The frequency and intensity of forest fires are related to climate and may roughly occur in cycles that can be plotted to give an idea of how forests develop, a scientist said Wednesday.

Intense forest fires primarily occur during patterns of warm, dry weather and might portend greater fire danger if the greenhouse effect, a global warming trend, occurs as predicted, said James S. Clark, a University of Minnesota ecologist."We know that one of the things that influences fire is weather," said Clark, who studied the patterns of forest fires dating back 750 years in what is now a Minnesota state park.

By examining deposits of charcoal in a lake catchment basin, he has been able to chart fire frequency in the Itasca State Park area, showing that fire scars on ancient trees help reveal the frequency of fires over hundreds of years.

His study, reported in the British journal Nature, shows that during the so-called Little Ice Age, between 1600 and 1900, intense forest fires averaged one about every 13 years.

During the warmer 15th and 16th centuries, "fires were averaging one every eight years, and that was an important difference for trees in living in this area," he said.

Climate along with an increase in fuels - the dry brush and grasses that add to a fire's intensity - work together in cycles to enhance fire danger, he said.

"In times of very intense fire the reasons seem to be related to the buildup of fuels. That is, if the climate changed a bit, that change mattered because fire frequency responds to climate and fuel."

"As fuels build up, the probablity of fire increases," he said.

Clark said his discovery of fire cycles in Minnesota cannot be extrapolated to other areas because conditions of climate and fuel differ from area to area. But, he said, his study suggests similar fire cycles can be plotted for other areas.

As for the potential of increased fire danger worldwide with the predicted global warming trend, Clark said it would be difficult to pinpoint which areas would be most vulnerable.

He said scientists still are uncertain which regions of the world would become warmest and for how long a period each year.

"It's safe to say some areas would be affected," he said. "It's difficult to say how much and which particular areas because we don't know which areas will be warming or how the global warming will affect the air masses."