Effects of Aspen and Spruce Density on Size and Number of Lower Branches 20 Years after Thinning of Two Boreal Mixedwood Stands

Abstract

This study examined branch diameter, number of whorls, and number of branches in the lower 2 m of the stems of white spruce growing in pure stands and in mixture with a range of densities of trembling aspen. Data were collected from two study sites located north of Big River Saskatchewan, which were regenerated following clearcutting in 1992 and thinned in 1996 to five aspen densities and two spruce densities. Results show significant decreases in spruce diameter at breast height (DBH), height, live crown ratio, maximum branch diameter in the lower 2 m, average branch diameter in the whorl closest to 1 m, and increases in tree slenderness with increases in aspen density. Increasing spruce density reduced the number of live branches and number of live branch whorls in the lower 2 m. Maximum branch diameter in the whorl closest to 1 m was influenced by both aspen and spruce density, with a significant interaction term resulting from effects of intraspecific competition being evident for aspen densities of 1500 stems ha−1 or lower. DBH and slenderness explained more than 49% of the variation in spruce crown width, maximum branch diameter in the lower 2 m, maximum live branch diameter in the whorl closest to 1 m, and average live branch diameter in the whorl closest to 1 m. DBH explained more variation in crown width and branch diameter than slenderness. This study demonstrates that growing spruce in mixture with aspen can lead to reductions in branch size and that early thinning to low aspen densities could lead to increases in size of knots and associated reductions in wood quality, particularly when spruce densities are low.