Context-dependent disturbance synergies: Subcortical competitors may constrain bark beetle irruption 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 (Pseudotsugae
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 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 of
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 of 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.