Unique growth strategy in the Earth's first trees revealed in silicified fossil trunks from China.

Cladoxylopsida included the earliest large trees that formed critical components of globally transformative pioneering forest ecosystems in the Mid- and early Late Devonian (ca. 393-372 Ma). Well-known cladoxylopsid fossils include the up to ∼1-m-diameter sandstone casts known as from Middle Devonian strata of New York State. Cladoxylopsid trunk structure comprised a more-or-less distinct cylinder of numerous separate cauline xylem strands connected internally with a network of medullary xylem strands and, near the base, externally with downward-growing roots, all embedded within parenchyma. However, the means by which this complex vascular system was able to grow to a large diameter is unknown. We demonstrate-based on exceptional, up to ∼70-cm-diameter silicified fossil trunks with extensive preservation of cellular anatomy from the early Late Devonian (Frasnian, ca. 374 Ma) of Xinjiang, China-that trunk expansion is associated with a cylindrical zone of diffuse secondary growth within ground and cortical parenchyma and with production of a large amount of wood containing both rays and growth increments concentrically around individual xylem strands by normal cambia. The xylem system accommodates expansion by tearing of individual strand interconnections during secondary development. This mode of growth seems indeterminate, capable of producing trees of large size and, despite some unique features, invites comparison with secondary development in some living monocots. Understanding the structure and growth of cladoxylopsids informs analysis of canopy competition within early forests with the potential to drive global processes.