The narrow rods peeking out from the walls of the clay mine didn’t initially look like much. But as more of the spindly fossils appeared, palaeobotanists Deming Wang and Min Qin soon realised they were in the midst of an ancient forest.
Discovered near the town of Xinhang, China, the fossilized tree trunks date back to about 365 million years ago, and cover at least 250,000 square metres, which is roughly the size of 47 American football fields. This means they now represent the oldest forest yet found in Asia, the researchers report today in the journal Current Biology.
These prehistoric trees would not have looked like the woody, branched plants that shade our modern landscape, instead growing as simple, straight stalks. Their root system, however, was a surprisingly advanced variety that paved the way for the swampy forests of the Carboniferous period, the buried remnants of which make up the vast majority of the coal humans burn today.
“This is what fired the Industrial Revolution,” says Christopher Berry, a palaeobotanist specialising in the rise of forests who was not part of the study team. “This is the basis of our present civilisation; this little [root] structure, which we see for the first time in this forest.”
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Stalking ancient stalks
Qin and Wang found the first fossils of the Xinhang forest in 2016 on a routine visit to the Jianchuan clay mine, where they were scouring the quarry walls for traces of past life. As they looked, they found stem after stem, many preserved as they likely once stood in life. (Learn about an ancient Antarctic fossil forest.)
These ancient plants belong to a group called the lycopsids, many of which grew straight upward like poles. The Xinhang trees started out as single stalks fringed with narrow leaves, but once a stalk reached adulthood, it would split into a crown of drooping limbs, each adorned with a spore-filled cone.
The ancient forest found in Xinhang would have featured trees that grew straight upward like poles, their stems fringed with narrow leaves. Once mature, the tree tops would have split into crowns of droopy branches, each tipped with a spore-filled cone.
IMAGE BY DEMING WANG
The exactly environment this languid-looking forest once inhabited remains to be seen, and more analysis of the sediments is necessary to tease out more clues, Berry says. But it’s likely these trees stood in what was once a coastal swamp that was frequently inundated by floods.
One such flood may have washed in a hefty load of sediment that quickly buried the trees. Though more saplings eventually took root, floodwaters soon buried them again. Parts of the trunks rotted away and then filled with sediments to create casts, while other parts, like the roots, mineralised into dark fossils that now stand in stark contrast to the muted rock surrounding them.
The events gives "us a ‘history book’ about that world," Qin, of Linyi University, says via email.
While the forest was expansive, the trees were probably relatively stumpy by modern standards. Based on the thickness of the trunks, the team estimates that most of these trees were less than three metres tall.
The challenge is that many of the lanky trunks were broken during fossilisation, so it’s difficult to know their height for sure. A conservative estimate is probably around 1.5 to 1.8 metres, says Patricia Gensel of the University of North Carolina at Chapel Hill, who specialises in the plants of the Devonian period, between 416 to 359 million years ago.
“It might be a forest that’s only as tall as we are,” she says.
The Xinhang forest dates to a transformative time in arboreal history, when trees were rapidly inventing new ways to thrive in different environments. In the early Devonian, flora largely creeped across the landscape. If you could travel back some 416 million years, “you’d be wandering around just towering above all the plants,” says Ellen Currano, a palaeobotanist at the University of Wyoming.
“The evolution of forests,” she says, “is one of the really transformative events in Earth’s history.”
The branching roots of the Xinhang trees, seen here in fossil form, were advanced for their time. Later versions of these roots allowed the larger trees of the Carboniferous to thrive in swampy environments.
PHOTOGRAPH BY DEMING WANG
Throughout the Devonian, plants figured out how to get bigger, developing ways to move water up and down their stalks, growing broader leaves for photosynthesis, developing structural support for their trunks, and more. They started reaching higher into the sky, and collecting in thickets. The oldest such forest yet found is the 385-million-year-old Gilboa fossil forest in the Catskill Mountains of upstate New York.
But to grow ever taller, trees also needed a better way to root themselves in place, which is why researchers are so excited that the Xinhang trees have such surprisingly advanced roots.
A similar stand of lycopsid trees in Svalbard, Norway, is a mere 20 million years older than the Xinhang forest, and those trees stood on bulbous bases rimmed with small roots, says Berry, who investigated these trees for a project funded by a National Geographic grant. By contrast, the giant relatives of the Xinhang trees, which dominated swamps millions of years later, commonly had what are known as stigmarian roots, which split into branches surrounded by tiny rootlets.
The latest discovery is the most complete evidence yet found of stigmarian roots at this early date. Such a branched structure provided a strong anchor for early trees, eventually helping them reach new heights.
“It’s going to rewrite the story of lycopod evolution,” Gensel says.
Coal’s ancient roots
These types of branching roots also contained cavities that helped oxygen reach the plants’ lower extremities, allowing them to thrive in water-logged swamps. This marked a turning point for future human populations around the world, Berry says.
Massive stretches of swamp-based forests exploded, sucking down carbon dioxide from the ancient atmosphere and storing it in their arboreal bulk. When members of these groves died, they’d pile onto the wet swamp floor, where the lack of oxygen slowed down decay. Eventually, this carbon-rich system transformed into the thick coal seams mined around the world today.
By burning that coal, modern populations are pumping ancient carbon back into our atmosphere, sending carbon dioxide levels soaring to heights not seen since modern humans first appeared on Earth.
And by clearcutting today’s forests and paving over the landscape, Gensel says, “we’re cutting off our source of CO2 drawdown at a time when CO2 levels are rising at the fastest pace they ever have.”
This all emphasises just how much we rely on plants for atmospheric balance, she says: “Without it, we’re really going to heat up and be in trouble.”