Effects of plantation intensity on longhorn and carabid beetles in conifer plantations mixed with broadleaved trees in northern Japan

Reconciling timber production with biodiversity conservation is essential. Increasing the mixture of broadleaved (BL) trees into conifer plantations increases the abundance of species that prefer natural BL forests but can reduce conifer yields. Therefore, modeling of the relationships of the abundances of various taxa with conifer and BL trees is necessary for effective biodiversity conservation. Longhorn and carabid beetles are useful ecological indicators; however, their responses to the amount (e.g. basal area and coverage) of conifer and BL trees within stands remain unknown. We surveyed the abundances of longhorn and carabid beetles in plantations of Todo fir Abies sachalinensis and Sakhalin spruce Picea glehnii mixed with various amounts of BL trees and in natural BL forests. We analyzed the response of each taxon to the basal area of conifer trees (CBA) within stands. Unexpectedly, for longhorn beetles, no effect of CBA was detected for species using BL trees as larval host plants and the effect was positive for generalist species. For carabid beetles, the total abundance of forest-dependent species decreased almost linearly with increasing CBA. The species-level mean response was decreasing abundance with increasing CBA, which occurred more rapidly in the lower range of CBA. These results suggest large negative impacts of conifer trees, even in small amounts, for many carabid species. To reconcile timber production with carabid beetle conservation, it is essential to maintain remnant natural BL forests and revert unsuccessful conifer plantations containing abundant BL trees into BL forests.