Impact of Initial Spacing on Plantation Black Spruce Lumber Grade Yield, Bending Properties, and MSR Yield
For decades, initial spacing of 2 m X 2 m has been used for black spruce (Picea mariana) reforestation in eastern Canada. In recent years, however, wider spacings for black spruce are being advocated to reduce establishment costs and accelerate tree growth. Wider spacings will affect not only return on investment but also the quality of products from the plantations, both of which are critical to the success of reforestation programs. As part of a multidisciplinary project, this study evaluated and quantified the impact of initial spacing on lumber grade yield, bending properties, and MSR yield in this species. Furthermore, visual grades of the plantation-grown lumber were compared for their bending properties and their compliance to the current grade requirements for bending stiffness. A total of 139 sample trees were collected from 4 different spacings (3,086, 2,500, 2,066, 1,372 trees/ha) in a 48-year-old initial spacing trial, and 849 pieces of 2-in.-thick lumber from the 4 spacings were graded visually and tested for bending strength and stiffness.
With decreasing initial stand density from 3,086 to 2,066 trees/ha, branch diameter showed a steady increase. However, the 3 higher stand densities (3,086, 2,500, and 2,066 trees/ha) had a comparable Select Structural (SS) grade yield thanks to the relatively small branches in this species. Lumber strength and stiffness in those 3 spacings were also quite comparable. When the initial stand density was further reduced to 1,372 tree/ha, however, a remarkable decrease in the SS grade yield due to knots occurred, and lumber strength and stiffness also decreased significantly. The real concern occurred when the plantation-grown lumber was compared to that from natural stands currently being processed in eastern Canada. On average, the plantation-grown black spruce lumber stiffness was 28.9% lower than that of lumber from the natural stands. As a result, a high percentage of the plantation-grown lumber did not meet the bending design values. However, the percentage of the compliance to the design values tended to increase with increasing initial stand density. This article discusses the possible causes for the significantly lower bending properties of the plantation-grown lumber, and potential solutions for increasing lumber properties and the percentage of the compliance.
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