Soil properties and understory species diversity at different stand densities in a tropical rainforest on Hainan Island, China Bodeneigenschaften und Artenvielfalt bei unterschiedlicher Bestandsdichte in einem tropischen Regenwald auf der Insel Hainan, China

Stand density has obvious effects on understory vegetation – an important part of forest ecosystems. Understory biodiversity in tropical rainforests is important for the natural regeneration potential, which is critical for improving the overall function of forest ecosystem. We investigated the effects on plant species diversity (separated in shrubs, interlayer plants and herbs) and soil physical and chemical properties at five levels of stand density (5000, 6500, 8000, 9500, and 11000 trees∙ha-1) in tropical rainforests on the Hainan Island, China. The results showed that soil water content increased with stand density, while organic matter content and both total and available phosphorus content was highest in the moderately dense stand. There was no effect of stand density on the diversity of interlayer plant species (climbing vines). The diversity of the interlayer was consistently greater than herbs, especially at high stand Seite 226 Benxue Chen, Yanbing Li, Shaohui Fan, Chao Peng, Biao Huang, Guanglu Liu density. Diversity of herbs was lowest in the high-density stand (11000 trees∙ha-1). Species richness and Simpson’s index of dominance of the shrub layer and species richness and its Shannon-Wiener diversity index of the herb layer were both highest at moderate stand density (8000 trees∙ha-1). Species richness and Shannon-Wiener diversity index of interlayer plants peaked at higher stand density (9500 trees∙ha-1), and the Simpson dominance index generally increased with stand density, indicating that climbers are more competitive in denser tropical rainforests. Stand density influenced the diversity and composition of understory species. We found positive correlations for understory plant species diversity with soil water content, organic matter content, total phosphorus, and available phosphorus content. Our results indicate that moderate stand densities of 8000–9500 trees∙ha-1 seem optimal for enhancing understory species diversity, soil fertility, stand stability, and natural regeneration. It could be used as target density for managing secondary tropical lowland rainforests.