Line thinning fosters the abundance and diversity of understory Hymenoptera (Insecta) in Japanese cedar (Cryptomeria japonica D. Don) plantations

We investigated the effects of line thinning on the abundance of Hymenoptera in two Japanese cedar plantations in northern Hyogo Prefecture, central Japan, in order to evaluate the effectiveness of this silvicultural treatment for ecosystem management. Line thinning is a silvicultural treatment practiced in Japan, where linear stand sections are cut (25%–35% of the total number of trees) retaining one to two rows of trees. We used Malaise traps to capture Hymenoptera from the line-thinned stand (treatment plot, including thinned and retained sections) and the unthinned stand (control plot). Overall, the total number of Hymenoptera was greater in the treatment plot than in the control plot. The treatment plot hosted more functional groups and families of Hymenoptera than the control plot in both plantations. In the Kuchiotani plantation (high-elevation site), the overall abundance of Hymenoptera and of many Hymenopteran functional groups were positively correlated with both species richness and biomass of understory plants. In the Sugi plantation (low-elevation site), understory vegetation was less developed and there was no correlation with abundance of Hymenoptera. Our study indicates that line thinning is an effective silvicultural treatment for future management of overstocked Japanese cedar plantations because it can fulfill biodiversity objectives, but the effectiveness may depend on silvicultural and landscape factors.