Periodicity of two-year cycle spruce budworm outbreaks in Central British Columbia: A dendro-ecological analysis

Abstract

An outbreak of the two-year cycle budworm (Choristoneura biennis Freeman) has caused defoliation damage to interior spruce (Picea engelmannii Parry x P. glauca (Moench)) and subalpine fir (Abies lasiocarpa [Hook.] Nutt.) forests of north central British Columbia for more than 10 yr and was ongoing in 1999. A sample of 429 increment cores from spruce, subalpine fir, and lodgepole pine (Pinus contorta var. latifolia Engelm.) was collected in areas of chronic defoliation, and used in a dendro-ecological study. The objective was to develop accurately dated ring-width chronologies of each species and, by comparing growth rates of the budworm host tree species (spruce and fir) with those of the nonhost pine, to determine the past history of budworm outbreaks in the region. This history would help in estimation of the potential duration and severity of the current outbreak in the region. Four periods of decade-long reduced growth attributable to budworm defoliation were identified in the increment cores from both spruce and subalpine fir. These occurred in the mid-1890s to the early 1900s, the mid-1920s to the mid-1930s, the 1950s to the early 1960s, and the late 1980s to present (1999). Outbreaks recurred approximately every 32 yr. The reduced growth period, indicative of past outbreaks, consisted of a growth reduction phase lasting 7 to 11 yr in which rings generally exhibited a pattern of alternating wide and narrow rings (a "saw-tooth" pattern). This pattern was attributed to the biennial nature of the life cycle of this budworm, in which severe damage is caused every other year. The growth reduction phase was followed by a growth recovery phase lasting 3 to 5 yr in which ring-width gradually returned to pre-outbreak levels. Thus, the entire growth loss period could last from 10 to 16 yr and cause an average annual loss in radial increment from 16 to 21%. The 32 yr cycle of outbreak recurrence was attributed to changes in forest structure in which the forest evolves from a nonsusceptible to a susceptible state as the proportion of subalpine fir present in the upper canopy increases relative to the spruce component. A 2 yr cycle budworm outbreak will selectively remove the subalpine fir component returning the forest to a less susceptible state. It was concluded that the 2 yr cycle budworm is an important disturbance agent of northern British Columbia forests causing significant growth loss.