Overlapping outbreaks of multiple bark beetle species are rarely more severe than single-species outbreaks

Abstract

Biotic disturbances that overlap in space and time may result in important shifts in forest structure and composition, with potential effects on many ecosystem services. Starting in the late 1990s, outbreaks of multiple bark beetle species caused widespread mortality of three co-occurring conifer species in the ca. 40,000-km2 subalpine zone of the southern Rocky Mountains (SRM), USA. To better understand the implications of such outbreaks, our goal was to determine if overlapping outbreaks of multiple bark beetle species caused greater tree mortality than single-species outbreaks in stands with multiple susceptible host tree species. We mapped stand susceptibility to outbreaks of spruce beetle (SB, Dendroctonus rufipennis), mountain pine beetle (MPB, Dendroctonus ponderosae), and western balsam bark beetle (WBBB, Dryocoetes confusus) by combining aerial survey data and forest composition variables in a random forest modeling framework. Then, we used existing maps of cumulative forest mortality from bark beetles to investigate the extent and severity of overlapping outbreaks from 1999 to 2019. We found that 46% of stands with two or more of the three studied hosts species—Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta var. latifolia), or subalpine fir (Abies lasiocarpa)—were susceptible to overlapping outbreaks (25% of all sampled stands). Of those stands, 31% experienced outbreaks of two or more beetle species. Stands affected by outbreaks of both MPB and SB had higher tree mortality than stands affected by one species alone, though stands susceptible to both MPB and SB were uncommon (<4% of all sampled stands). No other combinations of beetle outbreaks increased tree mortality above levels caused by single-species outbreaks. Thus, contrary to expectations, overlapping outbreaks were rarely more severe than single-species outbreaks in the SRM. This suggests that diverse forest communities may buffer against the most severe effects of bark beetle outbreaks, even during warm, dry conditions.