Effects of repeated thinning at different intensities on the recovery of hardwood species diversity in a Cryptomeria japonica plantation

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ABSTRACT To explore how and to what extent repeated thinning at different intensities affects the recovery of species diversity, we investigated the recruitment patterns of hardwoods (height ≥1.5 m) from 2003 to 2017 after first (2003) and second (2008) thinnings in 67% (Intensive), 33% (Weak), and 0% (Control) thinning treatments in a Cryptomeria japonica plantation in northern Japan. With intensive thinning, the numbers of individuals and species both increased rapidly after the first thinning, but slowly after the second thinning, irrespective of successional status (i.e., early, mid, and late succession). With weak thinning, both numbers increased monotonically but slowly in most years. With time after the second weak thinning, the numbers of early and mid-successional species both decreased, while the numbers of late-successional species increased monotonically, suggesting a replacement of shade-intolerant early and mid-successional species by shade-tolerant late-successional species. As a result, little differences were observed in both the numbers between the two thinning intensities. These traits suggest that thinning intensity has a minimal effect on species richness, but affects the future species composition (i.e., the dominance of late-successional species with weak thinning and co-dominance of all successional states with intensive thinning). For large individuals (diameter at breast height ≥5 cm), the numbers of both individuals and species were greater with intensive thinning. Therefore, although recovery of the species diversity in the forest understory can be realised with weak thinning, a mixed conifer–hardwood forest at the canopy level is more likely to be achieved with repeated intensive thinning.