Effects of species mixture on understory vegetation, soil properties and bacterial diversity of Acacia cincinnata, Eucalyptus robusta and Acacia mangium plantations in Southeastern China

Abstract

The establishment of mixed forests is gaining increasing attention as a way to optimize forest production, improve ecological benefits, and serve as a safety net for impacts of future climate uncertainties. However, practical knowledge about which species and what proportion of them should be mixed is still lacking. Thus, this study was conducted to identify suitable species for a mixture with Acacia cincinnata. The mixtures tested in this study were A. cincinnata and Eucalyptus robusta (6:4), A. cincinnata and Acacia mangium (3:1), and monoculture plantation of A. cincinnata established in 2014. After seven years of growth, we analyzed the effects of species mixing on tree species growth, understory vegetation, and soil physicochemical properties as well as bacterial community structure and diversity. The results showed that species mixing had no significant effect on the diameter and individual volume of A. cincinnata. However, mixed planting increased the total stocking volume compared to monoculture plantation of A. cincinnata. The species diversity, biomass and nutrient stocks in the understory vegetation were significantly increased by stand mixing. The soil of mixed stand of A. cincinnata and A. mangium had the highest C and N contents, whereas the soil of monoculture A. cincinnata stands had the highest P content. The diversity of soil bacterial community was highest in the mixing stand of A. cincinnata and E. robusta, followed by monoculture A. cincinnata stand and mixed stand of A. cincinnata and A. mangium. The relative abundance of Proteobacteria and Actinobacteria was highest in mixed stand soil of A. cincinnata and E. robusta. Furthermore, the relative abundance of Firmicutes was high in the mixed stand soils of A. cincinnata and A. mangium while the relative abundance of Verrucomicrobia was high in the mixed stand of A. cincinnata and E. robusta stand. In general, the study revealed that the establishment of mixed-species forest enhances the diversity and composition of understory vegetation, soil physicochemical and soils bacterial community; thereby increasing biodiversity, nutrient cycling and carbon sequestration in the biomass and soil. From the viewpoints of forest productivity and ecological benefits, it is advisable to establish a mixed forest of A. cincinnata and A. mangium in southern China. Overall, our work revealed that the sustainability of mixed-species plantations relies on the interactions between soil attributes, vegetation, and bacterial community.