Evolution

The World's First Trees Didn't Have Rings

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Every child knows that you can tell the age of a tree by counting its rings. For scientists studying a 374-million year old tree, that posed a problem — and not because it's really easy to lose track of your count when you pass 20 million or so. The trunk of this tree didn't have any rings at all, and that completely changes our understanding of how trees evolved.

A foreman uncovers a fossilized tree trunk, thought to be more than 350 million years old, at a quarry in upstate New York in the 1920s.

Cladoxlopsid Park

In May 2017, a team of researchers from Cardiff University in Wales, Nanjing Institute of Geology and Palaeontology in China, and State University of New York announced that the trunk of an ancient tree belonging to the group called cladoxlopsids — the ancestor of modern trees and ferns — looks nothing like the trees of today.

Tree rings are composed of xylem, a tissue that transports water from a tree's roots to its tips. In most trees, xylem grows in a single cylinder just under the bark. New wood grows on top of that, and the tree gets ever bigger in a predictable yearly cycle. (Interestingly, palm trees don't have rings. Their xylem grows in strands, which is why a cross-section of a palm tree is covered in dots instead of rings.)

But this ancient tree fossil looked nothing like that. Instead of growing xylem in yearly rings or in strands distributed throughout, this tree grew narrow strands in the outer five centimeters of its trunk, connected together in a web formation. That means that instead of rings, the cross-section of the fossil is covered in what like Dalmatian spots. Each of those spots, or strands, grew its own xylem rings, creating what was essentially tree trunks growing within a tree trunk. As those interconnected mini-trees got bigger, their connections split apart and repaired themselves to keep the tree from breaking under its own weight. The result was a huge tree with a flat base and a bulbous trunk.

It's Complicated

"There is no other tree that I know of in the history of the Earth that has ever done anything as complicated as this," said Cardiff University researcher Chris Berry in a press release. "The tree simultaneously ripped its skeleton apart and collapsed under its own weight while staying alive and growing upwards and outwards to become the dominant plant of its day."

What's unusual for evolution (but not unheard of) is the fact that these trees aren't any less complicated than modern ones. Why would the oldest trees use such a complex growth strategy? To answer this question, the researchers will need to find more fossils just as well-preserved as this one.

To learn more about how modern trees came to be, check out The Tree: A Natural History of What Trees Are, How They Live, and Why They Matter by Colin Tudge. The audiobook is free with a 30-day trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Why Do Trees Have Rings?

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Written by Ashley Hamer December 27, 2017

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