Upland black spruce stand development 17 years after cleaning and precommercial thinning

Abstract

Density management is often used by silviculturists to guide stand composition and development. We examined the effects of cleaning (hardwood removal) and four levels of precommercial thinning (0, 20, 35 and 50% basal area removal) on stand development in a dense, 24-year-old upland black spruce (Picea mariana [Mill.] BSP) plantation near Beardmore, Ontario. Immediately before treatment, stand densities and basal areas for all species and for black spruce averaged 7375 and 6415 stems ha-1, and 27.9 and 20.5 m2 ha-1, respectively. Seventeen years after treatment, black spruce total stand volume (VT) was higher in the cleaned, unthinned plots (243 m3 ha-1) than in the untreated controls (171 m3 ha-1) while total stand volume increment of all species combined was similar in these two treatments. Compared with cleaning alone, thinning cleaned plots from below increased quadratic mean diameters (DQ) by up to 9% but decreased VT by up to 28%. At plantation age 41, increases in black spruce densities of 1000 stems ha-1 resulted in mean decreases of 0.6 cm in DQ and mean increases of 43 m3 ha-1 in VT. Endemic black spruce stem mortality rates decreased with thinning intensity, with mortality concentrated in the smallest size classes. In some plots, mortality was increased by wind or snow damage, and by root rots. Height increment of dominant trees was unaffected by thinning. Projected yields at age 55 (the physical rotation age—the age at which maximum mean annual increment occurs) suggest the heaviest precommercial thinning could increase quadratic mean diameter from 16.1 to 17.7 cm, but decrease merchantable stand volume from 292 to 225 m3 ha-1. Results indicate that total black spruce fibre yields and product value on these sites will be maximized in denser stands. Cleaning appears to offer greater benefits for black spruce fibre production than precommercial thinning. Key words: black spruce, precommercial thinning, cleaning, density management, projected yields