BLUE RIVER, Ore. — Ambrosia beetles have a nose for alcohol. It's what attracts them to downed Douglas firs, which give off ethanol — ethyl alcohol — on the forest floor.

The beetles are among the first insects to move into dead trees, and they bring along a host of other invading critters.

Think of them as veritable taxicabs, said Mark Harmon, an Oregon State University professor of forest science.

"A Douglas fir, when it dies, hosts thousands of species," said Harmon, who is involved in a long-term study of rotting trees at the Andrews Experimental Forest, a 16,000-acre tract in the Willamette National Forest about 50 miles east of Eugene.

Researchers are wrapping up the first 20 years of the project but are only a tenth of the way done. They plan to track the decay process in 530 downed logs for 180 more years.

Two centuries of rot might seem a bit extreme, but the timeline reflects the speed at which things unfold in forests.

An old-growth stand near Lookout Creek demonstrates his point. There, giant Douglas fir, hemlock and cedar tower over lacy vine maple, ferns and Pacific dogwood. In the duff below, a dozen logs of various diameters slowly decay. Many of the living trees in the grove have been around for centuries, and some of the skeletons will linger that long, too.

But not all of them will, Harmon said, and that's one of the interesting things discovered in the first 20 years of the project.

Tree rot progresses at unique rates, depending on the species, the various fungi that move in, the moisture content, the geographic location, even the size of the tree.

The results run contrary to expectations, he said. Foresters had assumed wood decomposed at a fairly steady rate. The new information has implications for forest management, he said.

There were other surprises for researchers, who believed it would take years before wood nutrients such as nitrogen began enriching the forest. In fact, fungi invade a log and, with the leaching effects of rain, quickly release nutrients.

Brown-rot fungi attack the cellulose in a tree, Harmon said, leaving lignin, a structural material that helps build soil. White-rot fungi, on the other hand, break down the lignin, leaving little structure behind.

There's a practical side to such knowledge. Nature isn't the only thing interested in breaking down a log's cellular structure. Humans do it, too, turning wood into paper.

Pulp mills use chemicals to break down lignin in a process that can produce pollutants such as dioxin, Harmon said. Finding a fungus that does the same thing without harmful chemical byproducts might prove useful, he said.

When the research first began in the mid-1980s, most logging operations cleared out a site, taking not only the trees but the woody debris as well. It was a practice common in North America and Europe.

When the research began, loggers hired to help place the downed trees at Andrews demanded an explanation for leaving the wood to rot, Harmon said. Twenty years later, it's an accepted forest practice, and Harmon, only half kidding, has coined a term for exploring the role of dead trees in forest health. He calls it morticulture.

Growing trees and allowing them to die and remain in the forest serves both aesthetic and pragmatic purposes, he said.

Aesthetically, people like a diverse forest, where species such as bluebirds and woodpeckers, which require dead trees, can thrive.

Pragmatically, there's no telling what researchers will find as they study the thriving habitat that dead trees become. Maybe it's a fungus that can help transform the paper-making industry. Maybe it's a substance that could become the basis for a useful medicine, such as taxol, a cancer-fighting compound found in the 1980s in the Pacific yew.

The study is funded by the National Science Foundation and the Forest Service. The researchers will try to establish just how many trees need to be left for the overall health of the forest.

"That's our next big area of research," he said.