The aim of the research project is to examine the long term (31 years) effects of late commercial thinning at wide spacing on tree growth, wood characteristics and product quality and value in a natural jack pine stand located in New Brunswick. Because mechanized commercial thinning has just recently become a more prevalent silvicultural prescription in the softwood forests and plantations of Eastern Canada, little information is available on the long-term effects of intensive silviculture on tree growth and concurrent changes in wood properties.
In 1976, when the even-aged jack pine stand was approximately 40 years old, Natural Resources Canada, Canadian Forest Service conducted an operational commercial thinning from below in a portion of the stand. The commercial thinning operations resembled current mechanized practices as the extraction trails were approximately 20 m apart and were 3 to 4 m in width. Trees were spaced to about 2.7 m to 2.7 m (50% removal of basal area) which represent the upper limit of what is done today in Eastern Canada.
Thirty-one years later in 2007, sample trees were collected from each tree DBH class in a control area (natural-origin) and the heavily thinned area for wood quality evaluation. A total of 85 trees was measured and bucked into 3.1 m (10-foot) long logs. Lumber conversion was carried out in two separate batches so that chip samples could be collected for fibre-based (MDF) panel manufacturing and quality evaluation. After kiln drying, 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 the DBH class level and at the stand level. Finally, a benefit/cost analysis was made for the two treatments.
Commercial thinning had a positive effect on tree growth 31 years after treatment. The average tree diameter increased from 16.2 cm to 22.2 cm (37%) following heavy thinning. Average tree height increased from 15.4 m to 17.5 m (12%) and merchantable stem volume per tree increased from 171.3 to 347.3 dm3 (103%). The lumber volume recovery per tree was higher in the thinned area (50.5%) compared to the control area (45.7%).
Commercial thinning had a positive impact on the Select Structural lumber grade recovery with 14.8% for the thinned area as compared to 6.6% in the control area. The total lumber value per tree was 146% higher in the thinned trees compared to control trees. The economic analysis indicates that it is profitable to perform a one-entry commercial thinning at wide spacing with the stand conditions and assumptions described in this study. The benefit to cost ratio is 1.20 in the thinned stand, considering 1976 and 2007 revenues and costs, as compared to 1.56 in the control area.
Lumber stiffness and strength decreased following commercial thinning. The lumber modulus of elasticity (MOE) were 9859 MPa and 8783 MPa and the modulus of rupture (MOR) 40.6 and 36.5 MPa in the control and thinned trees, respectively. This represents a 12% and 11% decrease in MOE and MOR after commercial thinning. The results clearly show that the dominant fast-growing trees sampled in the largest diameter classes (32-34 cm) of the thinned area had the lowest mechanical properties.
In conclusion, based on this long-term study, commercial thinning operations should include removal of trees in the largest DBH classes, along with the more traditional removal of smaller diameter, suppressed trees and damaged trees. This prescription would make commercial thinning even more economically profitable at the time of treatment, while minimizing its effect on lumber mechanical properties. The resulting stand, comprising fewer fast-growing dominant trees and more co-dominant trees, will grow moderately and maintain desirable lumber mechanical properties.