Climate influence on forest fires.

Climate influence on forest fires 

Through the commanding figure of Smokey the Bear, millions of visitors to U.S. forests have learned that during dry times such as this summer, fire danger increases in the woods. But while forest managers understand how daily weather can raise or lower the chances of fire, scientists have little information on how long-term shifts in climate affect wildfire frequency. Now, using evidence of ancient forest burns, an ecologist has traced the year-by-year relationship between climate and fire in northwestern Minnesota for the past 750 years.

Results of this study, published in the July 21 NATURE, "establish the fact that the climate changes over the last few centuries have been accompanied by changes in the fire regime," says James S. Clark, now at the New York State Museum in Albany, who did the research while at the University of Minnesota. "It suggests that the influence of climate is very strong."

Clark found that during the relatively warm, dry 15th and 16th centuries, the forest in the study area burned roughly once every nine years. Over the next three centuries, during a cool period known as the "Little Ice Age," blazes were less frequent and less intense, occurring every 14 years. In light of predictions concerning the "greenhouse" warming of the climate (SN: 7/2/88, p.4), says Clark, "fires will presumably become more frequent in the future."

Previously, scientists had studied only the general relationship between fire and climate; they had not been able to resolve the frequency of fires occurring before human recordkeeping began. Clark constructed his chronology by measuring the amount of charcoal in sediments at the bottom of a lake in Itasca State Park. The lake he studied sets down an annual sediment layer, making it possible to trace fires to individual years and to identify cycles in the fire history.

This kind of information will help those who supervise forest lands, says Harvard ecologist David Foster. "If you're really going to manage an area, you have to have some idea of what the long-term frequency of fire was," he says.

Traditionally, people have attempted to suppress wildfires, but this practice often allows wood to accumulate on the forest floor, forming fuel for some future blaze, says Clark. "With continued fire suppression and the further [climate] warming, fuel buildup will result in more intense and/or more frequent fire," he says. In the last few decades, in areas such as Yellowstone National Park, forest managers have experimented with controlled burns, designed to emulate a more natural system.