Mangrove trees growing in a very saline condition but not using seawater

Abstract

Mangrove trees, which develop along tropical coasts, are known to use saline water uptake. In French Guiana, the high salinity condition is the result of seawater evaporation on mud banks formed from the Amazon sediment flumes. In the back mangrove a few kilometres inland, groundwater, soil water and the xylem sap uptake in the trees remain highly salty, and only very tolerant plants like Avicennia germinans can flourish, whereas the less salt-tolerant Rhizophora mangle is more difficult to find. Curiously, the same Avicennia trees propagate on the seafront. However, stable isotope ratio mass spectrometry (IRMS) measurements and ion analysis (high-performance liquid chromatography (HPLC) and inductively coupled plasma atomic emission (ICP-AES) spectroscopy reveal that the origin of the water in the back mangrove is not seawater. It is freshwater percolating into the sand bars from the inland marshes and rainwater during the wet season that redissolves a marine evaporite and gives a saline groundwater. The absence of barren saltine areas ('tanne') in French Guiana could be explained by this freshwater inflow, the aquifer being no longer linked with the ocean.

Seawater and Freshwater Circulations through Coastal Forested Wetlands on a Caribbean Island

“…Conversely, during the rainy season (and especially during strong tropical depression events), low atmospheric pressure causes mean sea level to rise while seawater flows far inland and lowers soil salinity (Figure 6d). This hydrological functioning differs strongly from that of other tropical coastal wetlands like mangroves in French Guyana, where the high tidal range (1.5 to 3.5 m) is the major hydrodynamic process [25,26]. As halophyte plants, mangrove trees have adapted to saltwater by means of various physiological mechanisms [27].…”

“…Leaf renewal and cambial growth restarting occur at the onset of the rainy season and later, in West Indian forested wetlands [21,[30][31][32]. Due to their superficial root system, these coastal wetland tree species are able to benefit from seasonal freshwater inputs [33,34] and from the subsequent lower salinity in the upper part of the soil [26,35].…”

The effects of salinity on growth and survival of mangrove seedlings changes with age

“…In addition to the salinity shift scenario, several other aspects of relevance to restoration work have also been revealed by this study. The mean percentage water content of plants grown under the low saline conditions was significantly higher than those grown in high and moderate saline conditions, implying mangroves tend to store more water under low saline conditions (Lambs et al 2008;Wei et al 2013). The cause of enhanced water absorption could be due to low osmotic potential under hypersaline conditions (Parida & Das 2005).…”

Variations de la consommation d’eau d’une mangrove adulte à Avicennia germinans, en Guyane française

“…In a recent complementary study based on stable isotopes and ion analysis, it was shown that local inflows of freshwater percolating into the sandbars from the inland marshes and reaching this mangrove forest could have contributed to the survival of trees (Lambs et al, 2008). This may explain the surprising absence of barren salty area (tanne) in the back mangrove along the coast of the Guianas.…”