Mass mortality of Abies veitchii caused by Polygraphus proximus associated with tree trunk diameter in Japan

Bark beetles (Curculionidae: Scolytinae) can cause widespread tree mortality, and change the structure, composition, and function of forests; however, limited data is available on bark beetle infestation of true fir forests. Although Polygraphus proximus Brandford (Coleoptera: Scolytidae) causes rapid degradation of large areas of natural and planted fir forests in Russia, the species is not present in large numbers and does not cause mass mortality in central Japanese subalpine/subarctic forests dominated by Abies veitchii. Furthermore, the relationship between P. proximus infestation and tree trunk diameter has not yet been studied to determine if the insect has a preference for trees of a certain diameter, although tree trunk diameter is often a crucial parameter affecting bark beetle colonization and tree mortality. We investigated P. proximus attack on A. veitchii and the association between the resulting tree mortality and tree trunk diameter at two sites in Honshu and Hokkaido, Japan. Polygraphus proximus colonized and utilized standing A. veitchii trees and fresh-cut logs. Both live and standing dead A. veitchii trees were attacked with high frequency: 331 (70.0%) of 473 trees in Honshu, and 121 (88.3%) of 137 trees in Hokkaido. Abies veitchii mortality was 38.1% in Honshu and 59.1% in Hokkaido. The proportion of trees with entry holes was significantly higher in small-diameter trees than in large-diameter trees in Honshu. Small-diameter trees also had significantly more holes than large-diameter trees in Honshu. On the other hand, there were no significant relationships between tree trunk diameter, and the presence and the number of holes in Hokkaido. Large-diameter trees were better able to survive P. proximus attacks in both sites. This study suggests that the beetles attack small-diameter trunk trees in early phase of mass mortality and then gradually attack large-diameter trunk trees with the increasing beetle attack. Polygraphus proximus may be influencing forest structure of A. veitchii. Enhanced colonization of small-diameter trees by P. proximus may act as a trigger of mass mortality and result in higher tree mortality might pose a threat to the regeneration of A. veitchii forests in Japanese subalpine/subarctic areas.