Influence of spacing and thinning on wood properties in conifer plantations

Abstract: As reported in four appended papers (I-IV), effects of genotype (half-sib family) and initial spacing (I) or initial spacing (II) and various thinning methods (III & IV) on stands and properties of sawn-products were investigated in Scots pine (I & IV), lodgepole pine (II) and Norway spruce (III) stands in southern Sweden. Most of the analysed growth parameters were found to be affected by interaction between genotype and initial spacing, but the treatments had no significant interactive effects on most quality parameters (I). Very wide initial spacing (4x4 m, 625 trees/ha⁻¹) was found to negatively affect most growth and quality properties of lodgepole pine. Differences in effects of the density currently used in forestry (2500 trees/ha⁻¹) and a tested density (1100 trees/ha⁻¹, 3x3 m), were quite small, but increasing the spacing decreased frequencies of potential high quality trees. The results indicate that Scots pine and lodgepole pine have similar volume production rates and external quality traits (II). External properties of trees in the stands, particularly stem straightness and frequencies of quality defects (e.g. spike knots), were affected by the selection strategies applied in the thinning treatments. Thus, selection in thinnings should focus on these traits as they affect both the quality and grades of logs (III). The applied thinning treatments did not significantly influence board properties, such as basic density, Modulus of Elasticity and Knot Area Ratio. However, differences in these variables between outer and inner boards indicate that quality improvement might be connected to improvement in the growth of individual trees (IV). In summary, use of improved planting material and wider than currently applied 2x2 m spacing in combination with selection of high quality trees might lead to production of higher quality timber. However, the scope for selection decreases with time and since the Scots pine boards from thinned stands and unthinned stands did not significantly differ the target traits and both objectives and market conditions must be carefully considered.