Importance of large-diameter conifer snags for saproxylic beetle (Cerambycidae) assemblages in old-growth remnants in Japan

Conversion of old-growth forests to managed forests reduces the amount and variety of dead wood, particularly large snags (standing dead trees) are disappeared. Therefore, organisms that rely on large snags are considered to be severely affected by forest practices that convert old-growth forests to managed forests. We examined the effect of snag characteristics (diameter, bark cover, and decay gradient) on longicorn beetle (Cerambycidae) assemblages to search for species that prefer large-diameter snags in two old-growth remnants in a cool-temperate mixed forest and a sub-boreal conifer forest in Hokkaido, Japan. We collected beetles using window traps attached to conifer snags belonging to genus <i>Abies</i> and <i>Picea</i> and evaluated how snag characteristics affected beetles that feed on conifers with considering spatial autocorrelation. Species richness and total abundance were not associated with any of the snag characteristics. However, permutation tests in redundancy analysis revealed that snag diameter and bark cover affected species composition in the mixed forest, while only snag diameter had a significant effect in the conifer forest. Two (<i>Macroleptura regalis</i> and <i>Prionus insularis insularis</i>) and one (<i>Sachalinobia koltzei</i>) species preferred large-diameter snags in the mixed forest and the conifer forest, respectively. These species should be used as conservation targets to reduce the impacts of forest practices on saproxylic beetles. In contrast, one species (<i>Molorchus minor fuscus</i>) preferred small-diameter snags in the conifer forest. To conserve species that rely on large snags, we should maintain large snags in harvesting operations and retain large living trees for the future recruitment of large snags.