The positive interaction between two nonindigenous species, Casuarina (Casuarina equisetifolia) and Acacia (Acacia mangium), in the tropical coastal zone of south China: stand dynamics and soil nutrients

The role of mixed forests in tropical coastal South China is unclear due to a long history of afforestation with a Casuarina (Casuarina equisetifolia) monoculture. In this study, we determined how the stand dynamics and soil nutrients in monoculture stands of Casuarina equisetifolia were influenced by Acacia (Acacia mangium), a fast-growing pioneer species, when the two tree species were combined in two initial proportions. We also compared the canopy conditions of mixed and monoculture stands of C. equisetifolia at the young stage. Over a period of ten years, the density of stems was relatively low in C. equisetifolia xAcacia mangium mixed stands compared to C. equisetifolia monoculture stands. By contrast, the aboveground biomass, understory diversity and soil nutrients were relatively high in C. equisetifolia xA. mangium mixed stands, particularly when the initial mixing proportion of A. mangium was greater. Moreover, C. equisetifolia can protect A. mangium in the windy coastal environment by ensuring evenly distributed crown growth, intact canopy conditions, and high leaf area index (LAI) during the young stage. In conclusion, the two species had a positive interaction in the mixed forests, which suggests that coastal conservation managers need to shift from their traditional focus on C. equisetifolia single-species afforestation to multi-tree species mixed afforestation.