Global warming could yield drier conditions on the northern Great Plains, triggering drastic ecological changes during the next century, a new study shows.

Botanists at Duke University who conducted the study declined to specify exactly what might be in store for one of the nation's productive grain and cattle regions, which also is a magnet for wildlife, especially migrating birds.But they said warmer, drier conditions probably would result in grasslands spreading east into areas that now are woodlands, with a corresponding increase in wildfires.

"What's important is that the sensitivity is there to global warming," Duke botanist James Clark said. "This system is really responsive, with the grasslands expanding eastward into forests and an increase in burning of this prairie."

Clark presented his findings Tuesday to the Ecological Society of America meeting in Baltimore. The convention runs through Thurs-day.

Other grasslands researchers said they generally agree with Clark's scenario but questioned whether ecological changes would occur in the order he described.

Forests that were established during moist periods can endure for centuries even when the climate turns drier.

"The next fire that comes along to take out the forests is what will allow the grasslands to expand," University of Colorado ecologist Tim Seastedt said.

The Duke study - which examined peat sediments, fossil pollen and charcoal deposits from ancient wildfires - encompassed an area that includes eastern Montana, North Dakota, South Dakota, Nebraska, Minnesota and Wisconsin. The region has flip-flopped between grasslands and forests during the past 8,000 years, depending on climatic changes.

Clark said the past 2,000 years have been relatively cool and wet, conditions favorable to woodlands.

That is, until the 20th century - the warmest in recent history, with the past several years being among the hottest on record.

Many researchers believe the rising temperatures are being driven to some extent by heat-trapping air pollution and other byproducts of human activities. Subtle atmospheric circulation shifts caused by global warming and other factors could favor drier weather, Clark said.

If the trend continues, today's woodlands on the fringes of the Plains could recede to a point where sufficient moisture is available - perhaps hundreds of miles to the east.

"We've seen this region getting cooler and moister until this century," Clark said. "We have seen a trajectory of cooler climate for over 4,000 years and there is good reason to believe it won't continue."

Other ecologists said the human-driven changes to the region's ecology already may be occurring.