Modern climate has roots in Early Devonian.

New fossils show that sapling-size plants with substantial roots grew on land at least 10 million years earlier than paleobotanists had thought. The advent of such deep-rooted land plants drastically reduced atmospheric concentrations of carbon dioxide, scientists believe, thereby turning down the greenhouse effect and cooling the climate.

Plants 2 to 3 meters tall with roots almost 1 m long lived 390 million years ago near what is now Gaspe Bay in Quebec, report scientists from the University of Tennessee-Knoxville in the February Geology. The researchers found fossil traces of the above-ground portions of plants attached to roots.

"We assumed we would find maybe some roots ... and lo and behold, we came across these traces that were just wild--they were just huge," says Jennifer M. Elick, a coauthor of the report. "These root systems are larger than anything that has been previously documented [for this period]."

Land plants with water-conducting vessels appeared even earlier, more than 410 million years ago. Those early plants, only 5 centimeters tall, shared the land with a variety of arthropods (SN: 11/10/90, p. 292). Vertebrates did not clamber onto land until 60 or 70 million years later.

Around 390 million years ago, many plants were still primitive and leafless, says paleobotanist William A. DiMichele of the National Museum of Natural History in Washington, D.C. Typical vegetation was "creeping ground cover, like a Pachysandra without any leaves," he says, "just branching sticks." Most plants were thought to be rootless and restricted to wet habitats, he says.

The recently found fossils have more complex structures, however. Elick says the deep-rooted plants lived along seasonal streams and had to tolerate prolonged dry periods. Fossils of these plants exist today because the streams sometimes flooded, flattening the plants and burying them in sediment. As the plants decayed, clay replaced the roots and stems, yielding a cast of the plant.

"The environment was pretty harsh," Elick says. Having large roots helped the plants withstand such severe conditions, she adds.

Several researchers have suggested that by accelerating the weathering of soil, deep-rooted plants drew down the atmospheric concentration of carbon dioxide and changed the climate (SN: 12/9/89, p. 376).

As plants grow, they capture carbon dioxide from the atmosphere and release some of it via their roots into the soil. There, carbon dioxide combines with water to form carbonic acid, which leaches out calcium, forming either insoluble calcium carbonate or soluble calcium and bicarbonate ions that dissolve into the groundwater and ultimately end up at the bottom of the ocean. Either way, the carbon dioxide in the carbonate is removed from the atmosphere.

"This is what made the Earth a habitable place--changed an intolerably steamy greenhouse climate ... into one that is tolerable to humans," says Gregory J. Retallack of the University of Oregon in Eugene.

Computer simulations by Robert A. Berner of Yale University show that burgeoning vegetation would have produced a big drop in carbon dioxide concentrations. Estimates of ancient concentrations, derived from analysis of fossil soils, agree with those predictions.

Finding such large roots at an early time validates his model, Berner says, noting that plants' ability to alter soil is the key to the carbon dioxide decrease predicted by the model.

Berner's findings may receive an additional boost when the Tennessee researchers analyze the fossil soils found with the root casts. Elick says her next step is to determine the ratio of carbon-12 to carbon-13 isotopes in carbonates from the ancient soil (SN: 8/28/93, p. 140). That ratio is a clue to the atmospheric concentration of carbon dioxide 390 million years ago, when the fossil roots were part of a small streamside forest.