Influence of past forestry practices on western spruce budworm defoliation and associated impacts in southern British Columbia

Abstract

The western spruce budworm, Choristoneura occidentalis Freeman, is a native defoliator of interior Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) forests in British Columbia. Repeated budworm defoliation causes tree mortality, reduction in growth rates, and reduced lumber quality. Eight hundred and fifteen (815) plots were established between 1992 and 2002 throughout chronically defoliated areas, and areas which until recently had never been defoliated, to assess defoliation and impact (growth reduction, top kill, tree mortality) in the Southern Interior of British Columbia. Stands within the Interior Douglas-fir (IDF) biogeoclimatic zone were most susceptible and suffered the greatest amount of chronic budworm activity. The driest, hottest ecosystems predominated by Douglas-fir had the greatest number of consecutive years of defoliation, as well as more frequent defoliation events (total years defoliation). Multi-storied stands with chronic budworm activity had on average 50% fewer Douglas-fir trees per hectare than stands with few or no budworm defoliation events. The understorey component (0.1–7.4 cm dbh and > 1.3 m height) of multi-storied and selectively harvested stands suffered substantial defoliation, often followed by tree mortality, due to intensive budworm feeding. Young, thinned stands and stands that had never been harvested suffered the least impact from budworm defoliation. In conclusion, changes in stand structure through harvesting have influenced the susceptibility of Douglas-fir to the western spruce budworm.