Research contrasts drought sensitivity of Eurasian and North American grasslands

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Grasslands in Asia and North America differ in their responses to drought, according to a new paper in the journal Nature led by faculty at Colorado State University. The findings show that differences in the dominant grasses and lower species diversity in the Eurasian Steppe grasslands may make it more vulnerable to drought than the North American Great Plains.

The findings have broad implications for land management practices on both continents and provide a valuable comparison point that was not previously available when addressing climate change.

The work at CSU was led by Professors Melinda Smith and University Distinguished Professor Alan Knapp in the Department of Biology. Smith said the multi-year project was built through ongoing collaboration with researchers in China, including first author and Smith’s former postdoctoral researcher at CSU, Qiang Yu. Together, the team established six sites on each continent and imposed extreme drought conditions over a four-year period. They found the Eurasian grasslands saw a 43% reduction in annual productivity – plant growth each year – compared to just a 25% reduction in North America under the same conditions.

Smith said the findings also show that the negative effects of drought in Eurasia increased over time. Meanwhile, the North American system was able to stabilize in the second year of the experiment. The paper explores that difference, specifically considering how plant diversity impacted each region’s ability to cope with prolonged extreme droughts.

Smith said the research shows the difference may be linked to the overall number of uncommon, subordinate plant species in each region. These species make up the bulk of plant diversity there and contribute to overall productivity – even as primary species struggle to do so in drought conditions. The findings show these key subordinate species declined in Eurasian grasslands during drought conditions but increased in North America – potentially stabilizing production losses there over time. 

“Particularly in North America, we show that subordinate species seem to be minimizing losses and picking up the slack in a way that is not happening in Eurasia where there is less species richness,” she said. “These species’ ability to step up – possibly, due to drought tolerance developed over time – provides a unique perspective on these two vast grassland regions and how they function in these conditions.”

The two regions also support different types of dominant grass species. The Eurasian grasslands are primarily suited to supporting C3 grasses, like wheat, that prefer cooler and wetter conditions, while C4 grasses, like corn, flourish in the warmer American Midwest. That allows for interesting comparisons related to agriculture and management practices under drought conditions, said Knapp.

“In general, C4 plants have more efficient photosynthetic pathways and are more productive with less water than C3 plants. Such differences may be especially important when trying to understand how productivity will change with increasingly more severe droughts,” Knapp said.

Plant productivity is a fundamental component of the global carbon cycle. That is because plant photosynthesis is the primary way that atmospheric carbon, as carbon dioxide, enters ecosystems and is made available for consumption by animals and storage as biomass. Because grasslands cover 40% of the Earth’s surface, they play a large role in balancing and facilitating carbon uptake and sequestration globally.

Smith said understanding how these similar, yet differing biomes contribute to that process has proven difficult in the past. That is because researchers around the globe struggled to standardize experiments in a way that allowed for clear comparisons. This work directly addresses that need by developing a large-scale, coordinated, multi-year experiment at multiple sites. The 12 total test sites for the project are collectively known as the Extreme Drought Grasslands Experiment (EDGE) and were chosen because they represent a variety of grasslands located along precipitation gradients. 

Smith said the work shows how vulnerable regions with lower species diversity can be to prolonged droughts. It also shows the need for management strategies that increase and maintain plant diversity to enhance resistance to extreme drought events under future climate change scenarios.

“Dryer grassland tends to be where a lot of cattle grazing happens and where many people live globally. They are also important when we consider how carbon is stored related to climate change,” she said. “So, there is a real need to develop management strategies in these areas, as climate change is expected to increasingly impact them with extreme, prolonged droughts.”
 

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