Mortality in black spruce stands of fire or clear-cut origin

The mortality pattern of 14 black spruce (Picea mariana (Mill.) BSP) stands from the Saguenay region (Quebec), originating from fire or clear-cutting in the early 20th century, was reconstructed based on the dendrochronological dating of dead trees. Most of the spruces died during the 1970s and 1980s, which suggests the possible impact of the most recent spruce budworm (Choristoneura fumiferana Clem.) outbreak. This hypothesis is supported by the observation of major growth reductions synchronous with known outbreak periods and by the existence of a significant partial correlation between the mortality rates and the occurrence of outbreaks. A significant partial correlation between the mortality rates and the relative density of the studied stands suggests that competition amongst individuals might act as a predisposing factor to the death of trees, while the outbreaks act as an inciting factor. The observed mortality pattern is similar to self-thinning, since most of the dead trees were from the smallest size classes.