Mangrove Ecosystems: Definitions, Distribution, Zonation, Forest Structure, Trophic Structure, and Ecological Significance
By Karen L. McKee

Limits on Mangrove Distribution
Extensive development of mangroves has occurred in the estuaries of large rivers flowing over shallow continental shelves, such as the Ganges in Bangladesh, Fly River in Papua New Guinea, and the Mekong Delta in Vietnam. The Amazon and Congo, the two largest rivers in the world, do not have extensive stands of mangroves primarily because of the huge outflow of freshwater.
The following factors are considered to be the major determinants of mangrove distribution:

1. Climate. Mangroves are tropical species and are not tolerant of freezing temperatures. Their latitudinal limits worldwide vary depending on air and water temperatures (Tomlinson 1986; Waisel 1972; Sherrod et al. 1986; Sherrod & McMillan 1985). The abundance of mangroves is also affected by aridity, and development is much greater along coasts that have high inputs of rainfall (Macnae 1968; Golley et al. 1975).

2. Salinity. Salt is generally not a requirement for growth, since most mangroves can grow in freshwater (Tomlinson 1986; Ball 1988). However, they do not develop in strictly freshwater habitats because of competition from freshwater species. Salinity is thus important in eliminating other vascular plant species that are not adapted for growth in a saline habitat.

3. Tidal fluctuation. Tidal influence is also not a requirement, but plays an important indirect role:
a. Inundation with saltwater helps exclude most other vascular plants and reduces competition.
b. Tides bring saltwater up estuaries against the outflow of freshwater and extend mangrove development inland.
c. Tides transport sediment, nutrients, and clean water into the mangrove environment and export organic carbon and reduced sulfur compounds.
d. Where evaporation is high, tides help flush soils and decrease salinity.

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