Soil physicochemical patterns and mangrove species distribution--reciprocal effects?

1 The distributions of two dominant mangrove species in a neotropical forest were associated with spatial variation in soil redox potentials (Eh) and interstitial water sulphide concentrations. Mangrove zones dominated by Rhizophora mangle L. alone or in combination with Avicennia germinans (L.) L. were characterized by moderately reducing soils (Eh = 100-300 mV) with low sulphide concentrations (less than or equal to 0.3 micromolar), whereas a zone dominated by A. germinans had strongly reducing soils (Eh less than or equal to 100 mV) with high sulphide (2-4 mM). 2 Less reducing soil conditions near the aerial roots of both mangrove species indicated that the relationship between soil physicochemical patterns and mangrove zonation reflected not only effects of soil factors on the vegetation, but effects of the adult root systems on soil Eh and sulphide. 3 A glasshouse experiment with seedlings demonstrated that although R. mangle and A. germinans are both flood tolerant, the species respond differently to flooding in terms of growth and biomass partitioning, particularly when the soil is strongly reducing and sulphide is present. Flooding under reducing soil conditions resulted in a 20-40% decrease in total biomass of A. germinans seedlings (relative to drained controls), whereas seedling biomass was increased 9-24% above drained controls in R. mangle. 4 A significant decrease in root oxygen concentrations in response to reducing soil conditions and sulphide was observed in A. germinans, but not R. mangle, and indicated that differential flood tolerance of seedlings is partly due to differences in root aeration. 5 The implication of these findings is that mangrove seedling dynamics are influenced by spatial and temporal variation in soil Eh and sulphide levels, but that these soil factors are also modified by the root systems of adult trees. These reciprocal processes may play an important role in the development of spatial organization in this intertidal community.