Plants with thick, fleshy, pointed green leaves in the foreground, brown fossil in the background
This Huperzia squarrosa (club moss) is a living relative of the ancient lycophytes whose fossils were analyzed in the study. Credit: Magnus A. Harding

Trees often are seen as great oxygenators, driving an early decarbonization of Earth’s atmosphere as forests first evolved.

But a new study published in Nature Communications goes back in time to show that in fact, the atmosphere may have had a lot less carbon dioxide (CO2) before this time, indicating that trees likely had less of an impact than previously thought. The study highlighted the role of small plants in shaping ancient environments.

Measuring Carbon from Long Ago

Previous studies of Earth’s early atmosphere suggested that before forests emerged, atmospheric CO2 levels hovered between 2,000 and 8,000 parts per million. When trees appeared in the Late Devonian (383–359 million years ago), CO2 levels may have dropped below 1,000 parts per million. However, direct measurements of the atmosphere from this period have high uncertainty, and just how much CO2 was present has remained unclear.

Looking to reconstruct atmospheric CO2 levels, researchers developed a method to estimate how much of the gas plants were taking in just before trees evolved. Because they could not directly measure the amount of gas in fossil plants from that time, the researchers calibrated their method using living descendants of Devonian lycophytes—club mosses physiologically similar to those that lived almost 400 million years ago.

The researchers grew lycophytes under different CO2 levels and learned that plants adapt to minimize water loss through their leaves. Such water loss is reflected in the abundance of carbon isotopes in leaves and in the density and size of their stomata—the tiny pores in plant tissue that regulate the gas and water that go in and out—explained lead author Tais Dahl, an associate professor of geobiology at the University of Copenhagen. When carbon dioxide levels are high, plants grow fewer and smaller stomata.

“Stomata density and size [in] ancient and modern lycophytes are very similar, so there’s no reason to think they would work too differently.”

“Stomata density and size [in] ancient and modern lycophytes are very similar, so there’s no reason to think they would work too differently, though early plants probably had to weather more in order to survive,” said Dahl.

With information about the relationship between carbon dioxide, lycophyte characteristics, and relative carbon isotope abundance, the researchers estimated the CO2 content of the air in which 66 fossil lycophytes grew. The fossils were diverse, representing three lycophyte genera from 13 geological deposits at nine distinct localities.

The data suggested that even before trees evolved, there was a lot less carbon dioxide in the air than previously thought: around 525–715 parts per million—just 30%–70% higher than today (~415 parts per million).

The evolution of forests, therefore, might not have driven a major decrease in atmospheric carbon dioxide. Shrub-like plants, on the other hand, might have received less credit than they are due.

Sequestering Carbon

The first plants with vascular tissue—a bundle of cells that allows plants to move liquid and nutrients internally—evolved millions of years before trees.

The roots of these ancient plants were crucial to their survival, Dahl explained. Because roots absorb most of the carbon dioxide plants take in, they are vital to pulling CO2 out of the atmosphere. By spreading into the soil, roots tilled land, exposing more of it to chemical weathering, the process by which rocks and minerals are altered at a molecular level. CO2 in the atmosphere combines with rainwater or groundwater to form carbonic acid, which slowly dissolves rock, trapping CO2 below the soil in the form of bicarbonate, until runoff carries it into rivers and eventually oceans, Dahl explained.

As ancient plants were mostly rooted in nutrient-poor soil, they demanded a lot of atmospheric carbon to grow and survive. Trees (and other modern vascular plants) recycle soil carbon and other nutrients such as phosphorus, Dahl said, and “for that reason, they do not need to weather new rocks.” Early vascular plants didn’t have this advantage.

These early shrub-like species, therefore, may have significantly contributed to driving down the amount of carbon dioxide in the atmosphere, Dahl said.

New Records

Combining carbon isotope measurements and stomata analysis in plant fossils “presents a new proxy for atmospheric CO2 in a period in which the data we have about CO2 levels are quite uncertain,” said Jeremy Rugenstein, a paleoclimatologist in the Department of Geosciences at Colorado State University who was not involved in the new research.

“But it might be a bold assumption to presume ancient plants worked in similar ways to their modern descendants.”

The problem, he said, is simply that there aren’t many vascular plant fossils from more than 400 million years ago. “It is really interesting that they have taken probably some of the oldest vascular plants we know to apply this method,” he said. “But it might be a bold assumption to presume ancient plants worked in similar ways to their modern descendants.”

Rugenstein pointed out that even if the modern lycophytes used for calibration were grown in different environments to test how they adapt to wet and dry settings, the conditions we have today might be completely different from those in the Devonian—including the soils of that time.

Also, as the model is calibrated for an 8-year period, it is not clear how plants would evolve in the long term under a high-CO2 setting, Rugenstein said. “It is impressive that this experiment was done in 8 years—but still, it is an 8-year experiment,” he explained. “This is the problem with CO2 proxies: We’re stuck on doing experiments in our human lifetime, but that’s not what we see in the geological record.”

To Daniel Ibarra, a biogeochemist and geoscientist at Brown University who was not involved in the research, the study added important data to paleoclimatology and geology research, but it does not clarify how Earth had such low CO2 levels. The actual impact of weathering and the role plants have in driving such weathering are phenomena to further investigate, he said.

“It would be interesting to see this method applied to the whole time series from the Devonian to our time,” Ibarra said. That would require great effort but would be an exceptional way to confirm the trends proposed by this study, he explained.

A good way to confirm the accuracy of the results would be to replicate the study for periods in which there are very solid atmospheric CO2 data, Rugenstein suggested. “Finding more lycophytes in other places and testing them would also be an interesting way to proceed.”

—Meghie Rodrigues (@meghier), Science Writer

Citation: Rodrigues, M. (2023), Small shrubs may have played a large role in decarbonizing the ancient atmosphere, Eos, 104, Published on 9 February 2023.
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