Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current year growth.

Xylem vessel structure changes as trees grow and mature. Age and development related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water-stress induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current year growth of young (1-4 yr), intermediate (2-7 yr), and older (3-10 yr) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding of plant hydraulic function and architecture requires the sampling of multiple aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.