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 d ominated by Rhizophora mangle L. alone or in combination with Avicenni a germinans (L.) L. were characterized by moderately reducing soils (E h = 100-300 mV) with low sulphide concentrations (less-than-or-equal-t o 0.3 mum), whereas a zone dominated by A. germinans had strongly redu cing 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 man grove species indicated that the relationship between soil physicochem ical patterns and mangrove zonation reflected not only effects of soil factors on the vegetation, but effects of the adult root systems on s oil Eh and sulphide. 3 A glasshouse experiment with seedlings demonstr ated that although R. mangle and A. germinans are both flood tolerant, the species respond differently to flooding in terms of growth and bi omass partitioning, particularly when the soil is strongly reducing an d sulphide is present. Flooding under reducing soil conditions resulte d in a 20-40% decrease in total biomass of A. germinans seedlings (rel ative to drained controls), whereas seedling biomass was increased 9-2 4% above drained controls in R. mangle. 4 A significant decrease in ro ot oxygen concentrations in response to reducing soil conditions and s ulphide was observed in A. germinans, but not R. mangle, and indicated that differential flood tolerance of seedlings is partly due to diffe rences in root aeration. 5 The implication of these findings is that m angrove seedling dynamics are influenced by spatial and temporal varia tion in soil Eh and sulphide levels, but that these soil factors are a lso modified by the root systems of adult trees. These reciprocal proc esses may play an important role in the development of spatial organiz ation in this intertidal community.