Interactions and impacts of multiple bark beetle outbreaks in the southern Rocky Mountains

2022 Spring.Includes bibliographical references.In conifer forests of western North America, outbreaks of native bark beetles are important biotic disturbances that influence forest structure and function. Subalpine forests in the southern Rocky Mountains (SRM) are home to multiple species of host-specific bark beetles that can affect the same stand concurrently or successively, potentially interacting to affect landscape patterns of tree mortality and subsequent disturbance dynamics. Despite their importance, the causes and consequences of interactions among bark beetle disturbances are poorly understood. To examine how outbreaks of different bark beetle species interact, we conducted two studies in the SRM. First, using broad-scale geospatial data, we explored the extent and severity of overlapping outbreaks of multiple bark beetle species across the SRM. We found that forest stand susceptibility to outbreaks of multiple bark beetle species is limited by host tree distributions, and that overlap of outbreaks was relatively uncommon. Furthermore, cumulative tree mortality was rarely higher in stands that experienced overlapping outbreaks compared to stands that experienced single-species outbreaks. These results suggest that forest trajectories in mixed-species stands will not be altered when multiple bark beetle outbreaks co-occur, compared to single-species outbreaks. Second, in a multi-scale study including both field and geospatial data, we explored how history of stand-replacing fires and a severe spruce beetle (Dendroctonus rufipennis) outbreak affected future susceptibility to subalpine fir (Abies lasiocarpa) decline (SFD) from outbreaks of the western balsam bark beetle (Dryocoetes confusus) and associated fungal pathogens. At the landscape scale, we found that disturbance history had limited effects on the susceptibility to future SFD, with a weak trend towards higher susceptibility in younger forests. At the stand scale, however, we found no effect of disturbance history on stand structure or composition traits that have been shown to influence susceptibility to SFD. Individual tree-scale analyses revealed that tree and local neighborhood traits were most important for determining likelihood of SFD. Weak connections between disturbance history, stand structure and composition, and SFD underscore the variability in forest recovery after disturbance, and highlight the need for future research. Together, these results from these studies indicate that, while climate change may be amplifying tree mortality events, subalpine forests in the SRM may be broadly resilient to co-occurring and successive disturbances