Global study shows extreme short-term drought has even greater impacts on grasslands and shrublands than previously thought

Students sit in a patch of dead dry grass with sensors taking notes
UC Santa Cruz students collected data on how simulated drought conditions affected grasslands and shrublands as part of the International Drought Experiment. Photo credit: Michael Loik.
Professional portrait of Michael Loik
UC Santa Cruz Environmental Studies Professor Michael Loik. Photo credit: Michael Loik.
Students look at meters and write notes next to a structure of plastic strips above grass
UC Santa Cruz students measured plant growth and photosynthesis beneath rainfall manipulation structures at Younger Lagoon Reserve. Photo credit: Michael Loik.

UC Santa Cruz scientists supported a new global study showing that the effects of extreme short-term drought—which is expected to increase in frequency with climate change—have been greatly underestimated for grasslands and shrublands.

The study was organized and led by Colorado State University (CSU) and published in Proceedings of the National Academy of Sciences. The findings quantify the impact of extreme short-term drought on grassland and shrubland ecosystems across six continents with a level of detail that was not previously possible. It is the first time an experiment this extensive has been undertaken to generate a baseline understanding of the potential losses of plant productivity in these vital ecosystems.

Melinda Smith, a professor in the Department of Biology at CSU, led the study and is the first author on the paper. Michael Loik, professor of environmental studies at UC Santa Cruz, is a co-author on the paper, a member of the steering committee that designed the experiment, and the lead scientist for three study sites at UC Santa Cruz. 

Known as the International Drought Experiment, the research originally dates back to 2013, as part of the National Science Foundation’s Drought-Net Research Coordination Network. Altogether, there are more than 170 authors representing institutions from around the world cited in the new PNAS study, which was completed over the last four years.

To gather their data, researchers built rainfall manipulation structures to experimentally reduce the amount of naturally occurring precipitation available to ecosystems for at least a full growing season. At UC Santa Cruz, rainfall structures were installed at the Coastal Science Campus, near the UCSC Arboretum and Botanical Gardens, and on UCSC Natural Reserve lands on upper campus. About half of the experiment’s participating sites imposed extreme drought conditions with these structures, while the rest imposed less severe drought for comparison.

Loik said a crucial component of the experiment’s plan was designing the standardized rainfall manipulation structures to be relatively small and easy and inexpensive to build, so that more researchers around the world could participate without concerns about financial capacity. As a result, the team was ultimately able to collect and analyze data from 100 sites.   

The results showed that, for a single year of simulated drought, loss of aboveground plant growth—a key measure of ecosystem function and an important carbon cycle process—was 60% greater at experimental sites that experienced extreme once-in-a-century drought conditions, compared to those where less severe drought conditions were imposed. These findings greatly exceeded previously reported losses for grasslands and shrublands. 

“Past studies suffered from methodological differences when estimating the impacts of extreme drought in natural ecosystems, but our standardized, distributed approach here addressed that problem,” Smith said.

Findings from the sites also provide insight into how specific climates, soil, and vegetation types broadly influence drought response. While the work shows that drier and less diverse sites are likely to be the most vulnerable to extremes, the severity of the drought was the most consistent and important factor in determining an ecosystem’s response.

Loik said studies such as this one, that identify how sensitive plants in different ecosystems are to drought, can be useful on many fronts. 

“The results should help to project drought impacts at regional and continental scales,” Loik said. “This knowledge can help land planners, wildland fire personnel, and natural resource managers who need to know how and when to intervene in a future climate that has much more variable precipitation patterns.”

The team is now examining data collected from the full four years of the project for the next phase of their research: assessing multi-year drought impacts globally.

This story was adapted from a Colorado State University press release.