Joseph Craine is a research assistant professor at Kansas State University. Joe Fargione is lead scientist for the North America region at The Nature Conservancy.
Half the United States is currently gripped by drought. And our grasslands are taking a beating. But how grasslands have historically coped with the kinds of extreme drought we’re seeing today has always been a bit of a mystery — until now.
A new study we co-authored for Nature Climate Change took on this question by gathering more than 400 species of grass from around the world and subjecting them to drought. The results — along with clues gathered way back during the Dust Bowl days of the 1930s — explain why grasslands like The Nature Conservancy’s Konza Prairie in Kansas can make it through a drought like this…but your lawn probably can’t.
Hidden Lessons in the Great Drought of the 1930s
To understand the depth of this problem, though, you have to step back to the research of John Weaver, a professor from the University of Nebraska whom some consider the father of grassland ecology in North America. Weaver’s careful observation of the grasslands of Nebraska and Kansas before, during, and after the Great Drought of the 1930s taught us an immense amount about how grasslands respond and recover to drought — and teach us important lessons for managing our grasslands in the 21st century.
During the Great Drought, the shortgrass of Nebraska and Kansas spread hundreds of miles to the east into the mixed grass region, while the mixed grass spread hundreds of miles into the tallgrass. But these expansions didn’t happen because individual species were migrating, Weaver discovered — rather, local populations were expanding and contributing to the diversity of individual communities.
Eventually, when the drought broke, the reverse occurred: the humid grasslands appeared to march back westward. Again, Weaver reported, this “march” occurred through the expansion of local populations or the resurgence of dormant seeds and roots. In many cases, species like big bluestem (Andropogon gerardii) recovered from roots that remained viable for almost a decade.
Weaver’s findings raise an interesting set of questions about how grasslands in different parts of the world respond to drought. When drought hits, how much of a grassland’s function can continue by relying on the local diversity of drought-tolerant species, like those in the Great Plains? Or will maintaining those grasslands require immigration of drought-tolerant species — which can be a long, slow process?
The Study: Bright Lights, No Water
To begin to answer these questions, we grew over 400 species of grass collected from around the world. The plants were grown up in a walk-in chamber with bright lights and controlled temperatures. To understand how tolerant each grass species was of drought, we stopped watering the plants after six weeks. When the soils in which grasses are growing begin to dry, some shut down quickly while others continue to grow. Some grasses roll their leaves, while others produce waxes to reflect light — which makes bluestem grasses become blue, for example. Yet, all of them eventually shut down and succumb to drought. When they do this, they can no longer photosynthesize and have to wait for soils to become wetter — if they survive at all.
When we looked at where different grasses lived, we found that dry grasslands had species that were tolerant of drought and also ones that were intolerant. But so did wet grasslands. Our conclusion: Almost all of the grasslands of the world should have a large diversity of drought-tolerant and -intolerant species.
For example, Konza Prairie, located in northeast Kansas, has almost 600 species of plants there. That’s almost one out of every 400 plant species in the world. Among those 600 species, 86 of them are grasses — about one out of every 125 grass species in the world.
Konza Prairie is a tallgrass prairie. With over 30 inches of rain a year, it’s on the wetter end of North America’s grassland spectrum — though it is relatively dry for a tallgrass prairie. Despite generally receiving abundant rain, almost the entire global diversity in drought tolerance can be found in Konza’s 90 species of grass. Its riparian areas and seeps have cordgrass and gamma grass, which have little drought tolerance. Its lowlands have bluestems and switchgrasses, which have intermediate drought tolerance. Its uplands have gramas, three-awns, and lovegrasses, which are extremely tolerant of drought.
The lesson from Weaver long ago was that this local diversity in grasses is critical and should be promoted. The diverse grasslands, like Konza Prairie and many other native grasslands throughout the world, hold species that can expand during the dry time until wetter climates return. Droughts knock back prairies like these, but they still produce grass for grazers, still hold their soil, and still moderate their climate by being green and photosynthesizing.
Yet, many other grasslands (and lawns) have been “improved” or restored at a much lower diversity than is found in native grasslands — often leaving out the most drought-tolerant species. If the drought we’ve been experiencing continues, many grasslands are going to need to rely on having drought-tolerant grasses present to continue to function until wetter times return. Yet, for those grasslands that don’t have those species on the landscape, what would be next?
Potentially, another Dust Bowl.
(Image: Konza Prairie, courtesy of Joseph Craine.)