Impact of a spruce budworm outbreak in balsam fir and subsequent stand development over a 40-year period

Nine 0.04-ha plots were established in 1956 (age 35 years) in a balsam fir (Abies balsamea [L.] Mill.) stand in northwestern New Brunswick, Canada to determine the impact of an uncontrolled spruce budworm (Choristoneura fumiferana [Clem.]) outbreak on stand development. The plots were measured annually from 1956 to 1961 and at five-year intervals from 1965 to 1995. Moderate to severe defoliation occurred from 1951 to 1957 and again in 1975 to 1977, 198 1, and 1986 to 1988. Budworm-caused mortality from 1956 to 1961 (age 35 to 40 years) varied considerably among plots, reducing volume by 35 to 113 m(3)/ha (34%-84%), and resulting in a wide range of post-outbreak plot densities. Plots were grouped into three post-budworm. outbreak (1965, age 45 years) basal area classes, of <= 20 m(2)/ha, 21 to 27 m(2)/ha, and >= 28 m(2)/ha, to examine stand recovery. Recovery of volume up to age 60 years ranged from 72 to 132 m(3)/ha, in the lowest to highest basal area classes, respectively. From age 60 to 75 years, five plots declined in volume due to the onset of stand break-up and four plots increased in volume. By age 60 years, survivor growth was greatest in the high basal area plots, ranging from 6.2 to 9.0 m(3)/ha/yr in seven plots, versus 2.6 to 3.2 m(3)/ha/yr in two low basal area plots. From age 60 to 75 years, survivor growth averaged only 2.8 to 5.2 m(3)/ha/yr, and the stand exhibited major decline, with 63%, 74%, and 78% mortality of fir <= 15 cm DBH in the low to high basal area plots, respectively. Budworm-caused thinning in the 1950s largely determined subsequent stand development and the rate of stand break-up 25 to 35 years later. The timing and rate of natural stand decline was strongly influenced by post-outbreak stand density.