Reconstructing grassland vegetation on the southern Great Plains of North America

Background/Question/Methods

The mid-continent of North America has experienced dramatic and abrupt climate change during the Holocene. Several lines of evidence indicate prolonged dry conditions that began during the early Holocene, but the response of grassland vegetation to this climate change has been difficult to quantify. To improve interpretation of tallgrass prairie vegetation in North America from pollen assemblages, we report data from a surface sample set collected from the Flint Hills ecoregion of Kansas, the largest contiguous remnant of tallgrass prairie in the U.S. We acquired 25 surface samples from sediments of small ponds (less than 10 ha surface area) in this region and quantified their pollen assemblages. We compared these tallgrass prairie assemblages to 476 modern pollen samples classified as “prairie” in the North American Surface Sample database. We then compared the surface pollen assemblages with fossil pollen assemblages from sediment cores at two sites in Kansas— Cheyenne Bottoms and Muscotah Marsh— using the modern analog technique.

Results/Conclusions

The surface sediments of the 25 Flint Hills tallgrass ponds contained 37 arboreal pollen taxa, 47 nonarboreal pollen taxa, and several spore types including Sporormiella. These sites are dominated by grassland vegetation: the percentage of nonarboreal cover within 1000 m radius of each pond averages 89%, ranging from 59 to 100%. Arboreal pollen percentages range from 17 to 62% and do not correlate with woody cover among sites, likely due to contributions from background pollen. Both Cheyenne Bottoms and Muscotah Marsh occasionally have analogs with the Flint Hills surface sample set, as calculated by squared chord distances, over the past approximately 29,000 cal yBP. However, the fossil assemblages of both sites have better analogs with surface samples from the northern Great Plains. This indicates that colder conditions than modern prevailed for much of the Holocene (and earlier) at these sites. Several fossil pollen assemblages from Muscotah Marsh have no analog with modern pollen assemblages, perhaps because of its location on the prairie-forest border.