Rhizophora mangle L. seedling development into the sapling stage across resource and stress gradients in subtropical Florida

Nutrient, light, salinity, soil redox potentials, and other soil characteristics were determined across a range of mangrove forest heights across the South Florida peninsula encompassing large-stature trees (> 10 m) on the west cease and dwarf forms (< 1 m) in the southeast Everglades. Relationships were established between abiotic factors and Rhizophora mangle seedling growth rates in four height (cm) classes, 25-40, > 40-55, > 55-70, and > 70-85. Light in open canopy gaps was an important determinant of seedling growth. Growth rates in gaps (0.32 +/- 0.04 to 1.89 +/- 0.18 mm/d) were two-to five-fold greater than adjacent closed canopy forests (0.14 +/- 0.01 to 0.40 +/- 0.07 mm/d). Among open canopy sites, labile soil phosphorus and soil redox potentials were significantly correlated to growth (P < 0.05; r = 0.98 and 0.89, respectively). Interstitial salinity ranged from 0 to 27 ppt across sites, well below hypersaline conditions. Thus, under low salinity stress and high light availability, soil fertility is proposed to be the dominant factor controlling R. mangle seedling development to a sapling stage (> 85 cm) in South Florida mangrove forests. In addition, soil anoxia is hypothesized to be an important stressor in lagoonal-bay estuaries and marsh-mangrove ecotones with minimal tidal exchange.