Variation in xylem anatomy was found both between and within clones of Picea sifchensis over successive years, and between sites supporting fast and slow growth rates. Increases in tracheid transverse area contributed most to increases in annual wood cross-sectional area. However, changes in wood density, associated with the development of juvenile wood, were not simply related to measured changes in anatomy, including mean cell numbers, tracheid area, tracheid shape, cell wall projection area, estimates of cell wall density, and ray sizes and areas. Interactions between anatomical development processes in the histogenesis of xylem tissue are discussed, and it n suggested that different compensatory processes may occur in histogenesis as the tree grows. Anatomical measures are less likely to be effective in selecting for high juvenile wood density than are differences in the relative rates of wood volume and wood weight growth along the main steam.