The Semi-arid Grassland Research Center in northern Colorado, USA. Credit: Colorado State University College of Natural Sciences
Intense, multi-year droughts have been found to have a significant impact on grassland and shrubland environments in new research led by Colorado State University (CSU) in the US.
The results reveal that plant production in grasslands is drastically impacted by extreme, sustained drought. These findings raise concerns about the long-term health of these environments, which typically survive several years of moderate drought.
“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,” says Melinda Smith, a professor in CSU’s Department of Biology who co-led the study.
“The Dust Bowl is a good example of this.”
Throughout the 1930s, a central area of the US called the Great Plains experienced an extreme drought known as the ‘Dust Bowl’. During this time, the land was so dry that severe dust storms frequently occurred and destroyed almost all crops.
Rainout shelters at the Purdue Wildlife Area in Indiana, USA. Credit: Jeffrey Dukes
“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,” says Smith.
“Now with our changing climate, Dust Bowl-type droughts are expected to occur more frequently.”
Smith was 1 of more than 170 researchers from around the world who worked on the study. They built rainfall manipulation structures over a select amount of grass and shrubland across the 6 continents involved.
These structures controlled how much rain the ecosystems received and mimicked 1-in-100-year extreme drought conditions over a 4-year period. This meant the team could study both the short and long-term impacts extreme droughts have on the ecosystem.
The researchers chose to focus only on grass and shrublands as they cover almost half of the planet and store more than 30% of the world’s carbon. These ecosystems are also central in supporting livestock and vegetation production.
Experiment site Allmend in Switzerland. Credit: Andreas Stampfli, Bern University of Applied Sciences
“An additional strength of this research is that the scale of the experiment matches the extent of these important grassland and shrubland ecosystems,” says co-author Alan Knapp, an expert in plant ecology and evolution and professor at CSU.
“This allowed us to show how widespread and globally significant these extreme drought impacts can be.”
Each site chosen for the study experiences different precipitation levels across an average year. Similarly, all the sites have varying soil and vegetation types.
This meant that the individual ecosystems all experienced a different combination of moderate and extreme drought and rainfall patterns, giving the study a unique experimental condition.
The findings, published in Proceedings of the National Academy of Science, show that losses in plant productivity, the rate at which plants create new biomaterials through photosynthesis, were more than twice as high after 4 years of extreme drought compared to similar ecosystems experiencing moderate drought.
An experiment test site in the cloud forests of the Kosñipata Valley, Peru. Credit: David Bartholomew
In moderate multi-year drought scenarios, productivity dropped drastically in the first year but then maintained the same levels across the following years. However, in extreme multi-year droughts, there was a steep drop in the first year followed by progressively larger declines as each year passed by.
On average, plant productivity dropped by 29% in the first year and by 77% in the fourth year in extreme drought sites. The researchers attribute the decline to factors such as changes in the plant community composition and species mortality.
For Smith, the research highlights the relationship between the extremity and duration of drought conditions, something that has rarely been considered in previous studies.
“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,” says Smith.
“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.”