This study examined the impact of initial spacing on various tree and wood characteristics, product quality and value recovery in black spruce. The study was based on the oldest initial spacing trial established in 1950 near Thunder Bay, Ontario. In 1998, all the trees in the trial were measured and sample trees were collected from 4 different spacings (3086, 2500, 2066, 1372 trees/ha). For each spacing, 6 trees per DBH class were selected to cover all DBH classes in 2-cm interval. For each sample tree, major tree characteristics were measured: 1) total tree height, tree height up to 5 cm diameter top, tree height up to 9.01 cm diameter top (10cm DBH class), 2) DBH and stem diameter from the stump to the top at 1-m interval, 3) live crown width and length, and 4) average diameter of the 5 largest branches. Based on these measurements, other tree characteristics were calculated: 1) stem volume, 2) stem taper, and 3) length of the log without live crown. Each sample tree was bucked to 8-foot-long logs for lumber conversion. From the top of each log, a 5-cm-thick disc was removed for the evaluation of wood characteristics. Lumber conversion was carried out in a way which allows to keep track of the provenance of each piece of lumber. Logs from each spacing were processed separately so that chip samples could be collected for the evaluation of chip quality and pulping properties. Each piece of lumber was visually graded both before and after drying. In-grading tests were also performed to determine lumber strength and stiffness. Based on the sample trees, impact of initial spacing was evaluated first at the diameter class level and then at the stand level. Finally, a cost/benefit analysis was made for the 4 spacings. In addition, special attention was paid on the impact of initial spacing on lumber strength and stiffness. Mechanical properties of plantation-grown black spruce lumber were also compared to those of black spruce from natural forest.
Initial spacing in black spruce has a considerable effect on diameter growth of individual trees. It appears that tree diameter increases moderately when stand density decreases from 3086 to 2066 trees/ha. However, when stand density decreases further to 1372 trees, tree diameter increases considerably. With decreasing stand density, average live crown size and branch diameter of the black spruce plantations show a steady increase, and consequently the best grade (Select Structural) recovery tends to decrease. However, when grades No.2 and Better are combined (current market practice), no significant differences were observed among the 4 initial spacings. With decreasing stand density, average stem taper for trees of the same diameter class tends to increase. As a result, trees of the same diameter class from lower stand density generally tend to have a lower tree volume, and thus a lower lumber volume and value recovery per tree. On the other hand, lumber volume/value recovery per tree increases dramatically with tree diameter. Consequently, at the stand level, the lowest stand density still have a considerably higher lumber volume/value recovery per tree due to an increased average tree diameter. Despite the fact that trees in the lowest stand density are larger, the total stand (product) value per hectare is lower than in the case of the denser stand (3086 trees/ha) because it has fewer trees. However, the lowest stand density (1372 trees/ha) generates a better economic return than the highest stand density because of the lower initial investment and reduced harvesting and processing costs per m3 of resource.
Lumber from stand densities of 3086, 2500 and 2066 trees/ha has a comparable strength and stiffness. However, lumber strength and stiffness from stand density of 1372 trees/ha are respectively about 30 and 18% lower on average, than the other 3 stand densities. The major source of concern arises when the 48-year-old plantations are compared to the black spruce from natural stands being processed across eastern Canada. On average, lumber stiffness from the natural stands is about 60% higher than the average lumber stiffness of trees from plantation-grown trees of initial stand density of 1372 trees/ha. Results also indicate that lumber stiffness generally increases with increasing log height in the tree. However, lumber strength does not follow the same trend. In general, it appears that lumber strength decreases from the butt log until it reaches its lowest point and then tends to increase steadily with height.
While we are confident that the results of the impact of initial spacing on tree/wood characteristics and product recovery and value at the tree DBH class level reflect the general trend for this species within these initial stand densities. We believe that the trends at the stand level reported in this study may not necessarily apply to other sites because the tree diameter frequency distributions for different spacings and growing conditions may vary, which will affect the overall economic return. Therefore, it is necessary to apply the economic variables (costs and values) obtained at the tree level to data from more sites across eastern Canada before a recommendation on optimal initial spacing could be made for black spruce.