Lessons and lies from Tree-Core Xylem Anatomy

The xylem conduits of annual rings in tree cores commonly extracted at breast height exhibit a classical pattern of progressive enlargement until conduit size approaches a maximum (Sanio’s first law), after which rings become markedly thinner, often described as juvenile and mature wood, respectively. This pattern is only apparently dependent on xylem age. Instead, it emerges from the cumulative growth of the tree through time and from the stable axial design of xylem architecture produced from the stem apex to the base. The axial widening pattern is remarkably consistent across years and undergoes only minor modifications under environmental stress. Analyses of whole-tree xylem architecture reveal that drought imposes carbon limitations, resulting in the formation of new xylem layers characterized by larger conduits near the stem apex and narrower conduits at the base. This suggests that, toward the apex, trees invest in sustaining leaf water supply by increasing xylem conductivity, even at the cost of higher embolism vulnerability. In contrast, at the stem base, conduits remain narrower because reduced non-structural carbohydrate availability and/or limited water supply constrain turgor, cell expansion, and the formation of larger cells. In this context, the hydraulic safety–efficiency trade-off read from tree rings risks being a compelling, but misleading, narrative.