Patterns of infestation by subcortical insects (Coleoptera: Buprestidae, Cerambycidae) after widespread wildfires in mature Douglas-fir (Pseudotsuga menziesii) forests

Abstract

Warming temperatures and changing weather patterns are causing more frequent and severe disturbances in western North American forests. The increasing length and severity of recent wildfire seasons have annually caused widespread injury to millions of trees, facilitating the subsequent outbreak of various subcortical insect species that infest damaged hosts. The subcortical woodboring beetle assemblage (Coleoptera: Cerambycidae and Buprestidae) comprises insects that feed primarily within the phloem and sapwood of defensively compromised trees, often causing severe degradation of high value stands slated for post-fire salvage logging. We studied three 2017 fire complexes within mature Douglas-fir (Pseudotsuga menziesii) forests in southern British Columbia to investigate the post-fire infestation patterns of woodborers. We observed the highest levels of woodborer infestation in forests with moderate to high levels of fire injury, rather than in extremely burnt stands with greater tree mortality. We also found that woodborer infestation likelihood could be predicted from a combination of tree height and fire injury for individual trees. In addition, infestation was more likely in large (>13.1 m tall) rather than small trees (<13 m tall) with the same levels of fire injury. We conclude that the woodboring beetle assemblage present in dry interior Douglas-fir forests is more aggressive and ubiquitous than previously observed, and these insects actively attack and kill living trees that may otherwise survive after fires with both economic and ecological ramifications. Economically, post-fire salvage may be more effective in preventing woodborer population build-up and timber degradation by targeting moderately scorched stands immediately after wildfire. Ecologically, woodborer impacts to trees that would otherwise survive may compromise their biological legacy and impair the diversity and composition of future Douglas-fir stands.