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.
Previously, we revealed opposing patterns of the vertical distribution of soil water salinity between the dry and rainy seasons in an estuarine mangrove forest under a tropical monsoon climate. This study clarifies the causes of such seasonal variation in soil water salinity distribution. We investigated the salinity of inundating water, and diurnal changes in soil water salinity and water table levels. Freshwater inundated the study plot in the rainy season, while saline water inundated it in the dry season. At the Sonneratia, Avicennia and Rhizophora sampling sites, soil water salinity measured in experimental pits fluctuated diurnally. Mean salinity significantly decreased in the dry season but tended to increase in the rainy season. The effect of evaporation on salinity was negligible. Water table levels at all sites primarily fluctuated within 0–10 cm from the surface and were generally not synchronized with tidal movements. We divided the soil profile into a thin tide‐sensitive layer and an underlying aquifer layer based on soil water movement pattern. The inundating water saturated the tide‐sensitive layer and then gradually infiltrated into the aquifer layer. The water in the tide‐sensitive layer was partially replaced by subsequent inundation. The water in the aquifer layer was gradually desalinized or salinized, depending on the salinity level of the inundating water, although soil water from the former season remained still in deep soil horizons. Thus, seasonal variation of the vertical distribution of soil water salinity was induced by daily inundation and subsequent infiltration processes in the mangrove soil.
Coarse woody debris (CWD) plays an important role in long-term carbon storage in forest ecosystems. However, few studies have examined CWD in mangrove forests. A secondary mangrove forest on an estuary of the Trat River showed different structures along vegetation zones ranging from the river's edge to inland parts of the forest (the Sonneratia-Avicennia, Avicennia, Rhizophora, and Xylocarpus zones, respectively). The mass distribution of CWD stock in downed wood and standing dead trees along these vegetation zones was evaluated. Most of the CWD stock in the Sonneratia-Avicennia and Avicennia zones was found in downed wood, while it mainly accumulated in standing dead trees in the Rhizophora and Xylocarpus zones. The total mass of CWD stock that accumulated in each zone ranged from 1.56-8.39 t ha À1 , depending on the forest structure and inundation regimes. The annual woody debris flux in each zone was calculated by summing the necromass (excluding foliage) of dead trees and coarse litter from 2010 to 2013. The average woody debris flux was 5.4 t ha À1 year À1 , and its zonal variation principally depended on the necromass production that resulted from forest succession, high tree-density, and lightning. Over all the zones, the above-and below-ground net primary production (ANPP and BNPP, respectively) was estimated at 18.0 and 3.6 t ha À1 year À1 , respectively. The magnitude of BNPP and its contribution to the NPP was markedly increased when fine root production was taken into consideration. The contribution of the woody debris flux without root necromass to the ANPP ranged from 12 to 28%.
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