Morphological and Physiological Adaptations

An environment where the water is filled with high concentrations of dissolved salts, water levels are constantly changing, and in oxygen deprived sediments would certainly exclude most plants. However, mangroves thrive in these conditions. They have evolved certain morphological and physiological responses, which allow them to avoid the pitfalls of these harsh conditions.

Tidal Inundation

Mangroves are facultative halophytes which means salt water is not a physical requirement for growth. Most can grow well in fresh water, but mangrove communities are not usually found in strict freshwater environments. There are two possible explanations. Most strict freshwater habitats exist where tidal inundation does not occur. Although not a direct physical requirement, tidal fluctuation plays an important indirect role in mangrove distribution. Tidal fluctuation results in the reduction of competition due to alternating wetting and drying, transportation of relatively clean water and nutrients in, exporting wastes, detritus, and sulfur compounds, effective dispersal of propagules. Where evaporation is very high, tidal fluctuation wash excess salt away preventing excessively high soil salinity concentrations. Due to the above factors, mangrove systems reach greatest development around the world in low-lying regions with relatively large tidal fluctuations. In Red mangroves,prop roots extend above most high tide levels. Black mangroves are excluded as water depths increase. In freshwater communities other species may out compete the mangroves for space.

Salinity Balance

Mangroves are found where salinity ranges from 0-90ppt. Red mangroves are found where soil salinities range 60-65ppt. Black and White have been recorded in soil salinities greater than 90ppt. In restricted bays and flats water salinities often range over 40ppt. For most species of plants these conditions would inhibit growth. On the other hand mangroves have evolved physiological responses to utilize these specific conditions to out compete other species under these usually harsh conditions.

Mangrove species utilize two major methods of internal ionic regulation. Salt excluding species do not take salt water internally. The Red mangrove is a salt excluder separating freshwater at the root surface by creating a type of non-metabolic ultra filtration system. Transpiration at the leaf surface creates negative pressure in the xylem. This causes a type of "reverse osmosis" to occur at the root surface. The salt concentration of xylem sap in the red mangrove is about 1/70 the salinity of surrounding seawater, but this is l0 times higher than in normal plants.

Black and White mangroves regulate ionic concentration by excreting salt through glands on the leaf surface. This temperature sensitive enzymatic process involves active transport with energy expended. Xylem sap is 1/7 concentration of salt water. This is l0 times the concentration of the salt excluders.

All three species exhibit to a small degree a combination of both methods of salt regulation. The Red mangrove stores and disposes of excess salt in the leaves and fruit. Black and White mangroves are capable of limited salt exclusion in the roots.

Anaerobic Conditions

Unlike most plants anaerobic sediments are not a problem and even assist in reducing competition. Lenticels and spongy tissue in roots and modified branches facilitates gaseous exchange. In Black mangroves, spongy pneumatophores (up to l0,000 per tree) extend up to 20 cm above the sediment. Prop roots in the Red mangrove possess many lenticels which allow O2 diffusion with passage to underground roots by means of open passageways (aerenchyma). In the White mngrove, lenticels in the lower trunk obtain 02 for aerenchyma. Peg roots and pneumatophores may be present when found in oxygen deprived sediments.

leaf glands