The distribution of different species of plants and animals in mangrove forests depends upon basic ecological factors such as grain size, the type of substrate, tidal period, pH, salinity, the level of the soil, and light.
In Thailand, especially around the upper Gulf coastline, mangrove forests consist of Avicennia alba as a pioneer species. It can be found on the outer limits of the shoreline. After Avicennia, the most frequent species is Rhizophora apiculata. Further inland, there is a mixture of different plant species such as Rhizophora mucronata, Xylocarpus moluccensis, Excoecaria agallocha, Bruguiera conjugata, and other Bruguiera spp. Nipa plants line the innermost part of the mangrove forests.
However, their predominance varies with the depth of the mangrove belt and the tidal characteristic, i.e. inundation pattern. This can be expressed by the Important Value Index (IVI) which is calculated from the total of relative frequency, relative density and relative basal width.
On the Andaman coast, in Phang-Nga province, the mangrove forest has about 50% of its total width under the tidal range and the Important Value Index of Rhizophora mucronata, Rhizophora apiculata and Sonneratia alba at 20 m from the sea was found to be more than 50%. Xylocarpus obovatus at the same distance from the sea had an IVI of 45.5. Other species growing at the same level had IVIs of lower than 30. At 60 and 100 meters Rhizophora apiculata had high IVIs between 116 and 128 (Kongsaenchai, 1976).
On coastal areas with a width of between 250 to 360 meters and a tidal inundation of about 130 to 200 meters, Rhizophora mucronata had an IVI of 70.2 and 89.3 at 20 and 60 meters respectively. But conversely, Rhizophora apiculata had an IVI of only 44.3 at 20 meters, and a higher IVI of around 131.5 to 175.4 between the levels of 60 to 340 meters. Other plants such as Sonneratia alba and Avicennia spp had a high IVI at 20 meters. A mixture of other plant species can be found in different zonations.
The relationship between the soil composition and the distribution of mangrove species at Amphoe Khao Saming, Trat Province, was shown by Patanapholphaibool (1979). He used the predominance of each species as distribution key of these mangrove species. At his study site, he found that Sonneratia caseolaris also predominated at the outermost part of the mangrove forest. Its presence decreased gradually until zero towards the landward site. The presence of Avicennia alba on the other hand was less at the outermost edge when compared with the landward zone. The small leave mangrove (Rhizophora apiculata) was found at the zone immediately following the previous mentioned two species.
Soil properties such as grain size, organic contents and salinity level, are directly correlated to the important species of plants growing at different zones, notably Rhizophora mucronata and Rhizophora apiculata.
In addition, various chemical properties of the soil, such as its pH and its phosphate and potassium content, may have some correlation with the distribution of other plant species. However, the measured variables in the soil composition did not allow any conclusion as to the controlling factors to the distribution of these plants.
The ecological relationships of animal species was studied by Frith et al. (1976) who reported on the zonation of macrofauna on a mangrove shore at Phuket Province in Thailand. They concluded that the substratum and the exposure period at low tide were the most important factors limiting the distribution of animals. Some well-known physico-chemical parameters such as temperature, salinity and pH were considered to be of minor importance.
In recent years, the Forestry Department has had a policy of giving long term contracts for wood cutting. For example, the contract may be for 15 years covering an area of 1 million rai or 1670 km2. The Department requires cutting in alternative strips, (i.e. shelter cutting). This method of cutting allows for natural reforestation.
In the upper portion of the Gulf of Thailand, such as the area around Samut Songkram province, mangrove forests are planted and allowed to grow for 8 to 12 years. Then, they are completely cut down (clear cutting). The amount of wood that is cut down for firewood is about 85 m3ha.
The clear cutting method destroys the habitats for certain plant and animal species that depend on the arial roots and trunks of the mangrove plants.
In addition, the clear cutting method changes many environmental factors. For example, it causes the air and soil temperature to rise and the soil to erode and harden which inhibits the development of certain animal larvae. These larvae need to bury themselves in soil and mud for feeding.
The study and data of the effects of the clear cutting method on various organisms in the mangrove area has not been done extensively, and consequently we cannot estimate the damage.
Mariculture and nearshore fisheries in Thailand need to be expanded to acquire more areas for shrimp and fish farming. To increase the yield of shrimp to at least 15 000 tons a year, the area has to be increased to 6 420 ha. Furthermore, we need to expand the areas for oyster and mussel farming.
In certain areas of Petchaburi province, 4 ha of mangrove forest have been allocated per person for shrimp farming by using a natural supply of shrimp larvae. The farming process starts by making the earthen dike around the mangrove forest, followed by cutting of all the trees to avoid rotting.
The obvious effect of the clear cutting method on shrimp or shellfish farming is that it necessitates irrigation. So water has to be trapped for farming purposes, which alters the environment greatly.
In addition, teaseeds or rotenone roots are used to kill fish that come in with the irrigated water. These fish are the main predators of shrimp (Piyakarnchana et al., 1975). For example the fish species Lates calcarifer can eat 5–7 shrimps of 4.1–0.5 cm in size per day.
The use of pesticides to eliminate fish in mari-farming destroys many of the economically important fish and therefore reduces their abundance in the area. This effect still needs to be studied more extensively.
To live or farm in mangrove forest is prohibited by law. In some areas it is illegal to engage in tin mining or to set up a charcoal kiln in the forest itself.
Certain governmental and quasi-governmental agencies have been pushing economic development in mangrove forests, such as the building of shopping centers, boat docks and customs' offices. Many of these programs are designed to promote tourism but their effects have already altered certain environmental factors.
Discharge of waste water from the various activities will not only have direct impact on the mangrove forests but will also effect the ecosystem in the wider area. The building of roads in these areas very often damming off parts of the swamps will certainly affect many living organisms, possibly to the point of extinction.
In the Gulf of Thailand, some fishermen use very fine fishnets to trap fish in mangrove forests at high tide and collect them at low tide. This method has greatly destroyed many post larvae of various organisms in the area.
Salt farming is another important activity that affects the ecosystem of estuaries. To make a salt farm, the soil has to be compacted in order to hold salt water for evaporation and saturation. This causes extreme salinity and high temperature in the area, and therefore is harmful to many organisms in the ecosystem.
During 1970–1973, pollution from the Mae Klong river greatly affected the mangrove ecosystem. In fact, the pollution in this river started in 1969. The main polluters were the sugar refineries situated on the Mae Klong river bank (see Fig. 15). The factories discharge waste water directly into the river without any treatment. With the reduced amount of water in the river during the summer months from March to April, the pollution accumulates and is highest in concentration.
In 1970, pollution from the Mae Klong river damaged cockle farms in the estuarine vicinity. In some areas, the farms suffered total losses. The damage to cockle farms alone at the time, in terms of monetary value was estimated at about 6.4 million baht (320 000 US$). When the damage on other organisms was also considered, the loss was estimated to be as high as 50 million baht (250 000 US$) (Ratasuk, 1973).
The Bangkok Post newspaper on April 9, 1973, reported that about 10 tons of shrimp and fish in the Mae Klong river had died. In addition, a very large “carplike fish” weighing about 52 kg was also found to have died.
Besides cockles, razor clam beds, one of the tourist attractions of the area, were also destroyed. The fishermen lost daily earnings of about 7 500 baht (375 US$). It was the effects of Mae Klong River pollution that mainly caused the decrease of the cockle productions in Samut Songkhram Province from 1971 and 1974 (see Table 8).
To this day, the lost marine life has not been replenished. Many cockle farms had to close permanently.
The Department of Industrial Works has established a central treatment plant to treat waste water before discharging it into the river. In 1975–1977, the Department requested all factories outside the central treatment plant to set up their own treatment plants, which has helped improve the quality of the river somewhat (Piyakarnchana et al., 1979).
Another important factor affecting the organisms in the estuarine of the Mae Klong river is the fast river flow. Piyakarnchana et al. (1979) reported on the decrease of benthic organisms in the river mouth area by this factor.
In 1975 it was reported that other rivers, such as Pranburi river, were also polluted by the canning factory and sugar cane mills, causing death of many organisms. In addition, Petchaburi river was polluted from the nearby sugar refinery and distillery plants. After the law for treatment of discharge water came into force, pollution from the sugar refinery has been reduced greatly. The distillery plants have solved the problem temporarily by depositing the slop wastes on land elsewhere, and not directly into river.
The current damage from pollution in the Petchaburi river cannot be estimated. The only indication of the extent of damage is that only 10% of the cockle farms now yield a production within a normal range.
Pollutants from the Chao Phraya river come from municipal waste (about 70–75%) and industrial waste (25 – 30%). An obvious effect from these pollutants is the more and more frequent occurrence of the red tides phenomenon in the upper part of the Gulf of Thailand during summer. The spread of these red tides is becoming wider (Piyakarnchana and Tamiyavanish, 1979), but the effect of red tides on the ecosystem has not been studied thoroughly.
The results of Silpipat's research (1979) show the amount of oxygen in the water around the five major river mouths is low in September, especially at the mouth of the Chao Phraya river (Fig. 17). In 1977, the amount of oxygen reached a record low to the point of being harmful to the ecosystem in the area.
The amount of suspended sediments being carried out to the sea by rivers (see Fig. 16), are directly correlated with the river flow. The highest amount of sediments was transported during June-December. The effects of excessive loads of sediments on the ecosystem, on nectonic as well as benthic communities, has not been studied in Thailand.
Although land reclamation has been carried out in many coastal areas in the southern part of Thailand, the effects on the surrounding biotope have not been carefully studied.
The major damage to the mangrove ecosystem was caused by tin mining. Mine tailing were discharged ill-considerately so as to accumulate and change the habitats. This in turn damaged the forest and the government was forced to reforest at a cost of 330 baht per rai ( = 100 US$/ha) (National Research Council of Thailand, 1977).
Tin mining in the sea has caused sediments to disperse over a wide area. In some parts, the sediments accumulated to the point of blocking the water way. Finer sediments can drift very far and settle on coral reefs. These finer sediments are believed to be the major cause of damage to the coral reefs around Phuket island.
In addition, tin mining also causes an increase in the amount of heavy metals in the Andaman sea. According to one estimate, one tin mining dredge can create about 1 000 000 cubic yards of sediment per year, which is dispersed over a wide area.
The effect of petroleum pollution originating from the sea on the ecosystem has not been studied extensively. Only the result of finding tar-balls on certain beaches during 1975 – 1977 was reported (Piyakarnchana et al., 1979). The only conceivable effects of tarballs at present are their annoyance potential on the beach, since they are possibly responsible for the bad small in donax shells. Therefore, fishermen and beach inhabitants in that area lose a portion of their income.
