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A global research effort led by Colorado State University shows that extreme, prolonged drought conditions in grasslands and shrublands would greatly limit the long-term health of crucial ecosystems that cover nearly half the planet. The findings are particularly relevant as climate change increases the possibility of more severe droughts in the future, potentially leading to a situation that echoes the Dust Bowl of the 1930s.

The new research published in Science shows that losses in plant productivity—the creation of new organic matter through photosynthesis—were more than twice as high after four years of continued extreme drought when compared to losses from droughts of moderate intensity. The work shows that these grassland and shrubland ecosystems lose their ability to recover over time under prolonged dry conditions.

"We show that—when combined—extreme, multi-year droughts have even more profound effects than a single year of extreme drought or multi-year moderate droughts," said CSU Biology Professor Melinda Smith, who led the study with Timothy Ohlert, a former CSU postdoctoral researcher.

"The Dust Bowl is a good example of this," she continued. "Although it spanned nearly a decade, it was only when there were consecutive extremely dry years that those effects, such as soil erosion and dust storms, occurred. Now with our changing climate, Dust Bowl-type droughts are expected to occur more frequently."

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Smith designed and led the International Drought Experiment with more than 170 researchers around the world. For the project, researchers built rainfall manipulation structures that reduced each rainfall event by a target amount over a four-year period in grassland and shrubland ecosystems across six continents.

The CSU research team includes University Distinguished Professor Alan Knapp, Professor Eugene Kelly, Associate Professor Daniela Cusack and Research Associate Anping Chen. Former Ph.D. student Amanda Cordiero and Postdoctoral Researcher Lee Dietterich also contributed to the study.

By simulating 1-in-100-year extreme drought conditions, the team was able to study the long- and short-term effects on grasslands and shrublands, which store more than 30% of global carbon and support key industries, such as livestock production. Variations in precipitation, as well as soil and vegetation across continents, meant different sites experienced different combinations of moderate and extreme drought years, providing unique experimental conditions that informed this study.

Smith said the paper highlights the interaction between extremity and duration in drought conditions and that this interaction has rarely been systematically studied using experiments. She added that the research suggests that the negative impacts on plant productivity are also likely to be much larger than previously expected under both extreme and prolonged drought conditions.

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Plant growth is a fundamental component of the global carbon cycle. That is because plant photosynthesis is the main way carbon dioxide enters ecosystems, where animals consume it and plants store it as biomass. Because grasslands and shrublands cover roughly 50% of Earth's surface, they play a large role in balancing and facilitating carbon uptake and sequestration globally. That means changes to these ecosystems caused by drought could have wide-ranging impacts, Knapp said.

"An additional strength of this research is that the scale of the experiment matches the extent of these important grassland and shrubland ecosystems," Knapp said. "This allowed us to show how widespread and globally significant these extreme drought impacts can be."

For more than a decade, Smith, Knapp and their colleagues have worked on similar research into grasslands at CSU. They often partner with agencies like the Department of Agriculture to develop a better understanding of the consequences of climate change to these ecosystems on topics such as species diversity.

The International Drought Experiment is a key example of this work. The team had previous published findings in PNAS from the same multi-site research network that quantified the impact of extreme short-term (one year) drought on grassland and shrubland ecosystems. Smith said the pair of papers now form an important foundation for further research into this topic.

"Because of the historic rarity of extreme droughts, researchers have struggled to estimate the actual consequences of these conditions in both the near and long-term," she said. "This large, distributed research effort is a truly a team effort and provides a platform to quantify and further study how intensified drought impacts may play out."