The objectives of the research project is to examine the short term (5 years) response of commercial thinning on tree growth, wood characteristics and product quality and value in a white spruce plantation located in Northeastern Ontario. While mechanized commercial thinning just recently became a more prevalent silvicultural prescription in the softwood forests and plantations of Eastern Canada, little information is available on the effects of intensive silviculture on tree growth and concurrent changes in wood properties.
In 1969, the study site was planted with bareroot white spruce seedlings at a spacing of feet. In 2003, when the plantation was 34 years old, a mechanical commercial thinning was conducted in a portion of the stand, and permanent sampling plots were established in both the control area and the thinned area. The thinning trails were 18 m apart and 5-6 m wide, representing approximately 30% of tree removal. At the time of thinning, the stand density was 2700 trees per hectare, of which approximately 60% were white spruce and the rest aspen, balsam fir and black spruce.
Five years later in 2008, sample trees were collected from each tree DBH class in the thinned area besides the permanent plots. Trees representing the control area (no thinning) were sampled from the buffer area of the thinning to maintain the integrity of the control area. The buffer area was a 15-20-m-wide strip, and trees were sampled in the middle of the strip and sampling was avoided in places where the strip was narrow (<18 m). The middle of the buffer area should represent the growth condition of the control area. A total of 56 trees covering 10 – 22 cm DBH classes was sampled and bucked into 2.5-m (8-foot) long logs. Lumber conversion was carried out with a portable sawmill. After kiln drying and planing, each piece of lumber was visually graded and tested in static bending to determine its lumber stiffness (MOE) and strength (MOR). Based on the sample trees, the impact of commercial thinning was evaluated at both the DBH class and stand levels.
White spruce responded moderately to commercial thinning 5 years after the treatment, in terms of individual tree growth. The average tree diameter increased from 13.1 cm in the control to 14.1 cm in the thinning, which represent about a 7% difference following the thinning. Merchantable stem volume per tree increased from 106.1 dm3 in the control to 125.1 dm3 in the thinned area, which is about 18% gain. No differences were observed in lumber volume, value and dimension recoveries between the control and thinned areas 5 years after commercial thinning.
For the sample trees, the Select Structural lumber grade recovery was slightly higher for the thinned area (29.1%) than for the control area (25.4%). Similar trend was observed for the No.2 & Better grade recovery. At the stand level, the Select Structural grade recovery and the No.2 & Better grade recovery were comparable between the two treatments. No differences were found in lumber stiffness and strength between the control and thinned area. The lumber modulus of elasticity (MOE) was 7.38 GPa and 6.92 GPa and the modulus of rupture (MOR) 35.8 MPa and 34.9 MPa in the control and thinned area, respectively.
In conclusion, based on this study, commercial thinning showed moderately positive effect on individual tree growth, however, no considerable difference in wood properties, lumber recovery and lumber quality was found between the control and thinning treatment 5 years after the commercial thinning. The effects of commercial thinning on tree and wood characteristics, lumber recovery, lumber quality, and economic return should be examined over a longer period of time.