Regeneration and growth of tree seedlings and saplings in created gaps of different sizes in a subtropical secondary forest in southern China

Abstract

Gap-based silvicultural systems are of particular interest in secondary forest management because gaps are associated with regeneration and tree species diversity. However, whether using a range of gap sizes to influence tree regeneration is effective remains unclear. In this study, gaps of different sizes were created to determine their effects on tree regeneration and growth in a secondary Pinus massoniana Lamb. forest in Hunan Province, China. A robustly designed experiment was used with over nine years of repeated measurements. Seedling densities were highest in small gaps, and seedling diversity increased with increasing gap size. Sapling densities and diversity were highest in medium gaps. Within medium and large gaps, saplings were heterogeneously distributed along gradients from opening to closed forest canopy, suggesting gap partitioning of saplings. Emergence rates of three dominant species (Pinus massoniana Lamb, Schima superba Gardn. et Champ., and Cyclobalanopsis glauca (Thunb.) Oerst.) were roughly equal in medium gaps. Regeneration of P. massoniana and C. glauca was spatially diverse as gap sized increased. However, there was little evidence of S. superba divergence within and among gaps, which indicated differential tolerance to shade. A nonlinear mixed-effects model was used to detect effects of microenvironmental variables on diameter–biomass relations. The model suggested that gaps combined with solar radiation, soil moisture, and soil temperature had various effects on sapling biomass accumulation. For saplings with the same basal diameter, estimated average aboveground biomass of S. superba and C. glauca was highest in medium gaps, whereas that of P. massoniana was highest in large gaps. The study suggests that tree regeneration and growth requirements are often not met with a single gap size alone. In practice, a single tree/group selection with medium gaps hybrid approach may be required in subtropical secondary forests to increase regeneration and ecosystem resilience, while also considering other factors such as microenvironmental variables, shade tolerance, and others.