Both the length and intensity of drought can work together to have cumulative, negative effects on grasslands, according to a new global study published in Science.
An international group of scientists ran the same drought study across six continents, including sites in Kansas, Minnesota, Oklahoma and Texas. They imposed extreme drought conditions for four years in a row to find out what these conditions do to a diverse array of grasslands and shrublands.
“With changing climate, particularly atmospheric warming, we're expecting to see more extreme events such as droughts, but those events are likely to be longer in duration,” said Colorado State University Professor Melinda Smith, who co-led the study and worked on multiple sites, including one near Manhattan, Kansas.
Smith and her team brought together a network of scientists from around the world to study drought, using a research coordination grant from the National Science Foundation. The International Drought Experiment, as it’s known, was born from that work and involved sites around the world.
It’s important to understand how more extreme drought events will affect ecosystems like prairies, said Cristy Portales-Reyes, an assistant professor of biology at St. Louis University in Missouri who studied sites in Minnesota.
“Grasslands are one of the most widespread ecosystems in our region,” Portales-Reyes said. “About 40% of North America used to be a grassland, and they're very important culturally and also for conservation purposes.”
Research has found evidence that longer, more intense droughts are already becoming more common and scientists expect climate change to increase that trend even more.
But the impacts of multi-year, extreme droughts have been understudied, said Timothy Ohlert, another co-lead of the study and a postdoctoral associate at the University of Colorado Boulder.
“Up until this point, there had been a lot of people doing their own individual studies, focused on their own sites, maybe their own regions,” Ohlert said. “So this was the first kind of coordinated effort to kind of time things all in one bin so that we can do this research together.”
At each site, scientists put up shelters with clear gutters to block a specific percentage of rainfall without blocking the sun. The amount of blocked rain depended on long-term historical records from the locations, to impose a 1 in 100 year drought that was unique to the site itself.
“You have to keep that infrastructure up for four consecutive years,” Ohlert said. “And so there's a lot of maintenance, there's a lot of wrenches and drills that go into this work.”
That was especially hard in the Flint Hills of Kansas, where researcher Seton Bachle conducted his work. Bachle now works for LI-COR, an environmental biotech company in Lincoln, Nebraska.
“Putting the roofing above the plant canopy was sometimes easier than other times,” Bachle said. “If anyone’s spent time in Kansas, they understand the amount of wind that is sometimes received.”
To measure the impact, the scientists cut plants from the site, dried them and weighed them to understand how the drought affected plant growth. Then, they put their data together.
“There were a few data synthesis meetings where they got as many people together as possible to bring their data or send their data, and it was like, you know, what I would imagine, space camp for kids is like,” Bachle said. “It was a super nerdy, super fun thing for everyone to get together and comb through data.”
Smith, the co-leader of the research from Colorado State University, said while the results may not seem surprising, it was important to systematically study drought severity.
“With this Science paper, we then show, as you increase the duration of drought, actually, duration doesn't matter so much when drought is moderate,” Smith said. “But when you go to those extremes, that's when the losses in plant biomass or plant growth are the most profound. The two together are really important in influencing drought response, and we show that those effects can be cumulative over time.”
In Oklahoma, Lara Souza had the Dust Bowl in mind as she studied a “beautiful mixed grass prairie ecosystem,” about 40 miles south of Oklahoma City. Souza is an associate professor and associate director of the School of Biological Sciences at the University of Oklahoma.
“The Dust Bowl is part of our heritage, our history here in Oklahoma,” Souza said. “Looking at the outcomes for this study in terms of declining productivity in those systems, I think it resonates, particularly, in a different way to Oklahomans.”
The study has implications for ranching, Souza said, because it may help scientists understand best practices for grazing their cattle on grasslands amid extreme drought. Next, she hopes to study how these ecosystems recover after intense dry periods.
“The results from this study would sort of highlight the importance of having this kind of practice of regenerative ranching or regenerative grazing approaches that will sort of promote the persistence of these grasses in our system,” Souza said.
There were some notable differences between sites in the study, Smith said. The scientists found that wetter grasslands were actually more resistant to drought, while drier grasslands fared worse.
“The wetter grasslands, like those in eastern Kansas, into Missouri, up into Minnesota, like the tallgrass prairies, they actually are quite resistant to these kinds of events and they can be responsive to a higher rainfall year within the context of a drought,” Smith said.
St. Louis University scientist Cristy Portales-Reyes was glad to see how some sites responded to even small breaks in the extreme drought.
“If you have, say, two years of extreme drought and then one year of medium levels of drought, or maybe a wet year, that essentially makes it so that those catastrophic impacts don't happen,” Portales-Reyes said. “That I found very encouraging.”
Those small breaks would mimic the Dust Bowl, Smith said, which was actually punctuated by periods of rain.
“And what our study shows is that there can be a rapid recovery if we have those kinds of intervening normal years within the context of Dust Bowl-type droughts,” she added.
Even for the more vulnerable sites, like shortgrass prairie in eastern Colorado, Smith said recovery will take time and patience.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
