Comparison of fire history, vegetation history, and cultural change during the past 2000 years to inform management at Effigy Mounds National Monument, Iowa

Effigy Mounds National Monument (EFMO) protects effigy mounds built by prehistoric people 1400 to 1000 calendar years BP.   Changing attitudes about cultural resource interpretation and the increasing popularity of savanna restoration expanded the park’s mandate to encompass restoration of the park’s landscape toward conditions at the time when the mounds were built.  The monument's resource managers funded pollen and charcoal investigations to help construct a narrative describing the interplay of vegetation, fire, and people on the landscape during the past 2000 years.

We processed charcoal and pollen samples from sediment cores taken from EFMO, counted charcoal at 125- and 250-micron size classes, and compiled characteristics of prehistoric cultures from archaeological literature. Six AMS dates provided an age model for the cores.

Between 2200 and 1600 calendar years BP, Early and Middle Woodland people flourished in the region.  Abundant oak and grass pollen suggest oak savanna vegetation. Charcoal accumulation rates were high, with peaks representing periods of fire activity separated by intervals of 30 to 110 years.  About 1600 calendar years BP, we noted dramatic shifts in culture, pollen, and charcoal.  Middle Woodland culture faded and Late Woodland culture appeared.  Grass pollen declined as sedge and oak pollen increased, suggesting wetter lowlands and more forested uplands.   Average charcoal accumulation rates dropped and remained low thereafter.  Four 100-year periods showed more frequent peaks in charcoal accumulation, and peaks were separated by as little as 15 years.  At least a century of low charcoal accumulation separated these high-frequency periods.