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CORVALLIS, Ore. - Researchers at Oregon State University have found that pine forests almost "hibernate" during hot, dry weather, losing their normal ability to sequester carbon dioxide and mitigate the greenhouse effect.

Based on research done near Sisters, Ore., as part of NASA's Terrestrial Ecology Program, a group of OSU scientists found that the ponderosa pine forest ecosystem of that area can absorb about five tons of carbon dioxide per acre each year.

But it appears this process, in which each day the trees absorb a little more carbon dioxide during the daytime than they give off at night, practically shuts down on hot, dry days.

"What we observed is a type of feedback loop, a defensive mechanism in which the trees close down their stomatal pores on hot days to avoid excess water loss," said Michael Unsworth, director of the OSU Center for the Analysis of Environmental Change. "This is a carefully-tuned response that the trees have evolved to help them survive.

"In an ecological sense," he added, "the forest basically holds its breath."

This process reduces to almost zero any absorption of carbon dioxide during hot weather in this particular type of forest, Unsworth said.

"This suggests that the growth and carbon-absorbing potential of the forest would be vulnerable to any climate change that resulted in higher summer temperatures and lower, or even unchanged, rainfall," Unsworth said.

To further compound the concern, Unsworth said, when the trees cut down their water loss, the local "microclimate" also gets hotter and drier. And a hotter, drier, local climate tends to further retard the normal carbon-absorbing potential of the forest.

These studies were done in the Metolius Research Natural Area in the Deschutes National Forest of the central Oregon Cascade Range. This forest is a mix of old-growth trees more than 250 years old, and younger regenerating trees about 40 years old.

In work done since 1995, OSU researchers have used sensors mounted on a 150-foot tower to detect the amounts of carbon dioxide absorbed on an hourly basis, throughout a growing season. It is the only study of its type operating in U.S. forests west of the Rocky Mountains, Unsworth said.

The research found that this type of ponderosa pine forest, similar to those occupying large areas in the western United States, can under current climate conditions play an important part in absorbing atmospheric carbon dioxide. This type of tree species and forest ecosystem, which is often found in drier climates, also directs much of a tree's growth into its root system, where carbon is even more effectively sequestered, Unsworth said.

"We found that this type of pine forest absorbed about 60 percent of the carbon dioxide into the root structure, compared to faster-growing forests west of the Cascade Range where only about 20 percent of the carbon dioxide goes below ground," Unsworth said.

But the recent studies show its less clear how such forests will grow, thrive and sequester carbon dioxide under the potentially hotter or drier climate conditions of the future, he said.

Some light may be shed on those issues, he said, if this area of Oregon gets a low snowpack this winter - as might be possible under El Nino conditions - and the trees face more drought stress than usual during their next growing season.

Considerable interest has been expressed in recent years in using forest ecosystems to sequester some of the carbon dioxide in the atmosphere that helps to create global warming, Unsworth said..

Various initiatives are under way to manage forests for this purpose or develop new forests in some areas, and studies such as this may help to determine their potential effectiveness, even in drier parts of the West.