GRAINS OF TRUTH: IOWA'S PLANT HISTORY PRESERVED IN POLLEN

Key Terms

Hypsithermal: A period in geological history spanning approximately 9,000 to 5,000 years before present.

Radiocarbon Dating: A technique that utilizes the minute amounts of radioactive carbon in organic matter to determine an approximate age for the matter in question.

Pinus: The genus encompassing pine trees

Abies: The genus encompassing fir trees

Macrofossil: A fossil that can be observed without a microscope

            Newcomers to Iowa might see nothing but endless rows of corn. But beneath the state’s surface landscape lies evidence of a diverse and fascinating floral history. Ancient plant material buried below the row-crops reveals a narrative of evolution and competition amidst geological and environmental changes. Studying the plant communities of Iowa’s past can provide us with clues on the history of climate change, glacier activity, and humans’ effect on the landscape.

DIGGING, DATING AND IDENTIFICATION

            The pollen left in soil by plants over the course of time can be identified to the species level to provide a picture of historical species presence and abundance. To study pollen history, scientists often take cores of sediments. In his 1979 study, “Late Glacial and Postglacial Pollen and Plant Macrofossils from Lake West Okoboji, Northwestern Iowa,” Ken Van Zant took a soil core reaching approximately 12m below the surface of Lake West Okoboji, Dickinson County, IA. The site “was selected as a coring site because of the reported presence of late glacial sediments and the reported ability to penetrate the Hypsithermal sediments” (Van Zant 1979). After pulling the core, Van Zant divided the soil into segments for radiocarbon dating, which allowed approximate time periods to be assigned to points along the core’s vertical axis. He then sifted through the sediment for plant macrofossils and pollen samples, which he identified using pre-established keys.

THE PROOF IS IN THE POLLEN

            By examining changes in the composition of the fossil and pollen records over time and correlating them with data from radiocarbon dating, Van Zant was able to reconstruct a history of the region’s floral composition. The product of Van Zant’s labor was a set of hypothetical plant communities that probably have existed near Lake Okoboji, as summarized below:

(1)  Around 14,000 years B.P: Coniferous forest with abundant spruce and larch

(2)  13,500 years B.P: Mixed forest with lots of black ash

(3)  11,800 years B.P: Forest dominated by birch and alder

(4)  11,000 to 9,000 years B.P: Deciduous forest with abundant oak and elm

(5)  About 9,000 years B.P: Prairie beginning to compete with the forest

(6)  7,700 to 3,200 years B.P: Prairie becomes dominant

(7)  3,200 years B.P: Return of some forest, prairie still present

(8)  About 400 years B.P: Row crops and humans

QUESTIONS REMAINING

            Some trends described by Van Zant conflict directly with other scientists’ hypotheses. Specifically, Van Zant’s “late glacial sequence from Lake West Okoboji contains lower percentages of Pinus and Abies pollen than Durkee (1971) or Brush (1967) found,” (Van Zant 1979) referencing studies conducted in in other regions of Iowa. Van Zant’s somewhat simplistic answer to this disparity was that “perhaps fir and pine pollen were misidentified by Durkee and Brush” (Van Zant 1979). Ideally all three authors in question would have preserved voucher specimens of the pollen and fossils they examined in their respective studies; unfortunately, no such records were kept, meaning there is no way to be absolutely sure as to who was correct in their identification, though it does seem somewhat unlikely that both Durkee and Brush would have independently made the same mistake.

PUTTING TOGETHER THE PUZZLE

            Despite the points of dispute between findings by Van Zant, Durkee, Brush, and similar authors, their works have contributed and continue to contribute to a broader body of knowledge. In fact, scientific progress requires conflicting hypotheses, because they provoke subsequent studies that build on existing knowledge while seeking to resolve the inconsistencies. In one such study, Baker et al. (1996) used inferences from Van Zant (1979) to inform their own broader study, “Holocene Paleoenvironments of Northeast Iowa.” Subsequently, Williams et al. (2008) paired data from Baker et al. (1996) and similar studies with modern spatial simulation techniques to generate approximations of climatic and floral history across the Midwestern United States.

Locations of the studies referenced in the text

          Ultimately we should treat Van Zant’s study as one of many building blocks forming the foundation of our modern understandings of environmental history. Van Zant’s work also might serve as a reminder that the study of something as seemingly inconsequential as ancient pollen might provide us with useful knowledge in unexpected realms.

Baker, R.G., E. A. Bettis III, D. P. Schwert, D. G. Horton, C. A. Chumbley, L. A. Gonzalez and M. K. Reagan. 1996. “Holocene Paleoenvironments of Northeast Iowa.” Ecological Monographs 66(2):203-234.

Brush, G.S. 1967. “Pollen Analyses of Late-Glacial and Post-Glacial Sediments in Iowa.” In “Quaternary Plant Ecology” (H.J.B. Birks and R.G. West, eds.), pp. 173-189. Blackwells: Oxford University Press.

Durkee, L.H. 1971. “A Pollen Profile from Woden Bog in Northcentral Iowa.” Ecology 52:837-844

Van Zant, K. 1979. “Late Glacial and Postglacial Pollen and Plant Macrofossils from Lake West Okoboji, Northwestern Iowa.” Quaternary Research 12:358-380.

Williams, J.W., B. Shuman and P.J. Bartlein. 2009. “Rapid Responses of the Prairie-Forest Ecotone to Early Holocene Aridity in Mid-Continental North America.” Global and Planetary Change 66:195-207.