these zones were regarded as progressively 

 later stages in succession, the entire 

 mangrove ecosystem was believed to be 

 moving seaward through a process of sedi- 

 ment accumulation and colonization. Davis 

 based his argument primarily upon the 

 sequence of observed zones and cores which 

 showed red mangrove peat underlying black 

 mangrove peat which, in turn, occurred 

 under terestrial plant communities. 



Unfortunately, this Clementsian in- 

 terpretation of mangrove zonation was 

 widely accepted, but rarely tested. For 

 example, Chapman (1970) expanded Davis' 

 original successional concept from south 

 Florida to explain zonation in mangrove 

 forests in other parts of the world. 

 Walsh (1974) thoroughly reviewed the man- 

 grove succession/zonation literature. 



Fortunately, not everyone accepted 

 Davis' point of view. Egler (1952) and 

 later Thorn (1967, 1975) argued that man- 

 grove zonation was a response to external 

 physical forces rather than temporal se- 

 quence induced by the plants themselves. 

 Egler (1952) showed that patterns of sedi- 

 ment deposition predicted by Davis' (1940) 

 theory did not always occur. He also 

 showed that in some cases mangrove zones 

 appeared to be moving landward rather than 

 seaward. Sea level has been rising in 

 south Florida at the rate of 1 ft (30 cm) 

 per 100 to 150 years (Provost 1974). 

 Spackman et al. (1966) emphasized the role 

 of sea level change in determining changes 

 in mangrove zonation, both through sea 

 level rise and land subsidence. Both 

 Egler (1952) and Spackman et al. (1966) 

 along with Wanless (1974) and Thorn (1967, 

 1975) suggested that mangroves were 

 reacting passively rather than actively to 

 strong geomorphologi cal processes. This 

 implies that mangroves should be regarded 

 as "land-stabilizers" rather than "land- 

 builders". 



Furthermore, field researchers fre- 

 quently noted that red mangroves were not 

 always the only "pioneer species" on re- 

 cently deposited sediment. It is not 

 unusual to find seedlings of black, white, 

 and red mangroves growing together on a 

 new colonization site. Lewis and Dunstan 



(1975) found that black mangroves and 

 white mangroves along with the saltmeadow 

 cordgrass, Spartina patens , are often the 

 pioneers on new dredge spoil islands in 

 central Florida. On the northern coast of 

 the Gulf of Mexico, where black mangrove 

 is the only mangrove species present, it 

 may be preceded by marsh grasses such as 

 saltmarsh cordgrass, S. patens , smooth 

 cordgrass, S. al terni fl ora , or the black 

 needle rush, Juncus roemerianus . In Puer- 

 to Rico, we observed that white mangrove 

 often pioneers and dominates sites where 

 oceanic overwash of beach sand has oc- 

 curred. All of these observations detract 

 from Davis' (1940) original contention 

 that red mangroves should be regarded as 

 the initial colonizer of recently de- 

 posited sediments. It appears that under 

 certain conditions, e.g., shallow water 

 depths, substrate type, and latitude, 

 white and black mangroves or marsh grasses 

 can be effective pioneer species. 



The work of Rabinowitz (1975) added a 

 new perspective to the mangrove zonation 

 debate. Through carefully designed recip- 

 rocal planting experiments in Panamanian 

 mangrove forests using species of Rhi zo- 

 phora , Laguncul a ri a , P e 1 1 i c i e r a and 

 Avicennia , she demonstrated that each 

 species could grow well within any of the 

 mangrove zones. In other words, physical 

 and chemical factors such as soil salinity 

 or frequency of tidal inundation, within 

 each zone, were not solely responsible for 

 excluding species from that zone. To 

 explain zonation, Rabinowitz proposed 

 tidal sorting of propagules based upon 

 propagule size, rather than habitat adap- 

 tation, as the most important mechanism for 

 zonation control. 



The most recent piece to be added to 

 the zonation/succession puzzle comes from 

 the work of Ball (1980). Based upon re- 

 search of mangrove secondary succession 

 patterns adjacent to Biscayne Bay, Flori- 

 da, she made a strong case for the impor- 

 tance of interspecific competition in 

 controlling zonation. She found that 

 white mangroves, which grow best in 

 intertidal areas, do not occur consis- 

 tently in the intertidal zone of mature 

 mangrove stands. Instead, white mangroves 



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