The periods of dry winters also contributed to dry vegetation, making the grasses more susceptible to fires set by Indians or caused by lightning.
Another contributing factor to sustaining the grassland was the frequency and amount of warm-season evapotranspiration -- a process in which moisture evaporates from the soil and transpires from plants.
Most of the tall-grass triangle had precipitation/evaporation ratios of .75 or higher, a number that suggests unusually wet warm-season conditions, in 70 percent of the years studied; adjacent regions in the high plains had much lower ratios. The researchers theorize that evapotranspiration rates played a major role in the formation and maintenance of the western boundary (Tulsa to Fargo) separating the short grasses of the plains and the tall grass triangle. The tall grasses needed enough warm-weather precipitation to produce the higher evapotranspiration rate, the researchers found.
The experience of the Midwest's prairie is one of extremes, including some of the very factors that could be more widespread as a result of global warming, said Changnon, chief emeritus of the State Water Survey.
"What this shows is that the whole environment of the Midwest has been very sensitive to certain extreme weather events," he said. "Having long-term data lets us talk more intelligently about potential changes in global climate. Most climate modeling generates average changes, not the frequency of extreme events. Talk of the occurrence of a 100-year flood really hasn't been based on 100 years of data; it may be extrapolated from just 40 years of records, so scientists must say that a 100-year flood will happen at least once, not necessarily only once, in 100 years."
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Contact: Jim Barlow, Life Sciences Editor
jebarlow@uiuc.edu
217-333-5802
University of Illinois at Urbana-Champaign
18-Aug-2003