Mangrove plantations can potentially restore the vegetation structures protecting coastal areas. In Avicennia alba plantations in central Thailand, we investigated the vegetation structures (trees, seedlings, pneumatophores, and belowground roots), sedimentation rates, and elevation changes over one year across the shore. The results showed a heterogeneous vegetation structure with an increasing tree basal area (BA) and seedling density towards the interior. The pneumatophore quantitative characteristics decreased towards the interior in association with the topographic gradient and inundation period. The sedimentation rates, which were greater in the plantation than on the mudflat, showed a negative correlation with the height, total surface area, and total volume of the pneumatophores. This indicates that the pneumatophores facilitated the transportation of fine sediments into the interior. Our results suggest that an optimal complexity of the aboveground vegetation structure might enhance the sedimentation rates. According to the gradient of tree BA and seedling density, the fine root density increased towards the interior. The monthly elevation changes in the plantation showed less fluctuation than those on the mudflat. The net elevation changes indicated sediment accretion within the plantation and erosion on the mudflat. Our results demonstrate the synergetic roles of mangrove plantations in which the aboveground structures facilitate sediment redeposition and the belowground roots stabilize sediment accretion in coastal areas.

Mangrove forests play crucial roles in the coastal ecosystems of the tropics. Few studies have addressed long-term changes in the net primary productivity (NPP) of mangroves in relation to the tropical monsoon climate. We conducted a tree census from 2008 to 2018 in a permanent plot at a secondary mangrove forest under the tropical monsoon climate of Eastern Thailand. During this period, the mortality of fast-growing species and the increasing number of newly recruited trees revealed a temporal change in the plant composition and distribution. Total tree biomass linearly increased from 283.64 to 381.72 t·ha⁻¹ during the study period. The NPP was calculated by using the summation method, which included fine root production. The NPP ranged from 21.19 to 29.04 t·ha⁻¹·yr⁻¹. The fluctuation in NPP and its components were analyzed in relation to climatic factors by the linear regression model. The NPP did not relate with the annual climatic factors, such as the mean temperature and annual rainfall. However, both increments in the basal area and living tree biomass, which is a major component of NPP, were negatively related with the maximum and mean monthly temperatures in the dry season. The annual mortality rate related positively with annual rainfall and the maximum monthly temperature in the dry season. Linear regression analyses showed that some major components of NPP were chiefly affected by the temperature environment in the dry season. These results indicated that the weather in the dry season was largely restricting the mangrove NPP due to effects on the saline water dynamics of the soils under the tropical monsoon climate, which were revealed by our recent study. It implies that the hot-dry season may lead to high mortality, long-term reduction in the increment of living-trees biomass, and thus lowered the ability to maintain high NPP of mangrove forests over the long-term.