Variation of tracheid length within annual rings of Scots pine and Norway spruce

Variation of tracheid length was studied within individual annual rings of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) from the pith to the bark. The material consisted of six Scots pine and six Norway spruce trees growing on sites of both low and high fertility. Microtome sections of 0.25 mm thick were cut from annual rings 7, 20 and 50 counted from the pith outwards, i.e., juvenile, transition and mature wood, respectively. After maceration, tracheid lengths were separately measured in each sample. In juvenile wood of Scots pine, tracheids were on average 17% longer in the latewood than in earlywood. However, in juvenile wood, the first formed earlywood tracheids were slightly longer than those in the middle of the earlywood zone. In the transition and mature wood of Scots pine, the increase in tracheid length was more gradual from earlywood to latewood, and no significant differences were found between earlywood and latewood. In Norway spruce, tracheids were 2–4% longer in the latewood than in earlywood. In general, tracheid length is highly variable within annual rings and the variation can differ from ring-to-ring even within the same tree.