Context-dependent disturbance synergies: Subcortical competitors may constrain bark beetle outbreaks following wildfires.

Wildfires and bark beetles have historically interacted to create complex and resilient forests. However, recent record-breaking wildfires in western North America raise concerns that the large areas of injured and dead trees could facilitate increases in insect populations that respond to resource pulses. Populations of Douglas-fir beetle (Dendroctonus pseudotsugae), the primary mortality agent of Douglas-fir (Pseudotsuga menziesii), often irrupt following fires due to the resultant ephemeral pulses of defensively compromised hosts. Other subcortical phloeophagous insects are also attracted to fire (e.g., woodboring Coleoptera: Buprestidae, Cerambycidae) and similarly colonize damaged trees. Although Douglas-fir beetle and woodboring beetle species are known to colonize the phloem of injured trees, the potential for interactions among them following fire is relatively unknown. Rapid colonization by woodborers of the bark beetle niche following fires could constrain bark beetle population growth, potentially suppressing population irruptions through subcortical competition. To evaluate this hypothesis, we studied three wildfire complexes in mature Douglas-fir forests that burned in British Columbia in 2017. We found that Douglas-fir beetle preferentially colonized mature stands containing large-diameter trees with moderate fire injury and that these trees were frequently co-colonized by woodborers. In the absence of woodborers, we found that potential rates of increase in Douglas-fir beetle populations (i.e., offspring per female) were sufficient to lead to a local population irruption. Conversely, when woodborers were common (>50% of trees infested per stand), potential rates of increase in Douglas-fir beetle populations fell below replacement. These findings suggest that competition by woodboring beetles may suppress irruptions of Douglas-fir beetle in fire-injured forests. Our results reveal complex, context-dependent interactions among disturbance agents and indicate that population irruptions by resource pulse-driven bark beetles following fire may depend upon the response of local subcortical competitors. Forest management practices that enhance the diversity and abundance of non-irruptive phloeophagous insects such as many woodboring beetle species may limit the potential for wildfires to contribute to subsequent bark beetle outbreaks.