Commercial thinning and its potential for contributionto the timber supply in British Columbia’s Interior forests

Thinning is the partial removal of trees in a forest stand prior to final harvest. The term can be divided in pre-commercial thinning where little if any volume is removed from the stand and commercial thinning where removals are intended to provide a positive economic result. From a silvicultural point of view, the goal of thinning is to enhance future crop tree quality by removing low-quality stems and providing sufficient space for the accelerated development of retained ones (Huuskonen & Hynynen, 2006). The goals of this study was to see if commercial thinning could positively affect the short and medium term timber supply (MTTS) in the Interior regions of British Columbia (BC), and whether or not Scandinavian forestry practices could be adopted in the BC context. The mountain pine beetle (Dendroctonus ponderosae Hopkins) has created significant forest planning problems in BC. The annual allowable cut (AAC) was raised to capture beetle-killed timber while still merchantable. These regions face drastic cuts in AAC in the mid-term, and the provincial authority is looking to mitigate the effects of this falldown, and commercial thinning has been suggested. The effects of commercial thinning on the provincial timber supply was analysed in three ways: a literature review, a series of stakeholder interviews, and a case study for the Alex Fraser Research Forest. A review of thinning regimes in Nordic countries shows thinning has beneficial consequences on timber quality, with small decreases in total volume. Merchantable volumes are similar to unthinned stands, ranging from a decrease (ca. 20%) in Scots pine (Pinus sylvestris L.) to a slight increase (0-5%) in Norway spruce (Picea abies L.). Average tree diameters are greater in thinned stands, a determining factor in stand culmination. Thinning provides intermediate sources of timber and income before final harvest. A series of interviews with important stakeholders in the Swedish forest sectors shows that commercial thinning plays an important role for the timber supply and the economy of their industries. All report that thinning is ubiquitous in “proper forest management”. On an area basis, two thirds of the yearly harvest is the result of thinning, producing one third of total volume. In comparison, final harvests are conducted over the remaining one third of the harvested landbase and account for two thirds of the timber supply. The thinning programmes in place are a result of the young age-class structure of the forest. The case study was conducted at The University of British Columbia’s Alex Fraser Research Forest (Beaver Valley, BC). A 2200 ha subset of the Gavin Lake block of was classified by maturity level, with a particular interest for stands suitable for a first thinning. A total of 957 ha have thinning opportunities, with an average of 120 m3/ha available for harvest during at first entry; a total of 102 458 m3 would be made available with thinnings. Partial harvests of these stands could improve the short-term and mid-term timber supply while providing beneficial effects (quality, age-class distribution) in the mid and long-term. Commercial thinning could play a part in filling a MTTS gap, by providing timber before final harvest, by controlling timber flows from thinned stands, by creating more merchantable sawlog volume, by accessing timber in visually sensitive areas, and by using the shelterwood regeneration method in hard to regenerate areas. Certain practices from Nordic countries are currently adaptable to the BC context, while others would require longer-term changes for the industry.

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