Forests soak up carbon, but for how long?

MISSOULA, Mont. - U.S. forests offset roughly 11 percent of the nation's industrial greenhouse gas emissions, storing "significant amounts" of carbon that would otherwise pollute the atmosphere, according to new government data.

We're aridifying our forests.  - Steven Running University of Montana

The findings, released last week, estimate the nation's expanding forests sequester an additional 192 million metric tons of carbon annually due to increases in both the total area of forest land and the amount of carbon stored per acre.

That's the equivalent of removing about half the cars on the roads nationwide, or almost 135 million vehicles.

But as emissions increase and the planet warms, that storage capacity could be compromised, scientists warn.

Warmer summers, changing precipitation patterns and a thinning snow pack are already "aridifying" Western forests, University of Montana Professor Steven Running said during a conference here last week. The combination imperils the health of vast swathes of the western landscape, he warned.

"We think of range as having a 'carrying capacity' - you put too many cows on a pasture and they all get skinny because they don't have enough to eat," he said at a meeting of the Society of Environmental Journalists. "It's the same principle for our forests."

In 40 years, for instance, Montana will likely see 5ºF warmer summers on average but receive 10 percent less rainfall, he said. "We're aridifying our forests."

But for now, the trend in carbon sequestration across the nation is up, according to the U.S. Forest Service data. On average, the agency said, the amount of carbon stored in forestland has increased since 1990.

Wet, temperate conifer forests along the Pacific Coast from Alaska to California store the most carbon - about 93 metric tons of carbon per acre. Arid pinyon-juniper forests of the Southwest store the least - 31 tons of carbon per acre on average.

Forests in the western United States store a greater proportion of carbon in the trees; other areas, such as the Great Lakes, have larger concentrations of peat in the soil, storing more carbon there.

Understanding those differences - and corresponding climate-induced changes - could have important ramifications as the agency assesses the carbon sequestration potential of the nation's forests, agency officials said.

"Forest management on all lands can contribute significantly toward cooling a warming planet," U.S. Agriculture Secretary Tom Vilsack said in a statement. "This new information will assist the public and policy makers as we work to address this significant issue."