The effects of commercial thinning treatments on the growth response of black spruce plantations in Northwestern Ontario

This study examined the temporal growth, yield and mortality responses of mid-rotation, black spruce ( Picea mariana (Mill.) B.S.P.) plantations to varying intensities of commercial thinning. We established 32 one-hectare experimental thinning plots that included replicates of four thinning levels (0%, 25%, 35%, and 50% basal area removals) at two sites that varied in soil type and site quality. Permanent fixed area (400 m 2 ) plots were established, and standard mensuration measurements taken at year 0, 5, 9, 14. Thinning had a variable effect on growth and yield depending on site, thinning intensity, and time after thinning. Periodic annual increment (PAI) of diameter and gross total volume were highest within 5 years of thinning, then declined gradually. Growth responses in diameter of individual trees were positively correlated with tree size, and larger diameter classes contributed more toward increased volume increments in the thinned plots. Although the growth response was relatively short-lived and PAI of gross total volume was negative in thinned stands on the better-quality site by the third measurement period, there was an overall positive effect on yield. Larger diameter trees in thinned stand should allow for size class upgrades and greater economic returns from harvested wood products at final harvest.