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CORVALLIS, Ore. - The sagebrush lands of the Great Basin, one of the largest ecosystems in the United States, may be reduced to a fraction of their current area due to ecological changes already under way and climate shifts that will hasten their demise, a new study suggests.

This vast, semi-arid region, dominated by frost-tolerant sagebrush and native grasses, is already suffering impacts from invasive species, fire suppression and the encroachment of other woody vegetation. The future will also bring increases in temperature that may allow frost-sensitive species from the Southwest to move hundreds of miles north and further displace the sagebrush, scientists say.

These findings were presented recently at a professional meeting and are being published in the Transactions of the North American Wildlife and Natural Resources Conference by researchers from Oregon State University and the U.S.D.A. Forest Service.

The hottest climate scenario would reduce sagebrush to about 20 percent of its current area in the Great Basin, a fairly rapid change in hundreds of thousands of square miles of the American West. Increases in woody vegetation and fire are predicted. Only a few small areas of sagebrush in southern Wyoming, the northern edge of the Snake River plateau, and small areas of Washington, Oregon and Nevada are expected to survive and persist under all scenarios, researchers say.

"Increases in temperature due to global warming will be the driving force in these changes, along with less-predictable changes in the summer rainfall regime," said Ronald Neilson, a professor of botany at OSU and ecologist with the Forest Service. "A major change will be that as the climate warms, woody vegetation now confined by cold temperatures to the Southwest may move into the higher plateaus of the Great Basin.

"Given the flat nature of much of this terrain, once the woody vegetation gets up and over the 2,000-foot elevation, it will be like opening the floodgates," Neilson said.

Changes in precipitation are most difficult to predict in future climate scenarios, said Neilson and Dominique Bachelet, an OSU associate professor of bioengineering. Earlier work by these researchers suggested both a decrease in frosts and increases in precipitation over much of the interior West, triggering a dramatic increase in wood expansion at the expense of sagebrush shrub land, and a corresponding increase in fire due to the increased fuel load.

More problems with fire in the wild land-urban interface are also probable, the researchers said. The amount of fire suppression conducted by land managers is an unknown variable that will affect total vegetation growth, and could result in a dramatic increase of the overall biomass of these regions in future years, Bachelet said.

"What's most certain is the rising temperature, which is going to allow a lot more oak, mesquite and invading grasses into new areas," Neilson said. "Precipitation is harder to predict and may be quite variable, due to inter-decadal climate patterns that appear to be getting even more volatile and intense. We could see some decade-long periods of drought during what should be a period of overall higher precipitation."

There may actually be more plant and animal diversity under the new scenario than the sagebrush ecosystems of the past, the study indicated. And the increased amounts of vegetation in the Great Basin, inadvertently, might support a U.S. policy of increased carbon sequestration in ecosystems. But as huge areas of the American West face these changes in their ecology, some existing sagebrush ecosystem species may also go extinct.

Seven different climate scenarios were considered in this study, and it's uncertain exactly which one will prevail. As models continue to get more refined they seem to be trending toward the hotter climate scenarios, Neilson said. And the rate of ecological change may be so rapid that the early winners will be invasive weeds that can travel easily and adapt to a wider range of conditions, he said.

The existing sagebrush biome of the western U.S. is one of the two or three largest ecosystems in the nation, comparable to the Great Plains and the eastern deciduous forest. The system tends to be very hot in the summer and subject to recurring hard frosts in the winter - a climate to which hardy sagebrush with its deep roots is particularly suited. Sagebrush and the species associated with it - such as sage grouse, sage thrashers and pygmy rabbits - dominate large areas of Nevada, Oregon, Washington, Idaho, Utah, Wyoming, Montana and other western states.

These ecosystems have already undergone intense changes since European settlement, with some estimates that intensive agriculture, grazing pressure and other impacts have reduced the sagebrush lands to less than half of their original size.

"Aside from the findings of this study in particular, one thing I find most striking is the overarching impact of humans," Bachelet said. "We've brought fire suppression, air and water pollution, we've introduced competitive exotic species, and we are responsible for the incessant expansion of agricultural and urban areas. This has created huge changes for natural systems to adapt to, and may be the ultimate cause of mass extinctions."