Quantification of Dryocoetes confusus-caused mortality in subalpine fir forests of southern British Columbia

Abstract

The western balsam bark beetle, Dryocoetes confusus Swaine (Coleoptera: Scolytinae), is considered the most destructive mortality agent in high elevation subalpine fir ecosystems, yet the actual impact of this beetle is not well quantified. Past estimates of in-stand losses from D. confusus in subalpine fir-dominated ecosystems have been unreliable due to historically limited and sporadic aerial survey coverage in British Columbia prior to the 1990’s. Tree level aerial assessment of mature subalpine fir in southern British Columbia in 1996–1997, and again in 2014, found mortality from this beetle to be as high as 70%, with mean annual mortality rates ranging from less than one percent to 1.6% in some of the ecosystems surveyed. Over 10% of 101–120year old stands surveyed in 1996–1997 had no visible mortality, and the mean percent mortality of this youngest cohort was 11% compared to over 17% mortality in all older age classes. By the second survey time, all age classes saw mean mortality estimates more than double, and all stands had some level of D. confusus attack. The highest increase in mortality was recorded in stands over 250years of age. Stands surveyed in the driest, coldest Engelmann Spruce-Subalpine Fir ecosystem in southern British Columbia sustained the highest levels of attack between survey times, averaging slightly less than 27% mortality, and by 2014 total subalpine fir mortality averaged more than 46%. The rate of mortality over the past two decades has increased. As extreme weather events become more common, particularly in wetter subalpine fir ecosystems, mortality from western balsam bark beetle will be amplified, causing both ecological and economic repercussions in these ecosystems. Results clearly show that subalpine fir stands over 100years in all ecosystems sustain continuous attack from D. confusus. To minimize future losses to this bark beetle, subalpine fir stands should be managed for a rotation age less than 100years and mixed species stands promoted where climatically feasible.