Regeneration of tree species after 11 years of canopy gap creation and deer exclusion in a warm temperate broad-leaved forest over-browsed by sika deer

Deer overpopulation is a major threat to forest ecosystems worldwide resulting in loss of natural vegetation cover and increased sapling mortality. To resolve this problem of deer overpopulation, different strategies such as deer exclusion and gap creation have been explored to determine more efficient methods to restore deer-damaged forest ecosystems. In the current study, we applied a 2 × 2 factorial design of four different treatment groups in warm temperate secondary forests: closed canopy with deer as control, closed canopy without deer, clearcut with deer and clearcut without deer. We compared the decadal change in tree foliar cover and tree species richness among treatment groups to assess tree regeneration success. We also selected six tree species (Abies firma, Quercus acuta, Eurya japonica, Cinnamomum tenuifolium, Castanopsis sieboldii and Neolitsea sericea) that are common in the studied region and compared their regeneration success among the treatment groups. In the absence of deer, clearcutting increased the diversity of tree species and accelerated sapling growth, while under closed canopy conditions sapling heights did not exceed two meters. Tree saplings tended to be less abundant in treatments with deer compared to their counterpart, suggesting limited successful recruitment of saplings at the current deer density (10–13.5 deer km−2). In clearcut-with-deer treatment, non-tree species became abundant, and negatively affected recruitment of tree species as was suggested by regression analysis. However, these general trends were not equal for all tree species. Although clearcut-without-deer treatment facilitated sapling recruitment of all six tree species, Q. acuta, C. tenuifolium and C. sieboldii required deer exclusion for sapling recruitment while A. firma, N. sericea and E. japonica required increased light availability. Consequently, informed decisions can be made by identifying whether certain tree species are capable of naturally recruiting without human intervention and how best to ensure successful recruitment if necessary. By implementing effective strategies, time and resources will be saved, and management goals such as reestablishing tree cover rapidly and increasing tree species diversity can be achieved.