Salinity plays a vital role on the distribution of species, their productivity and growths of mangrove (Twitlley & Chen, 1998). Changes in salinity are controlled by climate, hydrology, rainfall, topography and tidal flooding. Mangroves generally tolerate higher salinity range when compared to terrestrial plants but tolerance capacity also vary among the different mangrove species. In general, mangrove vegetation is luxuriant in low salinity levels. Experimental evidence indicates that in high salinity, mangroves spend more energy to maintain water balance and iron concentration rather than primary production and growth (Clough, 1984, Kathiresan, 2000). Therefore, mangrove show some adaptations to regulate excess irons. Some mangrove species exclude salt by having significantly impermeable roots which act as an ultrafiltration mechanism to exclude sodium salts from the rest of the plant. Another strategy for coping with high concentrations of salt is the secretion of salt through special salt-excreting glands on leaves (Plate 3).
Some mangroves uptake salt water and store excess salt in old leaves a the bark (Tomlinson 1994)
Adaptations of Leaves
Leaves of mangroves show many adaptations to survive in harsh conditions. One of the prominent adaptation is thick leaves. It helps to increase the photosynthesis efficiency as well as to store more water. This fleshy structure of leaves is a result of special modification of cells and tissues to store water. Thick cutinized outer epidermal wall is also present in many mangrove leaves to reduce water loss by evaporation. The stomatal structure of mangroves shows special features to minimize water loss by transpiration. They have a highly developed thick guard cells on both sides of stomatal cells to reduce the size of the stomatal pore when needed.
Mangroves are distributed in 123 countries and territories. Global coverage has been estimated at a variation of 10 million hectares (Bunt, 1992; Kethiresan & Bingham, 2001), 14 -15 million hectares (Schwanthom & Saint-Paul; 1996 Kethiresan Bingham, 2001) and 24 million hectares (Twilley et al. ; 1992; Kethiresan Bingham, 2001). Spalding (1997) gives a recent estimate of cover as 18 million hectares. The majority (41.4%) of them are in south and Southeast Asia. (Kethiresan &a Bingham, 2001). Approximately 45 genera and over 70 species of mangroves are found in the Indo- Pacific region which is the most extensive and diverse region of the mangrove areas in the world (Unesco, i 987; Amarasinghe, 1997).
There are six main mangrove zones that can be identified in the world. They are Western American Zone, Eastern American Zone, Western African Zone, Eastern African and Middleast Zone, Asian Zone and Australasia Zone (Fernando, 2007).
The mangals are always found in tropical and sub tropical countries (Figure 1.1).
Figure 1.1 Distribution of Mangrove Ecosystems in the World
Distribution in Sri Lanka
Sri Lanka is a continental island of 65,610 km which is situated in the Indian ocean between latitude 5″.55′,-, 9″.51 N and longitudes 79″.41.’ 81°.53′. close to south eastern India and share the same continental plate, Mangals are found along the sheltered lagoons and estuaries in Sri Lanka (Kariyawasam, 1998) (Figure 1.2).
The extent of mangals in Sri Lanka was estimated to be between 10,000 ha to 12,000 ha. This is less than 0.1% of the total land area of Sri Lanka (CCD, 1985, Amarasinghe, 2003). In many parts of Sri Lanka mangrove ecosystems are being destroyed at an alarming rate due to construction of saoltrns, shrimp farming and urban and village expansions. there fore the current extent of mangroves of Sri Lanka could be well below 1 2 , 0 0 0 ha. (Amarasinghe, 2003). But according to unpublished data of Department of Forest, it was approximately 15,668 ha in 2010. The current annual global rate of loss of mangals vary from 1- 20 % (Guebus, et. al, 2005 therefore it is a great challenge to protect the world mangrove community for the future.
Figure 1.2 Distribution of Mangroves in Sri Lanka (CCD, 1984.)
Sourced from published literature