Logging dead trees after a wildfire and planting new ones can make future fires worse, at least for a decade or two while the young trees create a volatile source of fuel, scientists found in a study that contradicts conventional practices.

The findings by scientists from the U.S. Forest Service and Oregon State University and published in this week's Proceedings of the National Academy of Sciences journal raise questions about the long-standing practice of salvage logging on national forests.

The study, the first of its kind, comes at a time when global warming is expected to increase the size and numbers of wildfires and the annual cost of fighting them is running around $1 billion.

Scientists examined satellite images, aerial photographs and records of logging and replanting to look at areas that burned in a 1987 fire in southwestern Oregon and again in a 2002 fire.

The 2002 fire, the largest in the nation that year, has been a battleground between environmentalists and the Bush administration over harvesting trees after fire. Only 5 percent of the area burned was logged afterward.

"It was the conventional wisdom that salvage logging and planting could reduce the risk of high-severity fires," said Jonathan R. Thompson, a doctoral candidate at Oregon State and the study's lead author. "Our data suggest otherwise."

The large stands of closely packed young trees created by replanting are a much more volatile source of fuel for decades to come than the large dead trees that are cut down and hauled away in salvage logging operations, the authors found.

"This isn't the full story," added Thomas Spies, a research forester with the U.S. Forest Service Pacific Northwest Research Station, who took part in the study. "The story could change if you look at a different place at a different time," particularly after young trees have grown larger and been thinned.

Agriculture Undersecretary Mark Rey, who oversees the Forest Service, noted that every decision on whether to salvage logs is specific to the wildfire site and based on a combination of economic and ecological factors.

He added that the study's findings indicated that on national forests that burn frequently, it would be a good idea to plant young trees farther apart and keep the lower branches pruned to reduce fire danger — something the Forest Service is starting to do.

Greg Aplet, staff scientist for The Wilderness Society, said a recent review of scientific evidence showed that economics — the value of the timber logged and the jobs that go along with it — is the only real benefit of salvage logging.

"There is no fuel reduction benefit. There is no ecological benefit to salvage logging," he said from Denver.

Recent studies suggest that as the climate warms and drought persists across the West, wildfires will become more common. Even the most severely burned forests will sprout plentiful seedlings on their own.