CHAPTER 2. AUTECOLOGY OF MANGROVES 



2.1 ADAPTATIONS TO NATURAL STRESS - 

 ANAEROBIC SEDIMENTS 



Mangroves have a series of remarkable 

 adaptations which enable them to flourish 

 in an environment characterized by high 

 temperatures, widely fluctuating salini- 

 ties, and shifting, anaerobic substrates. 

 In this section we review a few of the 

 most important adaptations. 



The root system of mangroves provides 

 the key to existence upon unfriendly sub- 

 strates (see Gill and Tomlinson 1971 for 

 an anatomical review of mangrove roots). 

 Unlike most higher plants, mangroves 

 usually have highly developed aerial roots 

 and modest below-ground root systems. The 

 aerial roots allow atmospheric gases to 

 reach the underground roots which are 

 embedded in anaerobic soils. The red 

 mangrove has a system of stilt or prop 

 roots which extend a meter (3 ft) or more 

 above the surface of the soil and contain 

 many small pores (lenticels) which at low 

 tide allow oxygen to diffuse into the 

 plant and down to the underground roots by 

 means of open passages called aerenchyma 

 (Scholander et al. 1955). The lenticels 

 are highly hydrophobic and prevent water 

 penetration into the aerenchyma system 

 during high tide (Waisel 1972). 



The black mangrove does not have prop 

 roots, but does have small air roots or 

 pneumatophores which extend vertically 

 upward from the underground roots to a 

 height of 20 to 30 cm (8 to 12 inches) 

 above the soil. These pneumatophores 

 resemble hundreds of tiny fingers sticking 

 up out of the mud underneath the tree 

 canopy. At low tide, air travels through 

 the pneumatophores into the aerenchyma 

 system and then to all living root tis- 

 sues. The white mangrove usually does not 

 have either prop roots or pneumatophores, 

 but utilizes lenticels in the lower trunk 

 to obtain oxygen for the aerenchyma sys- 

 tem. "Peg roots" and pneumatophores may 

 be present in certain situations (Jenik 

 1967). 



Mangroves achieve structural stabili- 

 ty in at least two ways. Species such as 

 the red mangrove use the system of prop 



roots to provide a more or less firm foun- 

 dation for the tree. Even though the prop 

 roots are anchored with only a modest 

 assemblage of underground roots, the hori- 

 zontal extent of the prop root system 

 insures considerable protection from all 

 but the worst of hurricanes. Other man- 

 grove species, including the black man- 

 grove, obtain stability with an extensive 

 system of shallow, underground "cable" 

 roots that radiate out from the central 

 trunk for a considerable distance in all 

 directions; the pneumatophores extend up- 

 ward from these cable roots. As in all 

 Florida mangroves, the underground root 

 system is shallow and a tap root is 

 lacking (Walsh 1974). As Zieman (1972) 

 found, individual roots, particularly of 

 red mangroves, may extend a meter or more 

 downward in suitable soils. 



From the standpoint of effectiveness 

 in transporting oxygen to the underground 

 roots, both prop roots and cable roots 

 seem equally effective. From the perspec- 

 tive of stability, the prop roots of red 

 mangroves appear to offer a distinct ad- 

 vantage where wave and current energies 

 are high. 



Unfortunately, as pointed out by Odum 

 and Johannes (1975), the same structure 

 which allows mangroves to thrive in an- 

 aerobic soil is also one of the tree's 

 most vulnerable components. Exposed por- 

 tions of the aerial root system are sus- 

 ceptible to clogging by fine suspended 

 material, attack by root borers, and pro- 

 longed flooding (discussed further in 

 section 12.1). Such extended stress on 

 the aerial roots can kill the entire tree. 



2.2 ADAPTATIONS TO NATURAL STRESS - 

 SALINITY 



Mangroves accommodate fluctuations and 

 extremes of water and soil salinity 

 through a variety of mechanisms, although 

 not all mechanisms are necessarily present 

 in the same species. Scholander et al. 

 (1962) reported experimental evidence for 

 two major methods of internal ion regula- 

 tion which they identified in two dif- 

 ferent groups of mangroves: (1) the salt 



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