Forest fire frequency and western spruce budworm outbreaks in western Montana

Abstract

Duration and intensity of western spruce budworm ( Christoneura occidentalis Freeman) outbreaks have increased with the decrease in forest fire frequency in western Montana since 1910. Frequency of budworm outbreaks, however, was not affected. Feeding activity and fire occurrence were measured in 20 mixed Douglas-fir ( Pseudotsuga menziesii var. glauca (Beissn.) Franco)-ponderosa pine ( Pinus ponderosa var. ponderosa Dougl. ex Laws.) stands representing dry Douglas-fir habitat types. Outbreak frequency, duration, and intensity were inferred from analyses of radial increment cores and fire history from basal fire scars on old-growth pines and firs. Data were compared between the pre-fire suppression period (1814–1910) and the fire suppression period (1911–1983). Since 1910, Douglas-fir, a shade-tolerant conifer and host for budworm, has invaded and dominated sites previously occupied by non-host ponderosa pine. This increase in the abundance of host is a response to decreased wildfire frequency and has resulted in stand conditions favorable for budworm populations, an important factor contributing to current widespread and damaging populations of western spruce budworm. Judicious use of ground-fire, thinning, and other silvicultural treatments that favor seral non-host conifers, reduce stand density, and minimize the number of canopy layers in managed stands would reduce the amount and quality of budworm habitat and significantly reduce the risk of damaging outbreaks.