Time of Year Important in Projections of Climate Change Effects on Ecosystems

Does it matter whether long periods of hot weather, such as last year's heat wave that gripped the U.S. Midwest, happen in June or July, August or September? Scientists studying the subtle effects of heat waves and droughts say that when such events happen makes a big difference.

Based on more than 25 years of data from the National Science Foundation Konza Prairie Long-Term Ecological Research site in Kansas--one of 26 such sites across the globe--ecologists looked at how droughts and heat waves affect grass growth during different months of the year

"We found that the dependence on the timing of climate variability was consistent across many lines of evidence, from plot-based measures of grassland productivity to observations from satellites," said study co-author Dr. Andrew Elmore of the University of Maryland Center for Environmental Science's Appalachian Laboratory.

"For example, across all of Konza, the 'greenness' of grasslands as viewed by satellites in the late summer consistently showed a strong relationship with early summer precipitation, but not precipitation in late summer."

The researchers found that droughts reduced grass growth most in early June, while heat waves reduced grass growth only during late July. Neither drought nor heat waves in August or September seemed to affect grass growth.

"Future projections need to incorporate predictions of not only how much climate will change, but when during the year changes will happen," says Joseph Craine of Kansas State University, the paper's lead author. "That the effects of climate change on grasslands depend on when they happen may not be much of a surprise--little snow in winter may have less effect than low rainfall in summer, for example," he says.

The sensitivity of grasslands to the timing of drought and heat waves was a big surprise, however.

"Heat waves mattering only during late July was not something we expected," says Craine. "Everyone seemed to think that August heat waves and drought would have major effects on grass productivity, but we couldn't find any."

The effects of drought and heat waves in fact declined over the summer season. Other studies showed that drought and heat waves affect parts of ecosystems differently. "For example, in some grass species, flowering is altered by drought in May, and in others by drought in August," Craine says.

Bison that graze the prairie don't seem to respond to heat waves, but may gain more weight in years with drier weather--provided that droughts come in late June or early July rather than in August or later. The researchers are looking at long-term records from other sites to determine whether there's a uniformity to the Konza findings.

"If these patterns are general across ecosystems," the scientists write in their paper, "predictions of ecosystem response to climate change will have to account not only for the magnitude of climate variability but also for its timing."

For now, says Craine, "the results will change the types of questions we ask about climate and ecosystems."

The study was published in the journal Proceedings of the National Academy of Sciences, was authored by Joseph Craine of Kansas State University, Andrew Elmore of the University of Maryland Center for Environmental Science’s Appalachian Laboratory, Jesse Nippert, Adam Skibbe and Stacy Hutchinson of Kansas State University, and Nathaniel Brunsell of the University of Kansas.

--from the National Science Foundation

Photo credit: NSF Konza Prairie LTER Site