This study examined the long-term effects of initial spacing on tree growth, wood characteristics and product quality and value in white spruce. The study is based on the oldest initial spacing trial established by the Ontario Ministry of Natural Resources in Thunder Bay. Three plantation spacings were examined: 1.8 m, 2.7 m and 3.6 m. 58 trees were sampled across the DBH classes present, bucked to maximize the production of 16 ft-long logs, and converted into lumber. Each piece of lumber was visually graded and tested in static bending to determine its lumber stiffness (modulus of elasticity, MOE) and strength (modulus of rupture, MOR), and MSR grade.
Sixty years after establishment, individual tree growth was greatly favoured by larger spacings. Tree diameter increased by 39% and 66% in the 2.7 m and 3.6 m spacings, respectively, compared to the narrowest 1.8 m spacing. This resulted in a remarkable volume increase of 129% and 159%. Stem taper increased from 0.93 cm/m in the 1.8 m spacing to 1.40 cm/m in the largest spacing. Lumber volume proportions of No.2 & Better grades were 85.7%, 86.8% and 80.4% in the 1.8, 2.7 and 3.6 m spacings, respectively. Expectedly, increased initial spacing favoured the production of large piece size. The volume proportion of 2x8 was 25.5% and 10.2% in the 3.6 m and 2.7 m spacings, but only 0.4% in the 1.8 m spacing. The 2.7 m spacing yielded the best total product value (lumber, chip, sawdust) per hectare ($50,817/ha), followed by the 1.8 m spacing ($44,043/ha). However, the 3.6 m spacing yielded the lowest plantation value ($37,231/ha) because of its low initial stocking.
Lumber stiffness decreased markedly with increasing spacing, from a maximum of 9427 MPa in the 1.8 m plantation, to 8031 MPa and 7476 MPa in the 2.7 and 3.6 m spacings. Similarly, through the same range of spacings, lumber strength decreased steadily from 37.2 MPa, to 30.1 MPa, and 27.1 MPa. The 2.7 and 3.6 m spacings had a major negative impact on lumber MOE (–15% and –21%), lumber strength (–19% and –27%), and a minor impact on wood density (–2% and –4%). MSR yields of No.2 & Better lumber grades decreased markedly with increased spacings. In the 1.8 m stand, 69.2 % of the lumber volume was graded MSR 1650f-1.5E, in comparison to 46.2% and 15% in the 2.7 m and 3.6 m stands, respectively.
In conclusion, this study indicates that initial spacing in white spruce should not exceed 1.8 m from a wood quality perspective, as wider spacings significantly decreased lumber stiffness and strength (MOE and MOR). However, if the silvicultural objective were to maximize economic value regardless of lumber mechanical properties, the 2.7 m spacing would give the highest plantation value.