A naturally regenerated jack pine (Pinus banksiana Lamb.) trial established in 1966 in New Brunswick was studied to determine how three precommercial thinning intensities (1.22 m × 1.22 m, 1.52 m × 1.52 m, and 2.13 m × 2.13 m) and a control (154 trees in total) affected tree growth and lumber quality. Mild (thinned to 1.22 m) and moderate (1.52 m) thinning had a modest impact on tree growth after 34 years (stand age 59). However, intensive thinning (2.13 m, or 2212 stems/ha) increased tree height by 13.1% compared with the control, whereas tree diameter and merchantable stem volume per tree increased by >20% and >75%, respectively. Yields of No. 2 and Better increased slightly with increasing thinning intensity, but lumber bending properties decreased with increasing thinning intensity. There was, respectively, >20% and >15% difference in lumber strength (modulus of rupture) and stiffness (modulus of elasticity) between the mild (1.22 m) and intensive (2.13 m) thinnings. Intensive precommercial thinning (2.13 m) is recommended for increased volume growth, but rotation age (>59 years) should not be reduced if lumber bending properties are of concern.

The objective of this paper was to investigate the impact of initial spacing and pre-commercial thinning (PCT) on tree growth, characteristics and stem quality in plantation-grown jack pine (Pinus banksiana Lamb.) after 58 years of growth. Four spacings including three initial spacings without PCT treatment, i.e., 1.52 ϫ 1.52 m (5 ϫ 5 ft), 2.13 ϫ 2.13 m (7 ϫ 7 ft) and 2.74 ϫ 2.74 m (9 ϫ 9 ft) and one initial spacing with PCT treatment (5 ϫ 5 ft thinned to 7 ϫ 7 ft at age 14), were chosen to analyze diameter at breast height (DBH) distribution, survival, and various tree and stem quality characteristics (e.g., DBH, total tree height, stem volume, crown size, branch, taper, sweep and wobbling). Tree mortality during the first 25 growth years was quite low for all four spacings, while after 25 years, survival for the four spacings rapidly declined. The decrease in survival for spacing 5 ϫ 5 was the steepest, followed by spacings 7 ϫ 7, 9 ϫ 9 and finally the PCT-treated stand. DBH distributions for all four spacings were significantly different from each other, and five years after PCT, the 5 ϫ 5 to 7 ϫ 7 spacings had a similar DBH distribution to that of the constant 7 ϫ 7 spacing. Mean DBHs differed significantly among the initial spacings. The 5 ϫ 5 spacing produced significantly smaller trees than did the other three spacings through the years, and the 9 ϫ 9 spacing produced significantly larger trees. PCT had positive effects on DBH growth, and yielded longer merchantable stems and better stem quality. PCT had little effect on crown development. Initial spacing significantly affected diameter growth, crown width and branch development, commercial stem length and volume, stem taper and stem deformation. Narrower spacing produced smaller but better quality stems. Spacing 7 ϫ 7 had the worst stem quality in terms of overall sweep and wave-like transversal deformation (wobbling). A narrow initial spacing followed by a PCT treatment may provide a compromise solution for both tree growth and stem quality.