Determinants of Mangrove Species Distribution in Neotropical Forests: Biotic and Abiotic Factors Affecting Seedling Survival and Growth.

Abstract

Mangroves are salt-tolerant trees and shrubs that dominate the intertidal zone in many tropical and subtropical coastal regions. These intertidal forests often exhibit relatively distinct and recurring patterns of species distribution. A series of field and laboratory experiments examined biotic and abiotic factors affecting survival and growth of three neotropical mangrove species: Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove), and Laguncularia racemosa (white mangrove). A field survey across a mangrove island range in Belize revealed six vegetation zones based on differences in dominant canopy species, size and vigor of the trees, spatial position relative to the shoreline, and hydro-edaphic conditions. Variation in relative densities of seedlings was correlated with physicochemical factors and distance from conspecific, reproductive adults. Differential recruitment occurred in areas of contrasting edaphic conditions. Biotic factors such as predation also varied spatially and temporally and contributed to seedling mortality. Seedling growth patterns indicated substantial differences among species in terms of relative growth rate, biomass partitioning, morphological and physiological flexibility and defensive properties. Field distributions and laboratory culture experiments indicated that the three species are naturally abundant in conditions that are sub-optimal for their growth, but where stress factors, resource availability, and herbivores prevent or limit the growth of other species. The results support the hypothesis that mangrove zonation is primarily the result of specialization by species to different portions of the intertidal. Predation pressure, resource availability, and stress intensity were found to vary across the intertidal and generated a series of contrasting conditions for mangrove seedling recruitment. Hypersaline conditions in the high intertidal favor Avicennia, which was determined to have a high salt tolerance. Rhizophora is dominant in the low intertidal since its growth and survival are less limited by low resource availability, high flooding, and predation compared to the other two species. Laguncularia is limited to areas where salinity and flooding stresses are both at a minimum. Differential seedling responses to biotic and abiotic factors coincided with adult distribution patterns, a finding consistent with the viewpoint that seedling dynamics is an important process structuring mangrove forests.