Trouble for forests of the northern U.S. Rockies?

From Acapulco, Mexico, at a meeting of the American Geophysical Union

Climate change expected to occur in the coming decades may cause forests in northern stretches of the U.S. Rockies to stop absorbing carbon dioxide and even to release some to the atmosphere, exacerbating the planet’s warming.

Trees pull carbon dioxide from the air as they grow. Much of the carbon from that gas is stored in wood and foliage, but some ends up in material littering the forest floor and in the underlying soil. From there, it can make its way back into general circulation, says Céline Boisvenue, an ecologist at the University of Montana in Missoula.

She and her colleague Steven W. Running used computer models to estimate how three climate-change scenarios might affect carbon storage at forest sites in Idaho, western Montana, and northwestern Wyoming.

The good news: By 2089, the growing season in the forests will be at least 3 weeks longer than it was in 1950. The bad news: Over that same period, higher temperatures will cause the trees to suffer water stress—slowing or stopping their growth—for an additional 8 weeks each year. Even under a climate scenario with higher precipitation than at present, trees will have insufficient water for 54 more days each year in 2089 than they did in 1950.

By the year 2020, under a scenario with reduced precipitation, dieback of trees and decomposition of leaf litter at three of the six studied sites will cause the forests to emit more carbon dioxide than they absorb. By the year 2070, the forests at five of those sites will be net producers of carbon, says Boisvenue.