Growth loss and recovery following defoliation by the balsam fir sawfly in young, spaced balsam fir stands

Two stands of intensively managed balsam fir [Abies balsamea (L.) Mill. (Pinaceae)] in western Newfoundland, located within an area infested by the balsam fir sawfly, Neodiprion abietis (Harr.) (Hymenoptera: Diprionidae) complex, were selected to study growth loss and recovery following severe defoliation by the balsam fir sawfly in the early 1990s. Four years after the start of the outbreak, volume increments were reduced by 78-81%. The decreased volume increment and recovery coincided well with the balsam fir sawfly outbreak and decline. The growth recovery rates following severe balsam fir sawfly defoliation were slow due to the absence of bud destruction, which triggers the release of suppressed buds, and thus increases foliage production. It is imperative that the severe losses in volume growth caused by balsam fir sawfly defoliation are incorporated into wood supply analyses so that future annual allowable cuts may be adjusted. In addition, to ensure maximum recovery rates at the end of an outbreak, future populations must be kept at a minimum to avoid additional foliage loss.