Pre-commercial thinning is a silviculture technique that involves removing undesirable trees from young forest stands before the trees are large enough to be of commercial value. The superior trees that are left behind have access to more space, sunlight, moisture and nutrients. Because of this, they typically stay healthier and grow faster than they would in an unthinned stand.
Pre-commercial thinning has become increasingly popular across Canada. The number of hectares tended using this method rose by 70% between 1990 and 2006. There are several reasons for this increase:
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- Thinning results in trees maturing more quickly than when they are not thinned. This means they can be harvested sooner, which gives forest managers more flexibility in managing wood flow across large forest areas.
- Thinned stands contain trees that are relatively uniform in size, which lowers harvesting and wood processing costs.
- Thinning allows forest managers to control the mix of tree species in a stand, which means they can maintain types of trees that are important food sources and habitat for wildlife.
These benefits have been well demonstrated in an ongoing study by the Canadian Wood Fibre Centre, Natural Resources Canada, where researchers have been looking into the long-term results of pre-commercial thinning. Findings to date of the trials at Green River, New Brunswick, confirm that thinning helps trees grow both substantially bigger and faster. Now, as the trials continue, the researchers are trying to determine whether thinning might also produce higher-quality wood.
Green River trials studying pre-commercial thinning
Between 1959 and 1961, stands of balsam fir and red spruce near the research site in Green River, New Brunswick, were pre-commercially thinned when the trees were young. For the next 40 years or so, the stands grew relatively undisturbed thanks to the collaboration of forest companies and government departments.
In 2004, when the trees were nearing typical harvesting age, the researchers set about gathering results. Trees that had been thinned to three different spacings—1.2, 1.8 and 2.4 metres apart—were measured. Their growth was then compared with that of unthinned trees.
The outcome of the study, published in March 2008, showed that thinning paid off well. Trees spaced at the two wider intervals (1.8 and 2.4 metres) had the greatest growth gains. Their average diameters were larger—21 and 23 centimetres, respectively—compared with those of trees from the unthinned stands, which averaged 18 centimetres. This greater size translated into significant volume gains by the individual tree stems: 33% for the trees spaced 1.8 metres apart and 62% for the trees spaced 2.4 metres apart.
Overall, stands that were thinned at between 2.1- and 2.4-metre spacing produced at least 21% more salable volume of wood than unthinned stands did. Another advantage: the thinned stands were available for harvest or as mature habitat at least 25% sooner than the unthinned stands were. And all of this was achieved without the loss of long-term plant diversity in the stands.
Further studies now underway at Green River will see a repetition of the treatments applied in the late 1950s. The aim is to determine whether stands treated the same way in succession will continue to produce the same volume over time.