Forests 'hibernate' during hot weather.

Pine forests "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 scientists from Oregon State University, Corvallis, found that the ponderosa pine forest ecosystem of that area can absorb about five tons of carbon dioxide per acre each year. However, it appears this process, whereby 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," indicates Michael Unsworth, director of the university's 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, 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. "This suggests that the growth and carbonabsorbing potential of the forest would be vulnerable to any climate change that resulted in higher summer...