mangroves, it plays an important indirect 

 role. First, tidal stress (alternate 

 wetting and drying), in combination with 

 salinity, helps exclude most other 

 vascular plants and thus reduces competi- 

 tion. Second, in certain locations, tides 

 bring salt water up the estuary against 

 the outward flow of freshwater and allow 

 mangroves to become established well 

 inland. Third, tides may transport 

 nutrients and relatively clean water into 

 mangrove ecosystems and export accumula- 

 tions of organic carbon and reduced sulfur 

 compounds. Fourth, in areas with high 

 evaporation rates, the action of the tides 

 helps to prevent soil salinities from 

 reaching concentrations which might be 

 lethal to mangroves. Fifth, tides aid in 

 the dispersal of mangrove propagules and 

 detritus. 



Because of all of these factors, 

 termed tidal subsidies by E.P. Odum 

 (1971), mangrove ecosystems tend to reach 

 their greatest development around the 

 world in low-lying regions with relatively 

 large tidal ranges. Other types of water 

 fluctuation, e.g., seasonal variation in 

 freshwater runoff from the Florida Ever- 

 glades, can provide similar subsidies. 



Substrate and Wave Energy 



Mangroves grow best in depositional 

 environments with low wave energy. High 

 wave energy prevents establishment of 

 propagules, destroys the relatively shal- 

 low mangrove root system and prevents the 

 accumulation of fine sediments. The most 

 productive mangrove ecosystems develop 

 along deltaic coasts or in estuaries that 

 have fine-grained muds composed of silt, 

 clay and a high percentage of organic 

 matter. Anaerobic sediments pose no 

 problems for mangroves (see section 2.1) 

 and exclude competing vascular plant 

 species. 



1.3 GEOGRAPHICAL DISTRIBUTION 



Mangroves dominate approximately 75% 

 of the world's tropical coastline between 

 25°N and 25°S latitude (McGill 1959). On 



the east coast of Africa, in Australia and 

 in New Zealand, they extend 10° to 15° 

 farther south (Kuenzler 1974) and in 

 Japan, Florida, Bermuda, and the Red Sea 

 they extend 5° to 7° farther north. These 

 areas of extended range generally occur 

 where oceanographic conditions move un- 

 usually warm water away from the equator. 



Although certain reqions such as the 

 tropical Indo-Pacific have as many as 30 

 to 40 species of mangroves present, only 

 three species are found in Florida: the 

 red mangrove, Rhizophora mangle , the black 

 mangrove, Avicennia germi nans , and the 

 white mangrove, Laguncularia racemosa . A 

 fourth species, buttonwood, Conocarpus 

 erecta , is not a true mangrove (no ten- 

 dency to vivipary or root modification), 

 but is an important species in the transi- 

 tion zone on the upland edge of mangrove 

 ecosystems (Tomlinson 1980). 



The ranges of mangrove species in 

 Florida have fluctuated over the past 

 several centuries in response to relative- 

 ly short-term climatic change. Currently, 

 the situation is as follows (Figure 1). 

 The red mangrove and the white mangrove 

 have been reported as far north as Cedar 

 Key on the west coast of Florida (Rehm 

 1976) and north of the Ponce de Leon Inlet 

 on the east coast (Teas 1977); both of 

 these extremes lie at approximately 29°10' 

 N latitude. Significant stands lie south 

 of Cape Canaveral on the east coast and 

 Tarpon Springs on the west coast. The 

 black mangrove has been reported as far 

 north as 30°N latitude on the east coast 

 of Florida (Savage 1972) and as scattered 

 shrubs along the north coast of the Gulf 

 of Mexico. 



Intertidal Distribution 



The generalized distribution of the 

 red and black mangrove in relation to the 

 intertidal zone is shown in Figure 2a. 

 Local variations and exceptions to this 

 pattern occur commonly in response to 

 localized differences in substrate type 

 and elevation, rates of sea level rise, 

 and a variety of other factors (see sec- 

 tion 3.2 for a full discussion of mangrove 



