Science News
Week of June 16, 2007; Vol. 171, No. 24 , p. 382

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

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.

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