Biological and ecological information

Types and the distribution of mangroves

Types and characteristics of mangroves

 The mangroves are made up of various different evergreen plants. They physiological adapt themselves to be able to survive in marine waters. They cannot directly use salty water as fresh water, so they have to absorb and store the water in the stems as much as possible and prevent the evapotranspiration by coating their leaves with Cutin, while, some species store the water within the special cells in the leaves. It is found that the concentration of minerals in the cells of mangrove tree is rather high comparing with other plants, so, salt glands the mangrove tree are developed in order to regulate the salt concentration within their cells.

Since the mangrove forest are located in the regular inundation areas, therefore, the areas are regularly flooded causing the lack of oxygen within the soil. Whiles, oxygen is still required for mangroves to be survived, pneumatophores or air roots are developed to have more oxygen by extending upward from the underground roots above the soil surface. The plants of genus Rhizophora are dominant and the other 78 species of mangrove trees.

Six types of mangrove structure have been suggested by Kathiresan (2013) (as shown in figure 2), can be summarized as:

Overwash mangroves are found on tidal flats and regularly influenced by tides; it looks like an island during high tide.

Fringing mangroves are situated on the coast with low steepness, generally found in exposed bays of mainland coast and large islands with mild influence of waves and storms. In the island coast, it is also found at the highest tide elevation.

Riverine mangroves are situated along rivers or creeks or fresh where the freshwater is flowing into the sea.

Basin mangroves are located at low and inundated areas connecting to the terrestrial forest. It is inundated by fresh and brackish waters with long period of time. Some epiphytes such as orchids could be found within this type of mangroves.

Scrub/Dwarf mangroves are found in the areas with the dryness that limit the growth of mangrove. Scrubs and shrubs of about 2 meters tall are dominated within the scrub mangroves.

 Hammock mangroves are quite similar to the basin mangroves except that this forest is occurred in more elevated area than the other types.

Species zonation of mangroves

Watson (1928) divided the zones of the mangroves at the Western Malaysia suggesting the five zones which were ranked by considering the frequency of tidal inundation as a main factor. It can be summarized as the following:

Zone 1: the areas are inundated by all high tides, only Rhizophora mucronata can be grown in this zone.

Zone 2: the areas are inundated by medium high tides, dominated by Avicennia alba, Avicennia marina, Sonneratia alba, and Rhizophora mucronata.

Zone 3: the areas are inundated by normal high tides and dominated with various species of mangroves tree, especially genus Rhizophora. Besides, Ceriops tagal, Xylocarpus granatum, Bruguiera parviflora can also be found in this zone.

Zone 4: the areas are inundated by spring tides, suitable for growth of genus Bruguiera, Xylocarpus granatum and Excoecaria agallocha.

Zone 5: the areas are occasionally inundated by exceptional or equinoctial tides, dominated by genus Bruguiera, Intsia bijuga , Heritiera littoralis, Excoecaria agallocha, and Nypa fruticans.

Figure  Different types of mangrove structure

Source: Kathiresan (2013)

The mangrove distribution in Thailand

The mangroves are distributed along the coasts of the eastern, central and southern parts of both the Gulf of Thailand and Andaman Sea. In the Gulf of Thailand, the mangroves cover along the coasts of Samut Prakan, Bangkok, Samut Sakhon, Samut Songkhram, Phetchaburi, and Prachuap Khiri Khan Province. The mangroves in Chumporn, Surat Thani, Nakhon Si Thammarat, Songkhla, and Pattani are mainly found in the estuaries. In Andaman Sea, mangroves are continuous distributed along the coasts the provinces included Ranong, Phang Nga, Krabi, Trang, and Satun.  The largest mangroves are in Phang Nga Province, followed by Satun, Krabi, and Trang Provinces. Besides, the mangroves in Thailand can be classified based on its structure as follows: (Figure ).

The mangroves along the estuary or brackish water

These mangroves are found along the rivers and creeks that are highly influenced by fresh water. The mangroves connected to the sea have more density than which that are found in the inner part of the river. For examples, the mangroves in Ao Pak Pha Nang of Nakhon Si Thammarat Province are found in the estuaries of Pak Pha Nang river, Klong Bang Chak, and Klong Pak Nakhon. This type of mangroves is also found in the estuaries of Kantang river, Pa Lian river of Trang River, and the mangroves in Ranong and Phang Nga etc.

The fringing mangroves

This kind of the mangroves is found along the coasts or small estuaries, which are insignificantly influenced by freshwater, where small amount of freshwater flows into the mangroves and most areas are dominated with seawater.  Sa Nga et al (B.E. 2530) classified the mangrove zone in Thailand as the followings:

Chumporn Province: the mangroves are landward distributed from the coast to the terrestrial forest zone. It can be concluded that at the front zone of mangroves are dominated by the mixed mangroves of the Sonneratia and Avicennia, while the inner zone is covered by the mixed mangroves of the Rhizophora and Bruguiera. At the back zone, the mixed mangroves of the genus Ceriops and Xylocarpus are found and followed by the mixed mangroves of the genus Excoecaria and Phoenix.

Surat Thani Province: the mixed mangroves of the genus Rhizophora and Avicennia are dominated at the front zone, by followed the mixed mangroves of the genus Ceriops, Xylocarpus which cover in the inner zone. Whilst, the genus Excoecaria and Lumnitzera are found in the back zone.

Nakhon Sri Thammarat Province: the front zone is dominated by Rhizophora apiculata. The inner zones are consecutively covered by the mixed mangroves of the genus Ceriops and Xylocarpus, and the genus Lumnitzera. Finally, back last zone is dominantly covered by the genus Ceriops.

Pattani Province: Rhizophora apiculata is dominant in the front zone, while in the inner zone is covered by the mixed mangroves of Rhizophora apiculata and the genus Bruguiera. Finally, the mixed mangroves of the genus Ceriops and Xylocarpus are found in the back zone.

Ranong Province: the mixed mangroves of the genus Agiceras and Kandelia are found as a dominant species in the front zone. The mixed mangroves of the Sonneratia and Avicennia, the genus Rhizophora and Bruguiera, the genus Ceriops and Xylocarpus and the genus Avicennia, are found in the other consecutive zones. Finally, the mixed mangroves of the gunus Lumnitzera and Phoenix are dominated in the back zone.

Pang Nga Province: The genus Sonneratia, genus Avicennia and Rhizophora mucronata are dominant at the front zone, while the inner zones are covered with the mixed mangroves of the genus Rhizophora and Bruguiera. After that zone, the areas are dominated with the mixed mangroves of the genus Ceriops and Xylocarpus. The final zone is dominated by the mixed mangroves of the genus Excoecaria and Phoenix.

Krabi Province: Rhizophora mucronata and R. apiculata are dominated in the front zone, followed by the genus Ceriops. The mixed mangroves of the genus Ceriops and Xylocarpus are found after the previous zone. The back zone is dominated by Lumnitzera and Phoenix.

Trang Province: The genus Sonneratia and Avicennia are found in the front zone while the genus Rhizophora and the mixed mangroves of the genus Ceriops and Xylocarpus are found at the middle and the back zone, respectively.

 Satun Province: The genus Sonneratia and Avicennia are found in the front zone while the other consecutive zones are dominated by the genus Rhizophora, the mixed mangroves of the Sonneratia and Avicennia, the mixed mangroves of the genus Ceriops and Xylocarpus, and the genus Lumnitzera, respectively. The back zone and the destroyed areas are dominated by Acrostichum aureum.

Figure  Types of mangroves in Thailand 

 

Sa Nga et al. (B.E. 2530) also concluded that the groups of plants in mangrove communities are highly related to the area condition and environmental factors, For examples, the mangrove genus Sonneratia and Avicennia are the pioneer species that grow on the sandy-muddy substrates near the river with regularly tidal inundation. Similar to the previous group, the thick muddy substrates near the river with regularly tidal inundation are also suitable for the genus Rhizophora to grow. The genus Bruguiera and Ceriops appropriately grow on the muddy substrates which are slightly hard with regularly tidal inundation. On the other hand, the genus Lumnitzera and Xylocarpus can grow on the hard muddy substrates with at higher elevation. Besides, some species can grow on the hard muddy substrates where the tidal inundation is occasionally occurred within a month including the genus Excoecaria, Melaleuca, and Phoenix. Furthermore, Acrostichum aureum can also found in the destroyed mangroves.

Environmental factors

Generally, the environment in mangroves is highly different from that of other forests, especially soil in mangroves which is abundant with nutrients from the coastal erosion, rivers and the organic matters that are derived from the decomposition process of the leaves, phytoplankton and algae in mangroves. The salinity of water in mangrove is quite low and varied by tides and amount of freshwater from the rivers and channels. Animals living in the mangroves are influenced by the environmental conditions that are different from terrestrial forests, especially, various benthic organisms which inhabit on the ground or in the mud as well as in the water column. These benthic animals such as shells, crabs, shrimps, worms, flat worms, polycheates, etc., have to adapt themselves to the changing environments that affect on their survivals like water loss, high temperature, low oxygen environment and salinity change. A high biodiversity of mangrove communities are resulted from a composition of various organisms that are able to adapt to survive in this environment, serving as nursery grounds and habitats for many important economic species.    The environmental factors which play an important role for their survivals are the geography that mangroves which are located in coastal muddy flat with regular tidal inundation. The important environmental factors include:

Climates including light, temperature, rain, wind; most of the mangroves are only found in tropical zone where the tropical climate is suitable for their growth.

Tides are the main factor influencing the zones of mangrove plants or aquatic animals. Tidal ranges influence on the physical characteristics of plant structures, especially, the aeration root which are big, above the ground. The height of this root is resulted from on the tidal range.  The more tidal ranges are wider, the higher roots are developed.

Waves and currents help blow the seeds of the mangroves into other habitats, distribute nutrients to the sea that benefit to other aquatic life and coastal aquacultures. Besides, waves and currents are an important factor for coastal sedimentation process.

Salinity of water and soil is significant to the distribution of plants and aquatic animals in the mangroves. A different salinity causes different distributions of plants and animals.

The dissolved oxygen is important to plants and animals; it also controls the types and growth of plants and distribution of aquatic organisms. Furthermore, The rate of decomposition is also affected by the amount of dissolved oxygen.

Mangrove soil is resulted from the deposition process of sediments flowing with water from various sources as well as the decomposition of the organic matter. Soil is important to the growth and distribution of plant and animal in mangroves, for examples, The Rhizophora mucronata is well developed in muddy substrate, while the sandy muddy substrate is suit for the Avicennia marina and Bruguiera gymnorrhiza to grow.

Nutrients, that is vital for the survival of organisms in the mangroves, are derived from both outside and inside the mangroves, that are, rain, rivers, sediments, sea, and the decomposition of organic matter in the mangroves from phytoplankton, diatoms, bacteria, algae, plants, dead animals, etc.