K-State researchers study response of prairie ecosystem

In face of changes in precipitation variability, climatic extremes

What does Kansas’ weather and life have in common? In the words of Forrest Gump, both are like a box of chocolates. “You’re never sure what you’re going to get.” Rain or drought. Drought or rain.
Concerns about future climate changes resulting from human activities often focus on the effects of increases in average air temperatures or changes in average precipitation amounts. But climate models also predict increases in climate extremes such as more frequent large rainfall events or more severe droughts. This aspect of climate change can lead to an increase in climatic variability without accompanying changes in average temperatures or total precipitation amounts, according to a report by a team of researchers at Kansas State University.

The team, headed by Alan Knapp, a university distinguished professor of biology, and John Blair and Phil Fay, professors of biology, has been studying how grasslands respond to increases in the variability of rainfall patterns to better understand how rapidly and to what extent ecosystems might respond to a future with a more extreme climate. Their findings appear in the latest issue of Science.

A key feature of the four-year field study is that the team was able to construct 12 rainfall “shelters” on the Konza Prairie — essentially greenhouses with clear plastic roofs but without sidewalls — that allowed the researchers to collect and store rainfall as it occurred and use it in turn to manipulate rainfall patterns in large grassland plots.

The research team was able to alter the rainfall variability by using the collected rainfall to increase the size of individual rainfall events and lengthen the periods of time between rainfall events by 50 percent, thus effectively increasing the severity of dry periods between storms without altering the total amount of precipitation received during the growing season in their experimental plots.

“All plots get the same amount of rain, but there are different durations in the dry periods between events and the size of the storms is different,” Knapp said of the. “We’re changing just the distribution of rain and the intensity of the rain, but we’re not changing the amount that occurs on the prairie.”

After four years, researchers discovered “a host of changes” in the tallgrass prairie as a result of just altering the patterns of rainfall and not the amount of rainfall. When intact, native grassland plots exposed to more variable rainfall patterns were compared to plots that received natural rainfall patterns, the researchers found that the physiological vigor of the grasses decreased as did the overall productivity or growth of all plants combined. More variable rainfall patterns led to lower amounts of water in the soil in the upper 30 centimeters of soil. Since this is the soil depth where most plant roots occur and where soil microbes are most abundant, grasses were more water stressed and the activity of below ground organisms was reduced. Overall, more extreme rainfall patterns reduced the rate of carbon cycling in this grassland by lowering the uptake of carbon dioxide by the plants above ground and slowing carbon dioxide release by roots and microbes below ground.

“We found a significant reduction in the amount of grass growth and productivity, just by changing the pattern of rainfall but not by changing the total amount,” Knapp said. “We saw significant reductions in below ground activity of the roots and microbes; we found significantly more stress in the grasses — they experience longer dry periods between storms.; they can’t use the big rainfall events as effectively as they can the more frequent smaller ones and so we see greater water stress in the dominant grasses.”

The group’s study as part of the Long-Term Ecological program at Konza Prairie, is the first to focus on and manipulate climate variability in an intact ecosystem without altering the climate average. Because all of the responses measured are similar to those that would occur under drought conditions, the results suggest that increased rainfall variability when combined with projected higher temperatures and decreased rainfall amounts may lead to even greater impacts on ecosystems than previously anticipated.

“This is a phenomenon of global importance, not just Kansas,” Knapp said. “An increase in precipitation extremes is likely to occur everywhere. The kinds of climate changes that are likely to occur here and elsewhere will have measurable effects upon the resources that we depend upon. Those resources can be grassland resources or they can be cropland resources but it’s very likely that this change in climate, this increase in extreme storm events, this increase in rainfall variability will have measurable effects in a fairly short period of time.”