Deterioration of severely defoliated balsam fir in relation to stand age, spacing, and foliar protection

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Abstract
Balsam fir (Abiesbalsamea (L.) Mill.) trees that had been heavily defoliated by the spruce budworm (Choristoneurafumiferana (Clem.) were felled and examined for decays and secondary insect activity during the declining phase of the budworm outbreak. Trees were sampled in (i) a 25- to 30-year-old stand that had been spaced a decade earlier to about 2.4 × 2.4 m and allowed to become defoliated, (ii) adjacent unspaced controls, (iii) a nearby mature (60- to 80-year-old) stand that had been defoliated, and (iv) and (v) spaced and unspaced plots that had been protected from defoliation by annual sprays of trichlorfon. Sampling was done annually over a 4-year period from preselected defoliated trees, and after the budworm population had collapsed from trees that had been intensively studied with respect to defoliation and mortality. Where defoliation was allowed to proceed unchecked (i) one-half of the spaced, (ii) one-third of the unspaced, and (iii) all of the mature trees died. The trees were attacked by wood wasps (Siricidae) and the sapwood-staining fungus, Amylostereumchailletii (Pers. ex Fr.) Boidin, followed by various stem-invading insects (Pissodesdubius Rand., Pityokteinessparsus Lee, Trypodendronlineatum (Oliv.), and Monochamusscutellatus Say) and by the sap-rot fungus Hirschioporusabietinus (Pers. ex Fr.) Donk. In surviving defoliated trees (i and ii) and in trees that had been protected by trichlorfon sprays (iv and v), numerous siricid injuries with associated pockets of stain caused by A. chailletii were found in the lower bole.

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