• Mithcell, M.D.
  School of Agricultural and Forest Sciences, University of Wales, Bangor, Wales
  
• Denne, M.P.
  School of Agricultural and Forest Sciences, University of Wales, Bangor, Wales
  

The influences of cambial age and ring width on density of Sitka spruce (Picea sitchensis (Bong.) Carr) were analysed in relation to within-tree trends in tracheid diameter and cell wall thickness. Discs were sampled at breast height from a total of 24 trees, from seven stands at three contrasting sites in Wales, and at breast height, 30 per cent and 60 per cent total tree height from one of the stands.

Across the juvenile wood, ring density decreased with ring number from the pith while radial tracheid diameter increased. These overall trends were considered to be inherent to tree growth, presumably associated with cambial ageing, since they occurred in all trees on all sites. In juvenile wood, density also varied with site growth rate (as indicated by ring width) at similar cambial age, wider rings being associated with more rapid rate of change in tracheid diameter with ring number and with decrease in tracheid wall thickness. Consequently, on a site having trees with high growth rate density decreased more rapidly across the juvenile wood, down to a lower minimum value, than on sites with a slower growth rate. In mature wood, the decrease in density with increase in ring width was associated with differences in both tracheid diameter and wall thickness. Density was slightly (though not significantly) higher at breast height than in comparable rings at 30 per cent total height, associated with significantly thicker tracheid walls at breast height.

Changes in radial tracheid diameter (with ring number, or with ring width) were associated with greater differences in the earlywood than towards the latewood end of each growth ring, while variations in wall thickness with ring width were associated with rate of increase in wall thickness toward the latewood. This may account for some previously conflicting reports on influence of silvicultural management on density, for density is likely to vary with influence of environment on the seasonal cycle of cambial activity.

The extent of the juvenile wood as delimited by the inner core of wide growth rings does not neccessarily correspond to the region of varying tracheid dimensions in Sitka spruce.