By Dennis Normile Oct. 23, 2017 , Science

Fossilized slices of a 374-million-year-old tree reveal a hollow core surrounded by numerous bundles of woody strands called xylem (the larger black spots), with soft tissue (in gray) between. The smaller black dots are roots. Dr. Chris Berry/Cardiff University

　　Scientists have discovered some of the best preserved specimens of the world’s first trees in a remote region of China. At up to 12 meters tall, these spindly species were topped by a clump of erect branches vaguely resembling modern palm trees and lived a whopping 393 million to 372 million years ago. But the biggest surprise is how they got so big in the first place.

　　Today’s trees grow through a relatively simple mechanism. The trunk is a single cylindrical shaft made up of hundreds of woody strands called xylem, which conduct water from the roots to the branches and leaves. New xylem grow in rings at the periphery of the trunk just behind the bark, adding girth so the tree can get taller.

　　This is not how ancient trees known as cladoxylopsids grew, however. Two specimens discovered in a desert in China’s northwestern Xinjiang province in 2012 were remarkably well preserved. That’s because they underwent a process in which silica—likely emitted by a nearby volcano—saturated the tree and took on the shape of the wood’s internal structure as it decayed, preserving its 3D cellular structure.

　　The fossils reveal that, unlike modern trees with a single shaft, cladoxylopsids had multiple xylem columns spaced around the perimeter of a hollow trunk. A network of crisscrossing strands connected the vertical xylem—much like a chain-link fence spreads from pole to pole—and soft tissue filled the spaces between all these strands. New growth formed in rings around each of the xylem columns while an increasing volume of soft tissue forced the strands to spread out.

　　All of this expanded the girth of the trunk, allowing for a taller tree. But it also split apart the tree’s xylem skeleton, which required the tree to continually repair itself, the team reports today in the Proceedings of the National Academy of Sciences. The weight of the tree squeezed tissue at the base of the trunk outward.

　　An artist’s impression of a stand of cladoxylopsida trees, which formed Earth's first forests. Peter Geisen

　　In the largest of the two fossil trunks, above the bulge, the xylem and soft tissue occupied a ring about 50 centimeters in diameter and 5 centimeters thick, with external roots making up the remainder of the 70-centimeter-diameter tree trunk. The scientists estimate cladoxylopsids could have been 8 to 12 meters tall.

　　This growth strategy has not been seen in any other tree in Earth’s history, says Xu Hong-He, a paleontologist at the Nanjing Institute of Geology and Paleontology in China who discovered the fossilized tree trunks. “It's crazy that the oldest trees also had the most complex growth strategy,” adds Christopher Berry, a plant paleontologist at Cardiff University in the United Kingdom who helped analyze the fossils.

　　The trees are particularly important, says Berry, because they dominated Earth during the Devonian period from 419 million to 358 million years ago. They formed the first forests and played a key role in absorbing carbon dioxide from the atmosphere. They also added oxygen to the atmosphere, affecting the climate and influencing conditions that fostered the emergence of other life forms, he says.

　　Despite their early critical role in the evolution of life on Earth, the cladoxylopsids do not have any modern descendants. They disappeared at the end of the Devonian period, perhaps because they were left in the shade of taller, more robust trees, or because changing environmental conditions may have favored Archaeopteris, the ancestors of modern trees that appeared about 385 million years ago.

　　The new study is an important step in solving several such mysteries about early Earth, says Brigitte Meyer-Berthaud, a paleobotanist at the University of Montpellier in France who was not involved in the research. To understand the role of cladoxylopsids on our planet’s past, she says, “it is essential to know how the trees are constructed.”