Utiliza Tion 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. 0'80 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. 0'80 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 0'80 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. The capacity to maintain favorable water relations along gradients of increasing salinity varies among different mangrove species (Ball, 1988), The responses of mangroves to such salinity gradients, however, are notoriously difficulty to quantify, not only because of spatial and temporal variation in soil salinity, but because the sources of water actually used by a plant may not reflect the entire body of water in the soil (Naidoo, 1985, 1989; Ball, 1988). Thus, it is still not clear whether mangroves utilize mainly surface water, as predicted from their shallow root distribution in anaerobic soils (Gill and Tomlinson, 1971, 1977; Hatchings and Saenger, 1987). Previous studies have demonstrated that in certain areas, mangrove species may shift their water sources from typical ocean water to rainderived freshwater along tidal zones or between the dry season and the wet season (Sternberg and Swart, 1987; Lin and Sternberg, 1992a). The variation in salinity of water available for mangroves, resulting from the shift in water sources, may significantly affect plant water relations, photosynthesis and growth, and thus may control plant distribution along tidal gradients and growth form differentiation in mangroves (Sternberg and Swart, 1987; Lin and Sternberg, 1992a, 1992b). Analysis of isotopic compositions of xylem sap water or stem water has been used to quantitatively determine the utilization of different water sources by both terrestrial and coastal plants (White et al., 1985; Sternberg and Swart, 1987; Ehleringer et al., 1991; Dawson and Ehleringer, 1991; Sternberg et al., 1991; Lin and Sternberg, 1992a). Such an approach is based on the observation that there are significant differences in isotopic composition between various water sources, and that there is no isotopic fractionation during water uptake (White et al., 1985; Sternberg and Swart, 1987). Isotopic analysis of stem water is the simplest possible technique to determine the utilization of freshwater and ocean water by coastal plants. Comparison of soil water salinity with that of stem water is not useful because some plants may exclude saline water by ceasing transpiration when