Daily inundation induced seasonal variation in the vertical distribution of soil water salinity in an estuarine mangrove forest under a tropical monsoon climate

Abstract

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.

Ten-Year Estimation of Net Primary Productivity in a Mangrove Forest under a Tropical Monsoon Climate in Eastern Thailand: Significance of the Temperature Environment in the Dry Season

“…The variation in inundation frequency and period affects the soil salinity distribution in the mangrove forest. At the same study site as that investigated in our study, Komiyama et al [28] reported that the horizontal distribution of soil-water salinity differed remarkably between the dry and rainy seasons. During the dry season, the salinity levels were high on the river edge occupied by Sonneratia and Avicennia trees and became low towards the inland areas dominated by the Rhizophora and Xylocarpus trees.…”

“…Thus, the soil-water salinity showed both spatial and seasonal fluctuations [28] corresponding to the species distribution in our study plot. Supporting the distribution of the Avicennia and Sonneratia trees in a wide range of soil salinities, Parida and Jha [38] reviewed that the two genera had three mechanisms of salt adaptation, namely, salt secretion, salt exclusion, and salt accumulation.…”

“…Mangrove zonation is a common characteristic of mangrove forests and manifests as a change in the dominant tree species inland from the seaward margins [34][35][36]. The surface elevation is a primary cause of zonation because of environmental gradients, such as inundation regimes [37], which might cause seasonal changes in the salinity environment [28,29].…”

“…The Rhizophora tree had a mechanism of salt exclusion and accumulation, while the Xylocarpus tree exhibited only salt accumulation [38]. Furthermore, an extremely high growth rate of A. alba trees during the wet season was reported [28]. These results suggest that the seasonality of soil-water salinity induced by the surface elevation might determine the distribution and growth of mangroves in the estuarine areas under the monsoon climate.…”

“…Under the tropical monsoon climate, we estimated forest productivity by using the data obtained from 2008 to 2018 in a long-term permanent plot at a secondary mangrove forest in Eastern Thailand. Long-term forest inventory data have been recorded annually, and several independent projects have been conducted in this plot under a series of mangrove carbon cycles, including soil respiration [16], fine root production [27], coarse woody debris flux [13], and seasonal tree-trunk growth related to salinity changes [28,29]. However, temporal variation in forest productivity over the decade of study has not been investigated for this plot.…”