Old Growth Strategy

A segment of a slice of fossilized Xinicaulis trunk showing several strands of cauline xylem (large dark circles in top right corner) and roots (smaller dark circles) amidst the tree’s soft tissue (grey).

Hong-He Xu and Christopher M. Berry

The first large trees appeared roughly 380 million years ago during the Devonian period. Fossils show that their internal structure was very different from and far more complex than that of most trees today—although similar to living palms. Instead of a single set of concentric rings, their trunks contained a ring of woody strands, each with its own set of concentric rings. New, uniquely preserved specimens have revealed the destructive growth process through which these ancient trees developed such a complicated inner structure.

Paleobiologists Hong-He Xu and colleagues at Nanjing Institute of Geology and Palaeontology, along with Christopher Berry, of Cardiff University, and William Stein, of the State University of New York at Binghamton, analyzed two 374 million-year-old Xinicaulis lignescens tree trunks found in the Xinjiang Province of northwest China. Unlike petrified trees, where all the organic material is replaced by stone, the Xinicaulis specimens were preserved by a process known as permineralization. Dissolved volcanic ash seeped into the wood, filling the spaces between and within cells with glass-like silica and leaving the hard cell walls to turn to coal. Washing thin cross-sectional slices of the trees in hydrofluoric acid made the tree’s black, coalified strands of xylem (water-conducting cells), clearly visible against the lighter silica, allowing the structures to be seen in detail—even down to individual cells.

Close observation of the tree slices showed that Xinicaulis contained two types of xylem: a ring of thick, stem-like strands of cauline xylem connected by a network of thin medullary xylem. While today’s trees grow by adding rings around a single trunk, Xinicaulis grew by adding new rings around each of its many cauline xylem strands, as if several tiny, thin modern trees were growing inside the ancient tree’s trunk. 

The permineralized fossils showed that, as the thick cauline xylem strands grew, the softer tissue between them also expanded, and the network of medullary xylem that held the tree together continuously broke apart and reformed. In other words, Xinicaulis trunks grew by continuously damaging and repairing its xylem skeleton. 

Now that Xu, Berry, Stein, and colleagues have established how Xinicaulis trunks grew, they hope to find fossilized branches of the species to determine how they were structured. (Proceedings of the National Academy of Sciences