Climate Factors that Caused the Unique Tall Grass Prairie in the Central United States

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Abstract
Throughout the 20th century scientists sought an explanation for the tall grass prairie in the central United States, a large area of tall grasses with a climate long believed better for forests than grasses. Scientists initially attempted to explain the anomaly-grasses where trees should have been-by offering a myriad of potential climate factors and physiographic differences. Quantification of the climate conditions that many identified as crucial to the prairie's existence was possible because of newly available 100-year climate records, and this study identified the thresholds of four conditions that define the boundaries of the prairie. The prairie experienced >90% more severe drought years than did the forested areas north and south. A second factor considered to sustain the prairie was frequent dry seasons. Fire was a third key to the prairie's presence-fires destroyed forests but grasses survived and were enhanced by prairie fires. Lightning, in addition to fires set by Native Americans, was a key factor behind the region's high frequency of prairie fires. West of the prairie was a short grass prairie, and the two prairies were separated by differences in warm-season evapotranspiration values. The climatic explanation for the tall grass prairie includes four climate extremes-more severe level droughts than north or south of the prairie, frequent very dry cold seasons, many more fires triggered by lightning, and more frequent low evapotranspiration rates in the prairie's west.