exclusion species and (2) the salt excre- 

 tion species. In addition, some mangroves 

 utilize succulence and the discarding of 

 salt-laden organs or parts (Teas 1979). 



The salt-excluding species, which 

 include the red mangrove, separate 

 freshwater from sea water at the root 

 surface by means of a non-metabolic ultra- 

 filtration system (Scholander 1968). This 

 "reverse osmosis" process is powered by a 

 high negative pressure in the xylem which 

 results from transpiration at the leaf 

 surface. Salt concentration in the sap of 

 salt-excluding mangroves is about 1/70 the 

 salt concentration in sea water, although 

 this concentration is almost 10 times 

 higher than found in normal plants 

 (Scholander et al. 1962). 



Salt-secreting species, including 

 black and white mangroves (Scholander 

 1968), use salt glands on the leaf surface 

 to excrete excess salt. This is probably 

 an enzymatic process rather than a physi- 

 cal process since it is markedly tempera- 

 ture sensitive (Atkinson et al. 1967). 

 The process appears to involve active 

 transport with a requirement for biochemi- 

 cal energy input. As a group, the salt 

 secreters tend to have sap salt concentra- 

 tions approximately 10 times higher (1/7 

 the concentration of sea water) than that 

 of the salt excluders. 



In spite of these two general tenden- 

 cies, it is probably safe to say that 

 individual species utilize a variety of 

 mechanisms to maintain suitable salt 

 balance (Albert 1975). For example, the 

 red mangrove is an effective, but not 

 perfect, salt excluder. As a result this 

 species must store and ultimately dispose 

 of excess salt in leaves and fruit (Teas 

 1979). Most salt secreters, including 

 white and black mangroves, are capable of 

 limited salt exclusion at the root sur- 

 face. The white mangrove, when exposed to 

 hypersaline conditions, not only excludes 

 some salt and secretes excess salt through 

 its salt glands, but also develops 

 thickened succulent leaves and discards 

 salt during leaf fall of senescent leaves 

 (Teas 1979). 



There appears to be some variation in 

 the salinity tolerance of Florida man- 

 groves. The red mangrove is probably 

 limited by soil salinities above 60 to 65 

 ppt. Teas (1979) recalculated Bowman's 

 (1917) data and concluded that transpira- 

 tion in red mangrove seedlings ceases 

 above 65 ppt. Cintron et al. (1978) found 

 more dead than living red mangrove trees 

 where interstitial soil salinities ex- 

 ceeded 65 ppt. 



On the other hand, white and black 

 mangroves, which both possess salt excre- 

 tion and limited salt exclusion mech- 

 anisms, can exist under more hypersaline 

 conditions. Macnae (1968) reported that 

 black mangroves can grow at soil salini- 

 ties greater than 90 ppt. Teas (1979) 

 reported dwarfed and gnarled black and 

 white mangroves occurring in Florida at 

 soil salinities of 80 ppt. 



There may be an additional factor or 

 factors involved in salinity tolerance of 

 mangroves. McMillan (1975) found that 

 seedlings of black and white mangroves 

 survived short-term exposures to 80 ppt 

 and 150 ppt sea water if they were grown 

 in a soil with a moderate clay content. 

 They failed to survive these salinities, 

 however, if they were grown in sand. A 

 soil with 7% to 10% clay appeared to be 

 adequate for increased protection from 

 hypersaline conditions. 



Vegetation-free hypersaline lagoons 

 or bare sand flats in the center of man- 

 grove ecosystems have been described by 

 many authors (e.g., Davis 1940; Fosberg 

 1961; Bacon 1970). These features have 

 been variously called salitrals (Holdridge 

 1940), salinas, salterns, salt flats, and 

 salt barrens. Evidently, a combination of 

 low seasonal rainfall, occasional inunda- 

 tion by sea water, and high evaporation 

 rates results in soil salinities above 100 

 ppt, water temperatures as high as 45°C 

 (113°F) in any shallow, standing water, 

 and subsequent mangrove death (Teas 1979). 

 Once established, salinas tend to persist 

 unless regular tidal flushing is enhanced 

 by natural or artificial changes in tidal 

 circulation. 



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