Ten 38-year-old trees of Piceamariana (Mill.) B.S.P., grown at each of three spacings (1.8 × 1.8, 2.7 × 2.7, and 3.6 × 3.6 m), at Stanley, 30 km west of Thunder Bay, Ont., were used to study the impact of spacing on growth rate, relative density, and tracheid length of juvenile and mature wood. Increment cores of 12 mm diameter were extracted from the south aspect of each tree at breast height. The boundary of juvenile and mature wood was demarcated by the variation in tracheid length, which varied among trees from ring 11 to 21. Average growth rate, relative density, and tracheid length were obtained between the pith and boundary point (juvenile wood) and beyond the boundary point (mature wood). Differences between the levels of spacing for the three response variables in both juvenile and mature wood were tested using contrasts. Properties of juvenile and mature wood were found to be affected differently by the plantation spacing. Juvenile wood has a faster growth rate and shorter fibres than mature wood. Relative density was similar in both wood zones. The growth rate in juvenile wood was found to be significantly different among the spacing levels. For mature wood, only the growth rate at the 3.6 × 3.6 m spacing was significantly different from the other two spacing levels. The highest relative density, 0.39, in both juvenile and mature wood was found at the 1.8 × 1.8 m spacing. No significant difference in the relative density between the two wider spacings was observed. At the widest spacing, the relative density was 8% lower than that at the 1.8 × 1.8 m spacing. The longest fibre lengths were found at the intermediate 2.7 × 2.7 m spacing, 2.05 mm in juvenile wood and 2.94 mm in mature wood. Tracheid lengths of the 3.6 × 3.6 m spacing were significantly shorter than those of the other two spacings. The relative density and tracheid length of plantation grown wood were lower than those of natural grown wood by at least 5% for relative density and 33% for tracheid length.
Variation in cell length and the relationship between cell length and ring width and circumferential growth rate were studied in jack pine (Pinus banksiana Lamb.), balsam fir (Abies balsamea Mill.), white spruce (Picea glauca Voss), black spruce (Picea mariana Britton, Sterns & Pogg.) and trembling aspen (Populus tremuloides Michx.) collected in the natural forest in Ontario, Canada. There was a negative relationship between cell length and ring width in jack pine, balsam fir and black spruce, and a positive relationship in trembling aspen. No relationship was found in white spruce. There was a negative relationship between tracheid length and circumferential growth rate in all conifers. In trembling aspen fibre length decreased in both higher and lower circumferential growth rate. Circumferential growth rate is a good index of the effect of tree growth on cell length.
Variation of sapwood thickness, in terms of a linear measurement (sapwood width) and a growth ring count (sapwood ring), in relation to age, height, aspect, and radial growth rate was studied in jack pine (Pinusbanksiana Lamb.) and tamarack (Larixlaricina (Du Roi) K. Koch). In general, jack pine has more sapwood rings and a greater sapwood width than tamarack. In jack pine, the number of sapwood rings steadily declined with increasing height, but in tamarack, the number of sapwood rings at first increased and then declined with increasing height. Sapwood width tended to show a species-specific constant thickness along the trunk, but both species exhibited a slight increase at the base and at the crown. The number of sapwood rings shows strong correlation with age, height, and sapwood radial growth rate, but not with sapwood width. In both species, the south aspect of the tree has wider sapwood and fewer sapwood rings than the north aspect. There is no statistical relationship between sapwood width and the number of sapwood rings.
The distribution and vertical variation of juvenile wood was studied in an 81-year-old dominant tree and an 83-year-old suppressed tree of Larixlaricina (Du Roi) K. Koch. Two criteria, growth ring width and tracheid length, were used to demarcate the boundary of juvenile wood. The width of juvenile wood, expressed in centimetres and the number of growth rings, decreased noticeably from the base to the top of the tree. The volume of juvenile wood decreased in a similar pattern. These decreasing trends had a strong negative correlation with the year of formation of cambial initials at a given tree level. The length of these cambial initials decreased with increasing age of formation of the cambial initials. In the juvenile wood zone, there was a positive linear regression between the growth ring number (age) and the tracheid length. The slopes of these regression lines at various tree levels increased as the age of the year of formation of the cambial initials increased. At a given tree level, the length of tracheids increased from the pith to a more uniform length near the bark. However, the number of years needed to attain a more uniform tracheid length decreased from the base to the top of the tree. These relationships suggest that the formation of juvenile wood is related to the year of formation of the cambial initials. Consequently, the juvenile wood is conical in shape, tapering towards the tree top.
Survival rate and the newly developed nuclear irregularity index (NII) of sapwood ray parenchyma cells were studied within single trees of four species: Pinusbanksiana Lamb., Piceamariana (Mill.) B.S.P., Abiesbalsamea (L.) Mill., and Populustremuloides Michx. The survival rate of ray parenchyma cells is defined as the number of living earlywood ray parenchyma cells in uniseriate rays, divided by the total number of dead and living ray parenchyma cells recorded, multiplied by 100. NII is defined as the ratio of the number of irregularly shaped nuclei of uniseriate ray parenchyma cells to the total number of the irregular and regular nuclei recorded in earlywood, multiplied by 100. The location where death of ray parenchyma cells was first seen in the sapwood varied with species from the second to the seventh growth ring, counted from the cambium. In general, the marginal cells in the outer sapwood died earlier in a given growth ring than the central cells. The survival rate of the sapwood ray parenchyma cells decreased curvilinearly from the outer or middle sapwood towards the boundary of sapwood and heartwood. Based on survival rate classification, Pinusbanksiana and Populustremuloides are type II species, in which some ray parenchyma cells die in the middle or inner sapwood and the number of dead cells increases from the middle sapwood towards the heartwood. Piceamariana and Abiesbalsamea are type III species, in which some ray parenchyma cells die in outer sapwood and the number of dead cells increases from the outer sapwood towards the heartwood. NII increased from the middle of the sapwood towards the sapwood–heartwood boundary and reached its maximum at the growth ring immediately adjacent to the heartwood. NII increased from May to a maximum in the middle of the growing season and then decreased sharply. The months of sharpest decline of the NII in Pinusbanksiana, Piceamariana, and Populustremuloides were August, July–August, and August–October, respectively. In Abiesbalsamea no sharp decline of NII was observed. The findings of this study are in agreement with those of other investigators who used different criteria to indicate the initiation time of heartwood formation. Thus it appears that NII can be added to the list of indicators that pinpoint the initiation time of heartwood formation.
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