Woodborers and wood-decay related beetle responses to a major forest disturbance event in the central and southern Sierra Nevada, California

Disturbance plays a critical role in the ecology of forests including influencing the abundance and diversity of fauna. Although numerous studies have focused on forest responses to various disturbance events, less attention has been given to arthropod community responses. California experienced an extreme, multi-year drought from 2012 to 2015 which severely stressed trees and incited epidemics of several bark beetle species (Coleoptera: Curculionidae: Scolytinae). Water stress and bark beetles contributed to a significant mortality event of hundreds of millions of trees in the central and southern Sierra Nevada, causing significant structural and compositional changes in forests. Our study sought to characterize woodborer and wood-decay-related beetle responses to various levels of tree mortality and snag (dead standing tree) retention resulting from this tree mortality event. Of particular interest were responses to differences in the orientation of dead wood, standing snags versus fallen snags. Ethanol-baited panel flight intercept traps were deployed for multiple weeks in 2022 and 2023 on plots representative of 3 disturbance classes: (i) low tree mortality (<30%), (ii) high tree mortality (>50%) with low snag fall (≤50%), and (iii) high tree mortality (>50%) with high snag fall (>60%). Woodborers and wood-decay-related beetle assemblages were compared at the family and species level. Our analyses revealed several significant differences in community assemblages among disturbance classes. Despite these differences, our results failed to reveal clear, qualitatively distinctive assemblages among disturbance classes. Rather, we could only conclude general patterns from the observed dissimilarities in richness and abundance. In general, we observed a greater diversity of woodborers on high-mortality plots than on low-mortality plots. Similarly, the diversity of wood-decay-related beetles generally increased with greater amounts (basal area) of snag fall. The amount of tree mortality and snag fall were positively related to several woodborer and wood-decay-related beetles. Observed beetle assemblages, their corresponding life histories, and the influences of altered habitat availability are discussed.