Grasslands, shrubs suffer 60% more plant growth loss in droughts: study

A comprehensive global study spearheaded by scientists from Colorado State University reveals that the impact of severe drought—a phenomenon predicted to become more common due to climate change—on grasslands and shrublands has been significantly underestimated.

The research, published in the Proceedings of the National Academy of Sciences, provides an unprecedented level of detail in quantifying the effects of short-term extreme drought on ecosystems spanning six continents.

This is the first time we have conducted such a wide-ranging experiment to establish a fundamental understanding of potential plant productivity losses in these crucial ecosystems.

The study was led by Melinda Smith, a professor in the Department of Biology at CSU, who is also the first author of the paper. She noted that the observed decrease in a critical carbon cycle process following a single 1-in-100-year drought event far surpasses previously documented losses for grasslands and shrublands.

"Through our research, we determine that the loss of aboveground plant growth, which is a crucial indicator of ecosystem function, was 60% greater in extreme short-term droughts than in less severe droughts that were historically more common," she said.

"Earlier research faced issues with methodological differences when estimating the impact of extreme drought on natural ecosystems. To overcome these, we used a standardized, distributed approach for our study" she added.

Collecting worldwide drought data on ecosystems

The International Drought Experiment, a newly published research initiative that began in 2013 as part of the National Science Foundation’s Drought-Net Research Coordination Network, involved over 170 authors from institutions worldwide. The study, completed over four years, used rainfall manipulation structures to experimentally reduce precipitation available to ecosystems for a full growing season.

With climate change, short-term extreme droughts, once considered 1-in-100-year events, could occur every two to five years. However, due to their historical rarity, the ecological impact of such droughts has been hard to estimate.

Professor Smith, the study’s lead, identified grasslands and shrublands as ideal research subjects due to their ease of manipulation, significant global carbon storage (over 30%), and support for key industries like livestock production.

Grasslands and shrublands ideal for study

Smith said grasslands and shrublands were perfect test areas to fill that research gap because they are easier to manipulate for study than other systems, such as forests. They also store more than 30% of the global stock of carbon and support key industries such as livestock production.

"Scalable to the globe, key ecosystems make them highly relevant for this kind of work," said Smith, the chair of the Faculty Council on campus. "Grasslands and shrublands, covering between 30% and 40% of the globe, frequently suffer from precipitation deficits. This makes them more vulnerable to climate change."

The study’s findings offer insights into how specific climates, soils, and vegetation types influence drought response. While drier and less diverse sites, like those in Colorado, are likely most vulnerable, Smith emphasized that drought severity is the most consistent and crucial factor in determining an ecosystem’s response.

"Our data suggests greater losses in drier sites, but if you are getting to the extremes—which is what is being forecasted—we can generally expect substantial losses no matter where you are in the world," she said. "We also found that even moderate losses from less severe droughts would still likely result in large impacts to the populations that rely on these systems. And then there is a combined loss of function across the globe to consider as well."

She noted that even moderate losses from less severe droughts could have significant impacts on populations dependent on these systems, and there could be a combined loss of function globally. The team is now analyzing data from the project’s full four years to assess the global impacts of multiyear droughts.

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