Mangrove distribution in relation to seasonal water salinity and ion compartmentation: a field study along a freshwater-dominated river

Abstract

This study was designed to survey zonation and ion compartmentalization of mangroves in relation to water salinity along a freshwater-dominated river to reveal controlling factors of mangrove distribution, and to define differences in the mechanisms of adaptation to fluctuating salinity between true mangroves and mangrove associates. The water salinity along the river exhibited significant spatial and temporal variations, and true mangroves did not occur where freshwater was permanent. Both the true mangrove Aegiceras corniculatum and mangrove associate Hibiscus tiliaceus showed signs of preferential uptake of Cl− and K+ at low-salinity sites, and Cl− and Na+ exclusion at high-salinity sites. However, they differed in their selectivity for K+ over Na+: A. corniculatum preferentially absorbed K+ via root absorption, whereas H. tiliaceus preferentially controlled Na+ and promoted the transport of K+ from roots to stems. These findings indicate that, under fluctuating salinity, mangroves along the river maintained ion homeostasis in their tissues via preferential uptake of Cl− and K+ under low salinity and exclusion of Cl− and Na+ under high salinity. Water salinity regime and salt tolerance of mangrove species can explain the mangrove distribution along rivers.