VARIATION IN DENSITY OF PICEA-SITCHENSIS IN RELATION TO WITHIN-TREE TRENDS IN TRACHEID DIAMETER AND WALL THICKNESS

The influences of cambial age and ring width on density of Sitka spruc e (Picea sitchensis (Bong.) Carr) were analysed in relation to within- tree trends in tracheid diameter and cell wall thickness. Discs were s ampled 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 a nd 60 per cent total tree height from one of the stands. Across the ju venile wood, ring density decreased with ring number from the pith whi le radial tracheid diameter increased. These overall trends were consi dered to be inherent to tree growth, presumably associated with cambia l ageing, since they occurred in all trees on all sites. In juvenile w ood, density also varied with site growth rate (as indicated by ring w idth) at similar cambial age, wider rings being associated with more r apid rate of change in tracheid diameter with ring number and with dec rease in tracheid wall thickness. Consequently, on a site having trees with high growth rate density decreased more rapidly across the juven ile wood, down to a lower minimum value, than on sites with a slower g rowth rate. In mature wood, the decrease in density with increase in r ing width was associated with differences in both tracheid diameter an d wall thickness. Density was slightly (though not significantly) high er at breast height than in comparable rings at 30 per cent total heig ht, associated with significantly thicker tracheid walls at breast hei ght. Changes in radial tracheid diameter (with ring number, or with ri ng width) were associated with greater differences in the earlywood th an towards the latewood end of each growth ring, while variations in w all thickness with ring width were associated with rate of increase in wall thickness towards the latewood end. This may account for some pr eviously conflicting reports on influence of silvicultural management on density, for density is likely to vary with influence of environmen t on the seasonal cycle of cambial activity. The extent of the juvenil e wood as delimited by the inner core of wide growth rings does not ne cessarily correspond to the region of varying tracheid dimensions in S itka spruce.