Early natural regeneration patterns of woody species within gaps in a temperate secondary forest

Forest gaps are important small-scale disturbances that initiate forest succession. However, natural regeneration patterns within gaps remain unclear, especially in temperate secondary forests. In this study, three-dimensional graphic systems were established to show the whole regeneration patterns of woody species with different shade tolerance in natural gaps. Furthermore, all gaps were classified into three size classes, and each gap was divided into two parts, canopy gaps (CGs) and gap peripheries (GPs). A total of 34 woody species were found in the 41 surveyed gaps, but almost no tree species had a high importance value index (≥ 0.100). Total plant density decreased from the small gaps to the large gaps, showing a density trend similar to that of shade-tolerant species. Total species richness peaked in the medium gaps, which indicated that medium gaps could provide moderate environments for most woody species. Both the species richness and plant density of shade-tolerant species were lower in CGs than in GPs, regardless of gap sizes. In contrast, both the species richness and plant density of shade-intolerant species were higher in CGs than in GPs. Interestingly, intermediate shade-tolerant species exhibited a “doughnut” regeneration pattern in the large gaps, which indicated that they could neither compete with shade-intolerant species in CGs nor invade GPs occupied by shade-tolerant species. Our findings suggest that gap formation could benefit woody species regeneration, but subsequent treatments based on gap silviculture are needed to eliminate low-value species and manage target tree species, which would help accelerate the restoration of secondary forests.