Distribution of mangroves with different aerial root morphologies at accretion and erosion sites in Ca Mau Province, Vietnam

Abstract

Aerial roots are a vital feature that allow mangroves to survive in anaerobic and unstable soil conditions. Mangrove species have different aerial root characteristics and show their own site-specific distribution. The distribution of three types of aerial roots with different morphological characteristics was investigated between accretion and erosion sites in Ca Mau, Vietnam. Stilt roots, pneumatophores and knee roots were the three main aerial root types in the study area. Stilt roots of Rhizophora apiculata BL. and pneumatophores of Avicennia species were the two most abundant root types at the study sites, dominating erosion and accretion sites, respectively. Stilt roots had a higher number of primary roots in seaward plots than in landward plots (p < 0.05), supporting the ability of R. apiculata to withstand erosion of the surface. Tidal erosion moved away sediment and exposed roots, which resulted in higher aboveground stilt root height in seaward plots than in landward plots at erosion sites (p < 0.05). The higher density and biomass of pneumatophores in seaward plots than those in landward plots at accretion sites (p < 0.05) confirmed the adaptation of Avicennia to newly accreted sites where tidal inundation and anaerobic conditions limited mangrove growth. The density of A. alba decreased with the distance from the sea at both erosion and accretion sites, implying that this species was a pioneer species adapted to newly accreted sites or disturbed sites among mangrove species. Bruguiera parviflora with knee roots was distributed in more stable conditions. Mangrove species with different root types displayed a zonal distribution with increasing distance from the shoreline. This distribution reflects mangrove species adaptabilities and species-specific roles in the context of coastal processes and site conditions, which should be considered in coastal mangrove restoration.