Effects of dispersed broadleaved and aggregated conifer tree retention on ground beetles in conifer plantations

Abstract Retention forestry is thought to contribute to the balance between biological conservation and timber production. However, its efficacy has mainly been investigated in natural or naturally regenerated forests, and there have been few studies in plantations and Asia. Conifer plantations are quite common in boreal and temperate regions, and native broadleaved trees in conifer plantations are a key structure for maintaining biodiversity in the plantations. We focused on two retention methods in conifer plantations in northern Japan: dispersed broadleaved retention and aggregated conifer retention. We examined the effects of dispersed broadleaved and aggregated conifer retention on forest ground beetles at both the species and community levels and compared the efficacies of these two retention methods under different retention levels. Our results demonstrated that the abundance of forest species in harvested areas increased with the level of dispersed retained trees. We also found that the abundance of forest species decreased in clear-cut areas of aggregated retention sites, whereas the abundance of most species did not decrease within aggregated retention patches of conifer trees and was similar to that in unharvested plantations. It was inferred from the comparison of efficacy between dispersed broadleaved and aggregated conifer retention that the efficacies of the two retention methods were comparable but that at high retention levels, the efficacy of dispersed broadleaved retention could be higher than that of aggregated retention with respect to the species richness of forest species. Our findings demonstrated that retention forestry mitigated the harvest impacts on forest ground beetles in Asian conifer plantations.