North American grasslands are of ecological, economic, and societal importance. However, climate-vegetation-fire relationships in grasslands remain poorly understood, and ecological knowledge from historical records may be inadequate for anticipating future environmental changes that exceed the range of observed variability. This presentation will highlight recent findings from Holocene paleoecological studies of grasslands in the Midwest and Northern Great Plains that reveal the long-term effects of climate variability on grassland ecosystems. 

Results/Conclusions

Climate and fire are major factors controlling the structure and function of grassland ecosystems today. New well-dated pollen records coupled with pollen-independent climate and fire history reconstructions suggest that, to varying degrees, these factors were also important drivers of grasslands on decadal to millennial time-scales during the Holocene. The eastward shift of the North American prairie-forest border during the early/middle Holocene in the upper Midwest was probably a response to warming and drying associated with high summer insolation and the retreat of the Laurentide Ice Sheet. In some regions pine forests apparently transitioned to mixed-grass prairie, implying steeper moisture gradients than exist in the region today. However, in the eastern Prairie Peninsula in Illinois fire-regime shifts were also important drivers of prairie expansion, consistent with speculations by early North American ecologists, such as Gleason. In the Great Plains fuel availability was the dominant control of fire regimes on decadal to millennial scales during the Holocene. Fire oscillated with climate and was more prevalent during wet periods when grassy fuels were most abundant; fire was uncommon when conditions were dry, weeds abundant, soils disturbed, and fuel production low. Pronounced centennial-scale droughts occurred throughout the Holocene, although moisture deficits were greatest during the middle Holocene. Changes in atmospheric ciriculation patterns and solar output may be related to drought occurrence, but the precise mechanisms remain poorly understood. Outstanding problems in the vegetational history of North American grasslands include insufficient knowledge of shifts in the abundances of cool-season and warm-season grasses (and thus forage quality and quantity), as well as how large grazers, such as bison, responded to and possibly influenced, shifts in grassland community composition. Recent technical advances for analyzing carbon isotopes in grass-pollen grains and thus distinguishing C₃ from C₄ grasses and for quantifying bison diets, mobility, population sizes, and fitness from paleontological and archaeological data are poised to help address these issues and thus further understanding of the ecological impacts of climate change in North American grasslands.