the nonspecial ized roots of young seed- 

 lings probably suffer from anaerobiosis. 

 The failure of young seedlings to de- 

 velop in deep water is probably the rea- 

 son why moderately shallow bays do not 

 become overgrown by red mangroves. 



Large areas of mangroves have been 

 killed in the past by reducing or block- 

 ing tidal flow to mangroves with highway 

 construction and diking. Indeed, the 

 limitation of tidal circulation by dik- 

 ing, usually combined with pumping water 

 to maintain continuously high water lev- 

 el, has been a standard means by which 

 mangroves were killed prior to filling 

 the land for real estate development. An 

 area in Dade County that was temporily 

 cut off from tidal flow was studied by 

 Teas et al. (1976). Mangroves occa- 

 sionally adapt to reduced tidal flow; 

 however, live mangrove forests are rarely 

 found where completely excluded from 

 tidal flushing. Stoddart et al. (1973) 

 reported one such situation on the is- 

 land of Barbuda in the Lesser Antilles. 



Mangrove species differ in their 

 response to altered tidal flushing pat- 

 terns. Both black and white mangroves 

 in Florida are typically more resistant 

 to the effects of diking and floodino 

 than are red mangroves. Noakes (1955) 

 reported that in the Malayan mangrove 

 forests channelization, which increases 

 tidal flushing, favors the development 

 of Rhiz ophora (the genus of Florida man- 

 grovej over several other genera. Evi- 

 dence from occasional survivors among 

 diked Florida mangroves suggests that 

 decreasing tidal flushing of a mixed 

 stand would favor black and white man- 

 groves over the reds, and conversely, 

 that increasing tidal flushing should 

 favor red mangroves over blacks and 

 whites (Teas et al . 1976). 



SALINITY 



Salinity is a factor in mangrove 

 growth. No mangroves are considered to 

 be obligate halophytes, that is, to re- 

 quire salt, although they may be facul- 

 tative halophytes, that is, tolerate 

 salt and even grow better with some salt 

 than without salt (Waisel 1972). 



It has been noted repeatedly in the 

 literature that mangroves grow larger in 

 the zone of lower, fluctuating salini- 

 ties some distance into an estuary from 



the shore than they do in saline waters 

 near the shore (e.g., Davis 1940). Giant 

 red mangrove trees on Molokai, Hawaii, 

 are not found at the shore, but rather 

 in the brackish zone some distance from 

 the sea (Teas et al . 1975). It has been 

 suggested that the lesser growth of man- 

 groves in the more saline waters may be 

 a metabolic "price" paid for salt toler- 

 ance (Carter et al. 1973; Teas 1974). 



Mangroves may grow fairly well in 

 freshwater. In 1933, Davis carried red 

 mangrove propagules to the National Bo- 

 tanic Garden in Washington, D.C., where 

 they were grown in greenhouses and 

 reached a height of more than 3 m (10 

 ft) (Figure 6). I was assured by a 

 long-time employee of the Garden that 

 these mangroves had always been watered 

 with tapwater. There are records of man- 

 groves having been grown in freshwater 

 for a century at Hamburg, Germany (Ding 

 Hou 1958). Mature mangroves of several 

 species can be seen growing in fresh- 

 water several hundred meters above sea 

 level in the Botanical Garden at Bogor, 

 Indonesia (H.J. Teas, unpublished). Ac- 

 cording to Ding Hou, mangroves at Bogor 

 have grown and reproduced in freshwater 

 for more than a century. 



Thus, mangroves tolerate, but do 

 not require saltwater. This tolerance 

 of saltwater is probably very important 

 to mangroves because it reduces competi- 

 tion from nonsaline tolerant species 

 (Teas 1977). As noted earlier, mangroves 

 do not prosper at low light levels. .Man- 

 groves are slow growing compared to many 

 nonsaline tolerant herbaceous and woody 

 plants that would overshadow and out- 

 compete them if saline soils did not 

 provide the mangroves a competitive 

 advantage. 



WAVE AND CURRENT ACTION 



Another factor in mangrove estab- 

 lishment and survival is shoreline ener- 

 gy from natural waves, currents, and 

 boat wakes. Wave action can wash out 

 well-established mangroves. Figure 7 

 shows a site along the Intracoastal 

 Waterway in Broward County, south of 

 Fort Lauderdale, Florida, where man- 

 groves (and other species)are toppling 

 into the water because of erosion caused 

 by boat wakes. A rock breakwater or 

 barrier of floating tires, of the type 



69 



