Quantification of Climate Conditions Important to the Tall Grass Prairie

The geologic history of the tall grass prairie in central North America reveals the prairie began developing 10,000 years ago after the Pleistocene ice sheets had retreated into Canada. The long-lived triangular shaped prairie survived several climatic shifts during the Holocene. Scientists have studied the anomaly—grasses where many climate conditions indicate forests should have been growing—by offering a myriad of potential climate, physiographic, and human factors. By the 1960s scientists recognized that fire was the key to the prairie’s presence—fires destroyed forests but grasses survived and were generally enhanced by prairie fires. Scientists have since believed that lightning, in addition to fires set by Native Americans, were the reasons for the region’s high frequency of prairie fires. Long records of various climate conditions collected since the 1890s now allow a quantitative comparison of the prairie versus non-prairie climate conditions. Assessment of drought conditions during the 20th Century shows that the prairie triangle experienced 50% to 200% more severe drought years than did the forested areas north and south of the prairie, an important factor behind the prairie’s presence. Cold season precipitation, averaging <34 cm, in the prairie was too low to sustain the deciduous forests that grew south and east of prairie, and resulted in a dry season necessary for frequent prairie fires. The rougher topography and numerous streams to the prairie’s south also served as firebreaks to contain fires, whereas the flat lands of the prairie with few major rivers had fewer natural firebreaks. West of the tall grass prairie was a short grass prairie, the climax vegetation in the more arid High Plains. Large differences in lightning activity exist along the prairie’s northern boundary during the prairie’s fire season (October-March), suggesting that the much lower storm incidences to the north help explain the presence of forests in that area. Long climatic records, which were not available for use in earlier studies, have allowed quantification of the climate factors that helped shape the Midwest’s tall grass prairie.