Some of those prehistoric droughts in the northern Great Plains of what is now the United States also lasted longer than modern-day dry spells such as the 1930's Dust Bowl decade, according to sediment core studies by the team.
The group's evidence implies these ancient droughts persisted for up to several decades each. At their heights, prairie fires became uncommon because there was too little vegetation left to burn. The ages of charcoal deposits suggest instead that prairie fires occurred during intervening wet periods, with each wet-dry cycle lasting more than a century each.
A report on the research will be delivered at a session at 1:30 p.m. on Wednesday, Aug. 4, in Meeting Room D136 of the Oregon Convention Center during the Ecological Society of America's 2004 annual meeting in Portland.
"We were looking for the effects of past climate changes on ecosystems," said Jim Clark, H.L. Blomquist Professor of Biology at Duke's Nicholas School of the Environment and Earth Sciences. But when Clark and his colleagues began examining evidence from the mid-Holocene period of 5,000 to 8,000 years ago in parts of the Dakotas, Montana and western Minnesota, "nothing seemed to make any sense."
"The question was: Could we look at the sediments for charcoal evidence of the amount of fire, for pollen evidence of the kinds of grasses that were growing then, for sediment chemistry to show how much erosion was going on, and be able to deduce climate changes -- or the lack of them -- under way at the time?" Clark said.