Mangrove zonation in the dry life zone of the Gulf of Fonseca, Honduras

The structural attributes and zonation patterns of mangrove forests in a dry environmental setting were evaluated in relation to gradients of soil resources, soil regulators, and hydroperiod between October 2000 and August 2001 in the Gulf of Fonseca, Honduras. Transects perpendicular to the edge of tidal channels were established at four mangrove sites, each sectioned into three mangrove zones (fringe, transition, and scrub) based upon distinct tree-height gradients. Porewater sulfide concentrations among zones were normally below levels of detection (< 0.03 mM) and mean redox values ranged from 163.4 +/- 9.9 to -42.4 +/- 15.8 mv, indicating slightly reducing conditions in all zones. Mean porewater NOx- and PO43- concentrations were not significantly different among zones and ranged from 3.3 +/- 0.5 to 4.5 +/- 0.4 mu M and from 0.05 +/- 0.02 to 0.18 +/- 0.04 mu M, respectively. Soil nitrogen concentrations were 1-3 mg g(-1) in all zones, and mean nitrogen:phosphorus atomic ratios were < 20 (range: 12.9-14.9), indicative of fertile wetland soils. Mean basal areas were significantly different among mangrove zones (fringe > transition) and were coupled with differences in height within species and zones. Avicennia germinans occupied areas with higher elevations associated with higher salinities ranging from 80 to 140 g kg(-1) in the dry season. Rhizophora mangle dominated lower elevations where salinities ranged from 38 to 57 g kg(-1) in both the dry and wet seasons. Spatial analysis on mangrove seedlings along transects confirmed that seedling distribution along the intertidal profile was statistically correlated with conspecific adults, indicating that survival and growth of seedlings in different intertidal locations are closely matched to canopy membership. The sharp transition (< 100 m) in forest structure and distribution of mangrove species along the intertidal zone of subhumid coastal environments demonstrate common zonation patterns where species distribution and species-specific habitat suitability are mediated closely with tolerance to soil regulators (high soil salinity) rather than resource availability.