Utilization of Surface Water by Red Mangrove (Rhizophora Mangle L.): An Isotopic Study
Physiological responses of mangroves to salinity changes in the field are difficult to quantify, partly because it is still not clear whether mangroves utilize mostly surface water, as believed by many researchers based on the shallow distribution of mangrove roots in anaerobic soils. To test this axiom, we analyzed oxygen isotope ratios of possible water sources in different soil layers and stem water from red mangroves (Rhizophora mangle L.), a dominant mangrove species in Florida, which occurs frequently in two distinct growth forms: dwarf and tall mangroves. δ18O values of stem water from both dwarf and tall mangroves always matched those of surface water, indicating that they utilize mostly the surface water. Consistently, most fine roots (about 70%) of dwarf mangroves occur in this upper soil layer. δ18O values of stem water from both dwarf and tall mangroves showed significant changes from the dry season (high values) to the wet season (low values), corresponding to the isotopic and salinity variation in surface water during this period. Fine root biomass also showed a significant increase in the wet season as response to the decrease in salinity of soil surface water. Predawn water potentials decreased with increasing δ18O values of stem water, while midday water potentials did not show such relationship. The dependence of mangroves on surface water as their sole water source has significant implications for plant water relations, and may explain growth form differentiation in some mangrove species of southern Florida.
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Document Type: Research Article
Publication date: 1994-01-01
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