Long-term outcome of precommercial thinning in northwestern New Brunswick: growth and yield of balsam fir and red spruce

A study was established between 1959 and 1961 to study the long-term responses of balsam fir (Abies balsamea (L.) Mill.) and red spruce (Picea rubens Sarg.) to precommercial thinning. Three nominal spacings of 4 ft (1.2 m), 6 ft (1.8 m), and 8 ft (2.4 m) were compared with an unthinned control in a randomized complete block design with five replicates. At the time of thinning, natural regeneration averaged 16 years of age, 8 years after harvest. Although thinning had minimal effect on gross total volume production over a 42 to 44 year observation period, actual spacings between 2.1 and 2.5 m produced an average of 360 m(3).ha(-1) gross merchantable volume (GMV), representing a 21% gain over unthinned stands. The same spacings produced quadratic mean diameters of 21 and 23 cm, respectively, compared with 18 cm in the unthinned stands. These size increases translated to individual stem volume gains of 33% and 62%, significantly reducing the age at which thinned stands would meet a specified minimum requirement for merchantability or habitat. The mean annual increment of GMV ranged from 6 m(3).ha(-1).year(-1) in unthinned stands, to more than 7 m(3).ha(-1).year(-1) in the thinned stands, and had not yet culminated an average of 50 years postharvest.