Mortality Patterns Following Spruce Budworm Infestation in Unprotected Spruce-Fir Forests in Maine

Abstract

Abstract Cumulative and annual mortality of red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] were examined over a 10 yr period to follow the mortality patterns in unprotected spruce-fir forests in northern Maine. Different mortality patterns were determined based on stand composition classes and merchantability classes. In general, balsam fir was more vulnerable to budworm attack, and reached 92–100% basal area mortality and 84–97% stem density mortality 12 yr after the start of the outbreak. Red spruce, in contrast, had approximately 32–59% basal area mortality and 30–66% stem density mortality during the same time period. Balsam fir mortality started 1 to 2 yr before spruce, while spruce mortality continued 2 to 3 yr after fir mortality was completed. Higher mortality was found in smaller trees than sawtimber-sized trees. Stands with hardwood components (30–70% in basal area) had the lowest mortality rate for both species. Furthermore, Schnute growth function (Schnute 1981) was used to characterize the cumulative mortality trajectories after the defoliation of spruce and fir by stand composition classes. The models estimated the time when annual mortality achieved maximum, the cumulative mortality at that time, and the asymptotic mortality over a long time period after the start of the attack. The information can provide guidelines for predicting protection strategies and scheduling salvage harvests.