Spruce Budworm-Caused Mortality to Balsam Fir and Black Spruce in Pure and Mixed Conifer Stands

Spruce budworm (Choristoneura fumiferana Clem.) (SBW) outbreaks have caused considerable damage to coniferous forests dominated by balsam fir (Abies balsamea [L.] Mill.) or black spruce (Picea mariana [Mill.]). Hence, forecasting SBW-caused mortality of these species is an important step in allowable annual cut calculations of the forest industry in northeastern North America. This study developed prediction models of balsam fir and black spruce mortality caused by SBW and compared SBW-caused mortality of fir and spruce that were growing under similar stand conditions. Zero-inflated models were estimated for the entire range of balsam fir and black spruce in Quebec, Canada, using historical records of insect defoliation and permanent sample plot inventories from 1970 to 2006. Fir and spruce mortality caused by SBW was successfully related to historical records of defoliation through a growth reduction index. Fir mortality increased with the number of years of severe defoliation and was always greater than that of spruce, which appeared to be weakly affected by SBW defoliation. In mixed spruce-fir stands, the total volume of mortality decreased almost linearly with increasing proportions of black spruce. This suggests that thinning aimed to increase spruce proportions could be efficient in reducing overall stand mortality during an SBW outbreak. FOR. SCI. 58(1):24-33.