Holocene history of prairie in midwestern United States: Pollen versus plant macrofossils

The lack of abundant pollen of unequivocal prairie taxa may lead to difficulties in recognizing prairie from the fossil record. Ambrosia, Artemisia, Poaceae, Chenopodiaceae-Amaranthaceae, other Asteraceae and Cyperaceae are taxa that define prairie in the pollen record. A transect of four sites from western Iowa to eastern Illinois shows that where only one or two of these taxa reach high pollen percentages, or where all show modest increases, interpretation is difficult. These are widespread taxa that are not limited to prairie, and even where all are abundant in the pollen record, they provide no information on the paleoecology or biogeographic history of modern prairie taxa. Plant macrofossils from sediment exposed in stream cutbanks provide a way of refining prairie reconstructions. These sediments are deposited in point bars or behind beaver dams and have yielded a rich Holocene record of prairie species. These sites are common well beyond the Wisconsinan glacial border, where lakes and wetlands are rare. Preliminary studies of modern analogs along prairie streams in western Iowa indicate that the richest deposits accumulate after prairie fires, and that representation of both upland and lowland vegetation is good.