Relationships between balsam fir mortality caused by the spruce budworm and stand, site, and soil variables in Michigan's Upper Peninsula

The association of various stand, site, and soil variables with impact of the spruce budworm, Choristoneurafumiferana (Clemens), on balsam fir, Abiesbalsamea (L.) Mill., was examined in the Hiawatha and Ottawa National Forests of Michigan's Upper Peninsula. Geographic differences in impact were due, at least in part, to differences in the duration of the budworm outbreak. Impact was extremely variable between stands. Linear relations observed between impact variables and stand, site, and soil variables were weak. The quantity of balsam fir in the stand (basal area per hectare or proportion of stocking) was positively correlated with dead basal area per hectare but not with percent basal area mortality. Increased impact was associated with increased quantities of white pine (Pinusstrobus L.), northern white-cedar (Thujaoccidentalis L.), eastern hemlock (Tsugacanadensis (L.) Carr.), sugar maple (Acersaccharum L.), and red maple (Acerrubrum L.) in one or both of the National Forests. Drainage and soil moisture appeared to play a role in determining the amount of spruce budworm impact in individual stands. Principal components extracted from the stand data accounted for variability between different stand types. However, the between-stand variability in impact did not coincide with the between-stand-type variability described by the components.