Do pachycaul trees have anything in common? Evidence from wood and bark anatomy

Summary Pachycaul trees are a distinctive group of woody plants with thickened stems that can store significant amounts of water and may have photosynthetic activity in some species. Anatomical observations provide direct evidence of the tissues involved in water storage, yet few studies have examined the wood and bark anatomy of these species. We hypothesise that pachycaul tree species exhibit convergent stem morphology with a specific water-storage tissue, reflecting adaptation to seasonal environments. Such convergent anatomies may occur in distantly related taxa, allowing the identification of groups with similar traits. We analysed anatomical traits of wood and bark in 20 pachycaul tree species from natural populations in Mexico. Our results indicate that the bark, with more than 70% non-lignified tissue, serves as the main site of water storage, although in several species, the wood also contributes to water storage. Non-lignified parenchyma in the wood, along with its cell division and elongation, represents an important trait in many pachycaul species. In the periderm, the phelloderm, characterised by multiple cell layers and the presence of chloroplasts, is a typical feature of most species. Variation in wood and bark traits, including ray dimensions, vessel diameter and frequency, total xylem parenchyma fraction, bark thickness, and cortical cell retention and division, allowed the identification of groups within the studied pachycaul trees. Overall, water storage emerges as a defining feature of this group, consistent with their highly seasonal habitats. However, xeromorphic or mesomorphic wood traits can also occur in pachycaul tree species.