth—and therefore carbon storage—based on such known or measured features as weather, light, tree ages and nearby tree canopies.

Under most of the conditions that the Idaho foresters simulated, carbon stores in beetle-hammered stands would return to pre-outbreak levels within 25 years. In some cases, a full restocking of a stand’s carbon might take as few as seven years, the researchers report.

“They make a good point,” says Barbara Bentz of the U.S. Forest Service’s Rocky Mountain Research Station in Logan, Utah. Because live trees remain in forests hammered by mountain pine beetle, she says, recovery of their carbon stocks should be quicker than following forest fires or clear-cut logging.

“But you can define recovery in several ways,” notes Pfeifer, now with the Forest Service in Challis, Idaho. For instance, “although the actual pool of carbon accumulated within that forest following the outbreak recovers, the rate at which it accumulates carbon never returns to what it had been before the outbreak,” he reports.

To establish that, his team compared the amount of carbon incorporated into the tissues of recovered stands with that in stands that had sustained no beetle mortality. They calculated that the annual carbon uptake rate would have diminished substantially after beetle outbreaks—by at least 12 percent and in some cases by as much as of 51 percent.

Think of a stand’s carbon uptake rate as the interest paid on a bank account, which compounds over time. The seven- to 25-year recovery of the initial carbon stores represents a loss of any interest for that period, Pfeifer says. Following this catch-up, when interest is again paid, it’s now at a lower rate than in forests that hadn’t been hammered by beetles.

The new data project that it would take affected stands 56 to 185 years to catch up to the carbon bank balance that had accumulated in stands that did not experience a beetle outbreak. Carbon stores in four of the modeled plots would never catch up.

By discriminating climate impacts of beetles on the basis of tree ages and the diversity of species within beetle-outbreak regions, “this is indeed a very nice and novel study,” says Werner Kurz of Natural Resources Canada’s Pacific Forestry Center in Victoria, British Columbia. It also confirms, he says, the importance of accounting for these factors when projecting the carbon-storage value of trees in helping to moderate climate.