Occurrence of seasonal water replacement in mangrove soil and the trunk growth response of Avicennia alba related to salinity changes in a tropical monsoon climate

Salinity is a crucial factor regulating mangrove growth. We evaluated seasonal variations in soil water salinity and the water replacement process in a tropical monsoon mangrove forest of eastern Thailand during 2015-2018. Trunk growth of Avicennia alba was monitored monthly using dendrometer bands and was analyzed in relation to water replacement. Soil water salinity showed remarkable seasonal variation that was influenced by the infiltration of inundated water from the river, with a salinity level similar to that of seawater during the middle of the dry season and to that of fresh water during the middle of the rainy season. Patterns of soil water salinity shifted seasonally in both horizontal and vertical distributions, highlighting soil water replacement between the two seasons. In the middle of the rainy season, soil water salinity across the horizontal distribution was nearly fresh at most sampling points on the river side but gradually increased landward along a 120-m transect. The vertical distribution of soil water salinity showed relatively low salinity at the surface horizon, which gradually increased downward to a 100-cm depth. In the dry season, the soil salinity gradient in both distributions was opposite to that in the rainy season. This seasonal change in soil water salinity was significantly related to the trunk growth of A. alba, which increased greatly during the rainy season. Seasonal water replacement causes fluctuations in soil salinity and probably nutrient availability. The combination of low soil water salinity and large nutrient influx might enhance trunk growth during the rainy season.