Some U.S. grasslands will become more diverse but less productive as predicted changes in precipitation patterns related to global warming become realized, according to the first results of a long-term grassland rainfall study conducted by Colorado State University researcher Alan Knapp. Reduced production of grasslands would have a negative economic impact on ranchers.
Global change models have consistently predicted that precipitation events will become more intense but less frequent. In 1997 Knapp, senior ecologist and a professor of biology at Colorado State, with his colleagues at Kansas State, New Mexico and Yale universities began a 15-year experiment on the native tallgrass prairie of eastern Kansas. The goal was to simulate predicted future rainfall patterns in which storms occur less often — about 50 percent longer between rain events than today — but with larger volume of precipitation during each rain event.
In 2003, infrared heat lamps were positioned above portions of the research plots to examine the combined effects of higher temperatures (2-3 degrees Celsius) with altered rainfall patterns. The longer periods between rain events leads to drier soils and areas underneath the heat lamps were found to dry out at an accelerated rate, according to researchers.
"One of the first patterns that became evident was of reduced productivity among varieties of grasses that had previously grown very tall," Knapp said. "This allowed other plant species that previously had been shadowed by these tall grasses to flourish."
"If you are a rancher, anything that reduces the total biomass produced by these grasslands would be an economic disadvantage," he said. "From an ecological perspective, the consequences of changes in the composition of the species of grasses and other plants will require further study."
To replicate altered precipitation patterns of less frequent, but more intensive storms, Knapp’s research team erected a dozen large greenhouse structures with only roofs and no sidewalls on the Konza Prairie in northeast Kansas. Naturally occurring rainfall that fell on the roofs of the structures were collected via gutters and stored in large tanks. All the rain collected in the tanks is then showered on the research plots at the appropriate time. The research is sponsored by the USDA’s Managed Ecosystems Program, the Department of Energy’s Climate Change Research Program and the National Science Foundation.
During the second half of the research study, Knapp said he and his research colleagues expect to better understand how their research discoveries will relate to grazing by cattle as well as affect the presence of invasive non-native species.