a future with more climate extremes. In the four-year field study, the researchers altered rainfall variability by increasing the amount of precipitation that falls in one storm, and lengthened the periods of time between rainfalls by 50 percent. That effectively increased the severity of dry periods between storms without altering the total amount of precipitation received during a growing season.

"When these native grassland plots, exposed to more variable rainfall patterns, were compared with plots that received rainfall in a natural pattern, the overall growth of all plants decreased," said Knapp. "More variable rainfall patterns led to lower amounts of water in the soil in the upper 30 centimeters. Since this is the soil depth where most plant roots occur, and where important soil microbes are most abundant, grasses there were water-stressed and the activity of below-ground organisms was reduced."

In contrast, said Collins, "the diversity of plants in plots with greater variability in rainfall patterns increased." Collins cites two possible explanations for this finding: "A high degree of variability in resources can lead to a greater number of co-existing species. Or reduced total productivity may have allowed less common species to increase in abundance." Regardless of the mechanism, said Collins, these results show that plant community structure can be significantly changed, and the cycling of carbon slowed, in as little as four years when grasslands are exposed to a more variable climate.

Concerns about predicted climate changes resulting from human activities often focus on the effects of increases in average air temperatures or changes in average precipitation amounts. But widely used climate models also predict increases in climate extremes, said Knapp, such as more frequent large rainfall events or more severe droughts. "It's important that we look at variability in a new way: not only from year to year or decade to deca
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Contact: Cheryl Dybas
cdybas@nsf.gov
703-292-8070
National Science Foundation
12-Dec-2002

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