Ancient grasslands lost to plows and development
When a 10,000-year-old prairie is plowed up to plant corn or a savanna is consumed by a strip mall, these once-grassy expanses, if left to regrow, can eventually look vaguely like they did before. But a growing body of research suggests that what comes back isn't the same as what was lost, even decades later. And now scientists are starting to understand why.
That's the takeaway of a new study of grasslands worldwide published in the journal Proceedings of the National Academy of Sciences.
The results are important because grasslands are more than just treeless expanses. They can look barren to the untrained eye, but they have extensive underground roots that reduce water runoff, control erosion, and store a third of Earth's terrestrial carbon. Grasslands provide fodder for livestock and habitat for unique plants and animals ranging from American bison and African wildebeests to the great bustards of Asia. Covering nearly a quarter of Earth's land, they also help support the livelihoods of more than a billion people worldwide. Think of the vast 5,000-mile belt of grasslands that is the Eurasian steppe, stretching from Hungary to China; the prairies of North America's Great Plains; the African savanna.
Yet over the past couple of centuries, ancient grasslands around the world have largely been converted into cropland, tree plantations, or development as cities expand. In Brazil, for example, a species-rich savanna known as the Cerrado loses an area the size of London every three months. India's savannas shrank from 100 million acres (156,000 square miles) in 1880 to 60 million acres (94,000 square miles) in 2010, according to one study. And in North America's grasslands, less than half of their historical acreage remains.
At the same time, millions of acres of farmland are abandoned worldwide, providing an opportunity for secondary grasslands to spring up in their place.
What the study found
Previous research by the lead author of the new study, Ashish Nerlekar, a postdoctoral researcher at Michigan State University, had hinted at how these newly regrown grasslands might differ from ancient ones. The new study analyzed data for 742 plant species from grasslands across six continents. It compared older grasslands that had never been plowed or converted to farmland with patches of younger secondary grassland at varying stages of regrowth.
The analysis revealed that the differences between old-growth and secondary grasslands are remarkably consistent around the globe, according to co-author Lars Brudvig. The plants that are lost when an old-growth grassland is plowed up tend to be long-lived perennials with tough, leathery leaves. Many of the original grassland residents are also very slow-growing, Nerlekar said.
In contrast, the new plants that grow back as the grassland recovers have features associated with faster growth and a greater ability to capture resources. Regrowing grasslands were distinguished by faster-reproducing annual species, such as white goosefoot (Chenopodium album), that are better at competing for water and nutrients. Plants in regrowing grasslands also tend to grow taller, which makes them better at stealing the sunlight they need to thrive. In one case, plants in secondary grasslands still towered over their old-growth counterparts even 300 years later.
Why this matters and what can be done
These findings are important because grasslands are far more than just empty fields. They play a critical role in storing carbon, preventing erosion, and supporting biodiversity and human livelihoods. The fact that secondary grasslands do not easily recover the full suite of original species means that simply allowing a plowed field to regrow may not be enough to restore a functioning grassland ecosystem.
Brudvig said that active intervention, such as sowing seeds and transplanting, is needed if secondary grasslands are to function like old-growth ones. Nerlekar emphasized that preserving existing old-growth grasslands matters because "you don't quickly get back what is lost."
The study was supported by grants from FAPESP, CNPq, and other organizations.