Differences in forest composition following two periods of settlement by pre-Columbian Native Americans

Temperate broadleaf forests in eastern North America are diverse ecosystems whose vegetation composition has shifted over the last several millennia in response to climatic and human drivers. Yet, detailed records of long-term changes in vegetation composition and diversity in response to known periods of human activity, particularly multiple distinct periods of human activity at the same site, are still relatively sparse. In this study, we examine a sediment record from Avery Lake, Illinois, USA, using multiple metrics derived from pollen data to infer vegetation composition and diversity over the last 3,000 years. This 3,000-year history encompasses the Baumer (300 bce–300 ce) and Mississippian settlements (1150–1450 ce) at Kincaid Mounds (adjacent to Avery Lake), and captures differences in the impact that these groups had on vegetation composition. Both groups actively cleared the local landscape for settlement and horticultural/agricultural purposes. Given the persistence of fire-tolerant Quercus in conjunction with declines in other tree taxa, this clearing likely occurred through the use of fire. We also apply a self-organized mapping technique to the multivariate pollen assemblages to identify similarities and differences in vegetation composition across time. Those results suggest that the vegetation surrounding Avery Lake was compositionally similar before and after the Baumer settlement, but compositionally different after the Mississippian settlement. The end of the Mississippian settlement occurred simultaneously with a regional shift in moisture characterized by drier summers and wetter winters associated with the Little Ice Age (1250–1850 ce), which likely prevented this ecosystem from returning to its pre-Mississippian composition.