CHAPTER 1 



INTRODUCTION 



1.1 "MANGROVE" DEFINITION 



The term "mangrove" expresses two 

 distinctly different concepts. One usage 

 refers to halophytic species of trees and 

 shrubs (halophyte = plant growing in 

 saline soil). In this sense, mangrove is 

 a catch-all, botanically diverse, non- 

 taxonomic expression given to approximate- 

 ly 12 families and more than 50 species 

 (Chapman 1970) of tropical trees and 

 shrubs (see Waisel 1972 for a detailed 

 list). While not necessarily closely 

 related, all these plants are adapted to 

 (1) loose, wet soils, (2) a saline habi- 

 tat, (3) periodic tidal submergence, and 

 (4) usually have degrees of viviparity of 

 propagules (see section 2.3 for discussion 

 of "viviparity" and "propagules"). 



The second usage of the term mangrove 

 encompasses the entire plant community 

 including individual mangrove species. 

 Synonymous terms include tidal forest, 

 tidal swamp forest, mangrove community, 

 mangrove ecosystem, mangal (Macnae 1968), 

 and mangrove swamp. 



For consistency, in this publication 

 we will use the word "mangrove" for indi- 

 vidual kinds of trees; mangrove community, 

 mangrove ecosystem or mangrove forest will 

 represent the entire assemblage of "man- 

 groves". 



1.2 FACTORS CONTROLLING MANGROVE DISTRI- 

 BUTION 



Four major factors appear to limit 

 the distribution of mangroves and deter- 

 mine the extent of mangrove ecosystem 

 development. These factors include (1) 

 climate, (2) salt water, (3) tidal fluc- 

 tuation, and (4) substrate. 



Climate 



Mangroves are tropical species and 

 do not develop satisfactorily in regions 

 where the annual average temperature is 

 below 19°C or 66°F (Waisel 1972). 

 Normally, they do not tolerate temperature 

 fluctuations exceeding 10°C (18°F) or 



temperatures below freezing for any length 

 of time. Certain species, for example, 

 black mangrove, Avicennia germinans , on 

 the northern coast of the Gulf of Mexico, 

 maintain a semi -permanent shrub form by 

 growing back from the roots after freeze 

 damage. 



Lugo and Zucca (1977) discuss the 

 impact of low temperature stress on Flori- 

 da mangroves. They found that mangrove 

 communities respond to temperature stress 

 by decreasing structural complexity (de- 

 creased tree height, decreased leaf area 

 index, decreased leaf size, and increased 

 tree density). They concluded that man- 

 groves growing under conditions of high 

 soil salinity stress are less tolerant of 

 low temperatures. Presumably, other types 

 of stress (e.g., pollutants, diking) could 

 reduce the temperature tolerance of man- 

 groves. 



High water temperatures can also be 

 limiting. McMillan (1971) reported that 

 seedlings of black mangrove were killed by 

 temperatures of 39° to 40°C (102° to 

 104°F) although established seedlings and 

 trees were not damaged. To our knowledge, 

 upper temperature tolerances for adult 

 mangroves are not well known. We suspect 

 that water temperatures in the range 42° 

 to 45°C (107° to 113°F) may be limiting. 



Salt Water 



Mangroves are facultative halo- 

 phytes, i.e., salt water is not a physical 

 requirement (Bowman 1917; Egler 1948). In 

 fact, most mangroves are capable of 

 growing quite well in freshwater (Teas 

 1979). It is important to note, however, 

 that mangrove ecosystems do not develop in 

 strictly freshwater environments; salinity 

 is important in reducing competition from 

 other vascular plant species (Kuenzler 

 1974). See section 2.2 about salinity 

 tolerance of mangrove species. 



Tidal Fluctuation 



While tidal influence is not a 

 direct physiological requirement for 



