This is how the world's earliest trees on Earth grew

New Delhi: How did the earliest trees on Earth grow? A joint research project by scientists from China, Britain and the US has unveiled the mystery.

A species of fern, called Cladoxylopsida, appeared on Earth around 390 million years ago and became more common over 100 million years ago before dinosaurs existed.

Researchers from the Nanjing Institute of Geology and Palaeontology, Cardiff University and the State University of New York conducted the research based on well preserved fossils of tree trunks unearthed in China's Xinjiang Uygur region.

The density of carbon dioxide in the air dropped at that time, allowing tetrapod vertebrates to transition to living on land.

The scientists found that the early tree trunks were composed of numerous individual strands of xylem, the plant's water-conducting cells. The structure allowed the tree to have several sets of annual ring networks, compared to just one set in modern trees.

Xu Honghe, from the Nanjing institute and chief scientist on the project, said each of the xylem strands could expand to add additional annual rings, or divide to produce a new xylem strand system.

"The extinct fern was the earliest tree species that formed forests. Its multi-xylem strand structure was capable of producing trees of large size that cannot be found today," Xu said.

He said the research would help scientists better understand the early evolution of the terrestrial ecosystem, and allow for further comparative study of early and modern trees.

The research was published in the science journal PNAS on Tuesday.

(With IANS inputs)