Understory Vegetation Structure in Remnant Natural Forests and Acacia Plantations on Coastal Sand Dunes in North Central Vietnam

In the coastal sand dune forests of North Central Vietnam, vegetation has been seriously damaged by war and overexploitation. To recover ecosystem functions, including sand stabilisation under harsh environments, exotic species like Acacia spp. have been planted as a monoculture. However, the long-term sustainability of this practice remains unclear. To assess the long-term effectiveness of revegetation with Acacia spp., this study aims to understand the differences and similarities in ecological characteristics of remnant natural forests and Acacia plantations on the coastal sand dune of North Central Vietnam by comparing understory vegetation structure and environmental conditions. We investigated the understory vegetation (height < 130 cm) in a total of 54 quadrants (1 m × 1 m), including nine natural forests and nine Acacia plantations. We compared diversity indices by mixed ANOVA and examined the differences in the understory vegetation structure between the two forest types through PERMANOVA. We also determined some abiotic environmental factors (e.g. light and soil water availability, and soil pH). We identified 951 individuals, with 792 found in natural forests and 159 in plantations. The species found in natural forests were well-distributed among Liana phanerophytes (Lp), Microphanerophytes (Mi), Mega-Mesophanerophytes (MM), and Cryptophytes (Cr). In contrast, species found in plantations were predominantly Cr, Hemicryptophytes (Hm), and MM. All diversity indices were significantly higher in natural forests (P < 0.05), and the NMDS analysis confirmed significant differences in the understory vegetation structure between natural forests and plantations. Only soil pH was significantly lower in natural forests (P < 0.05), while none of the environmental factors had a statistically significant impact on the variations in understory vegetation structure. Our results indicate that succession by native tree species does not seem to occur naturally in Acacia plantations. Hence, to restore and sustainably develop coastal sand dune forests in North Central Vietnam, it is essential to establish a scientifically based strategy for managing and protecting the remaining natural remnant forest areas.