Indirect effects of low-impact thinning on insect communities in forest floor of coniferous plantations

Abstract

ABSTRACT To reveal the effects of low-impact thinning of coniferous planted forests on insects’ diversity, we investigated the relationship among cover degree and the number of plant species and the number of insect species in eight functional groups of herbivorous beetles, ants and carabid beetles at 63 sites of Japanese cedar and cypress which were thinned or unthinned and located in the middle of Japan. We found that when low thinning was applied to approximately 30% of the stand density, the number of plant species increased from the following year, and cover degree moderately increased in the years following thinning, although canopy openness did not significantly change. Using structural equation modeling, we found that the number of herbivorous beetle species increased with the cover degree increment of feeding-intended plants (woody-plants and herbs/vines) in the 4–8 years after thinning. The number of species of woody-plant feeding beetles also increased with an increase in the number of herb/vine plant species. For ant species, the number of habitat generalist species increased in the 4–8 years after thinning or with cover degree increment, although no response was observed in woodland specialist and open-habitat specialist species. For carabid beetle species, only the number of large-size species, not including open-habitat specialists, increased with cover degree increment. It was considered that there were moderate, and yet sustainable, positive effects of low-impact thinning on species diversity of forest-floor-inhabiting insects via understory vegetation increment with a lag-time, and no promotion of migration of non-forest-habitat ant and carabid beetle species.