Dense stands of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and amabilis fir (Abies amabilis Dougl. ex Forbes) are a significant source of timber supply for British Columbias coastal forest industry. Following clearcut harvesting, naturally regenerated stands have very high densities, which leads to questions about whether and at what densities these stands should be thinned to meet management objectives. An 18-year-old naturally regenerated mixedspecies stand in the Callaghan Valley near Whistler, B.C. was studied (Experimental Project 1211). Surveys and destructive sampling were used to examine species composition, density, height, and age of the naturally regenerated stand at 18 years. A completely randomized experiment was then established to test five levels of pre-commercial thinning (PCT): 550, 800, 950, 1200, and 1600 stems per hectare (sph) against an unthinned control. Over the 15-year study period, competition-induced mortality reduced the density in the unthinned control from 22 000 to 5900 sph (73%), while mortality in the PCT stands remained low (26%). As expected, the PCT stands had total volumes that were significantly less (3260%) and average quadratic mean diameter that was significantly larger (up to 100%) than the unthinned control. Of the thinned stands, the 1200 sph treatment had 52152 m3/ha more total volume in diameter classes > 25 cm than more widely spaced stands (550800 sph). Pre-commercial thinning treatments to between 950 and 1200 sph appear to provide the best trade-off between piece size and volume.
de Montigny, Louise, Noble, Sophie Le; Nigh, Gord. 2019. Effects of Pre-commercial Thinning after 15 Years on Growth and Yield of Mixed Western Hemlock and Amabilis Fir Stands in Coastal British Columbia (EP1211). Ministry of Forests, Lands, Natural Resource Operations and Rural Development. Research Report (FLNRORD). TR119