Masayasu Tatani1,Yuko Tanaka1, Keiji Arita 2 and Eiji Yauchi 3
5-banchi,
Yonbancho, Chiyoda-ku, Tokyo, 102-8451, Japan
1Toa Corporation, Environment Dept.
2 Toa Corporation, Overseas Dept.
3 Dr. Eng. Toa Corporation, Design Dept.
TEL +81-3-3262-5105, FAX +81-3-3239-2793
3E-mail:e_yauchi@toa-const.co.jp
Abstract: This paper reports on the results of a mitigative mangrove replanting project carried out in Singapore. Some 13 ha of mangrove forests were lost due to a project to create a disposal area for incinerated ash and other waste materials. Some time after replanting, survival of 80% to 90% of the planted mangroves was confirmed over the overall replanted area. These results are attributable to efficient supply of saplings, advance consideration of distribution tendencies in areas of mature mangrove growth, and the formation of a buffer zone.
Keywords: mangrove, mitigation, replant, oil spill
Mangrove forests are among the most productive wetlands on earth, representing specialized communities of organisms adapted to the saline environment of the tropics. However, they are also rapidly disappearing throughout the world.
In 1819, mangrove forests covered some 7500 ha, or roughly 13% of the land area of the Singapore island. Today, less than 500 ha remain, despite government recognition of the value of these ecosystems from ecological, education, and recreational viewpoints.
In 1996, the government of Singapore began work on a project to create a site for the disposal of incinerated ash and other waste materials. However, as this project would displace a mature stand of mangroves on the island of Pulau Semakau, one of the Pulaun Islands, the Singapore Ministry of the Environment chose to replant a replacement mangrove forest equal to the area (13 ha) of the mangrove forests lost.
This study reports on the outcome to date of the replacement project.
Located about 8 km to the south of the Singapore main island, Pulau Semakau island is wholly covered with mangrove forest (Figure 1). This project connects two islands through the construction of strong granite bunds to form a central basin between the islands for the disposal of incinerated ash and other waste materials, as shown in Figure 2. The incineration ash is scheduled to be accepted for 30 or more years into the future. The revetment is stone-tiled, and a shielding membrane is paved at the basin side.
The new mangrove forest is intended to replace the 13 ha destroyed by construction. The replant area is divided into two sites: a north area of 5.8 ha and a south area of 7.2 ha. An existing mangrove forest is to be preserved behind the new forest.
Fig. 1 Pulau Semakau
Fig. 2 Disposal Site
Singapore has a tropical rainforest climate with high temperature and humidity. The westerly winds blow in a rainy season from November to February, with lower temperatures during this period. The yearly average temperature is about 27 centigrade degree. Daily squalls are common. The annual precipitation is 2,171 mm.
Temperatures and precipitation levels following the start of the replant were higher than average. In particular, average annual temperature for 1997 and 1998 were 28.0 and 28.1 centigrade degree, respectively, as shown in Figure 3 and 4. Levels of precipitation were some 1.5-2.7 times that of the average year.
MHWS is +2.8m and MLWS is +0.5m in the tide level.
Figure 5 shows the significant wave height and period in the replant area. These values indicate a very calm sea.
Fig. 3 Singapore climate (1997) Fig. 4 Singapore climate (1998)
Fig. 5 Wave height and period
The flora of existing mangrove forests were examined before the replant. The site was divided into four districts on the basis of the physical condition and botanical aspects, as shown in Figure 6.
The flora of each district mangrove is shown in Table 1. District 1 was a wetland with sparsely distributed shrubs. The dominant species was Rhizophora apiculata in district nos. 1, 2, and 3, and other species were common in those districts. More species were found in district no. 4 than in other districts; and the dominant species were Avicennia spp. and Sonneratia alva. These two species often form a seaward colony in mangrove forests. Given the characteristics of each district, the mangrove forests appear to have expanded from 4, to 3 and 2, to 1.
The values for the area and density of the replant zone – 13 ha and 25,000 boles – are based on these field surveys.
Table 1 The mangrove flora
|
District |
No. of Main Composition |
Size |
|
1 |
7 |
3-4.5m |
|
2 |
5 |
3-4.5m |
|
3 |
6 |
5-10m |
|
4 |
16 |
10-15m |
Fig.
6 Survey area
The environment of the replant site was designed based on the existing growth environments, as follows.
The C.D.L was set at from +2.2 m to +1.8 m, which correspond to +0.6 m or +0.2 m in the upper M.T.L since mangrove forests needed to be submerged 50 times per month.
Since muddy soil is suitable for the growth of mangroves, a sand mound was created at first, then mud soil from existing mangrove forests was moved to the replant site.
It is well known that waterways are very important for mangrove forests. The artificial waterway installed at the replant site circulates water using river inflow and tidal flow. The north district and the south district were divided into blocks of 18 and 25, respectively, with these waterways.
R. apiculata was selected as the dominant species in the initial period, based on the transition process of existing mangrove forests. The main composition species mangrove forests in Southeast Asia, and R. apiculata it reaches about 40 m in height. R. apiculata was replanted on about 90 % of replant site, and R. mucronatum, R. stylosa, B stylosa, B. gymnorthiza and Avicennia spp. were planted as other species. The replant density was set at 3 trees/m2 based on densities observed in other mangrove forests.
It is difficult to replant a grown tree. Therefore, viviparous seeds gathered from existing mangrove forests were used. R. mucronatum was planted in littoral zones, because it is seed often found in such areas. Figures 7 and 8 show the arrangement of R. apiculata and R. mucronatum, selected as dominant species. R. apiculata was planted in from +2.2 to +1.9m C.D.L blocks of both the north and south districts. R. mucronatum was planted at four +1.8 m C.D.L blocks and two wave-affected blocks in the north district, and in six +1.8 m C.D.L blocks in the south district.
However, some viviparous seeds flew out without taking root, while other seeds withered after planting. Therefore, the nursery was installed on the floor of an existing mangrove forest, and the resulting saplings after two months planted. Floating nurseries and the sufficient space for nursery transport were prepared to supply saplings efficiently, unaffected by tidal effects.
Growth was improved by this treatment, but parts of the seedlings still died. This was discovered to be due to the admixture of acidic soil from a deeper layer during digging. In particular, the growth of R. apiculata is impeded when soil pH drops below 6.3. Upon determination of the cause of this problem, the affected soil was repeatedly exposed to sea water, raising pH. The adverse effects on plant growth disappeared within several months.
In all, some 400,000 seedlings were finally planted. They were tended to prevent the adhesion of acorn barnacles or seaweed, the weight of which might bend and break the saplings.
Fig.7 Arrangement of replant mangroves (North)
Fig.8 Arrangement of replant mangroves (South)
The area of replanted mangroves was affected by two oil spills.
The first occurred in 1997 in the Malacca Strait, with the spilled oil reaching the replanted area. However, this oil was washed off in a mater of two weeks, since the spill had occurred far from the replant site.
The second spill occurred 1.5 km from the site in 1998. All mangroves from a height of 70 cm to 100 cm were covered with the black oil slick, and no small animals such as crabs or gobys could be seen.
We first planned to wash the mangroves with a neutralizing agent. However, given the potential harmful effects of this treatment on wetland recovery, no treatment was applied. Instead, the mangrove growth was carefully monitored.
Within three months, sprouts had returned to the mangroves. Within six months, the blackened mangrove had turned a light brown, the growth rate of the leaves had recovered, and the oil slick had been flushed from the soil. Within one year, nesting crabs were observed, and recovery was observed over almost the entire replanted area.
Fig.9 Mangroves after oil spill
Fig.10 Growth and ground level
When mangrove growth was inspected 25 to 26 months following planting, R. apiculata saplings that had been 0.25 meters at planting were found to have grown to 0.9 m to 1.3 m in the north district, while R. mucronatum planted at a height of 0.45 m were found to have grown to 1.3 m to 1.4 m. The average growth rate per month was 0.036 m in the former and 0.037 in the latter species. Average growth over twenty months for the two species was 1.9 m for R. mucronatum and 1.2 m for R. apiculata.
In the south district, 0.25 m R. apiculata saplings were found to have grown to a height of 0.9 to 1.2 m, while R. mucronatum seedlings that were 0.45 m at planting were found to have grown to 1.2 m to 1.4 m. Average monthly growth rates were 0.036 for the former and 0.037 for the latter. Average growth over twenty months was found to be 1.0 m in R. mucronatum and 1.2 m R. apiculata. Figure 10 shows the relationship between growth and elevation for R. apiculata by planting district. The growth was large as ground level being low in the south district.
Fig.11 Just after replant
Fig.12 After two years
The study confirmed the survival of replanted mangroves on at least 80% to 90% of the replanted area. The most important factors for these results are summarized below.
(1) Efficient supply of sapling
High-density planting of saplings was made possible by the efficient supply of saplings through the floating nursery. This single factor is thought to have improved the survival rate of the entire replanted mangrove system.
(2) Consideration of distribution tendency
Mangrove growth was also helped by careful consideration of the optimal density following mature grow-out, as well as careful consideration of the site beforehand.
(3) Securing a buffer zone
A river flows between the replant site and an existing mangrove forest. A brackish water zone was created here, and nutrients were supplied from existing mangrove forests to the replant site. Waterways were also created in the replant site to enable natural interaction between existing mangrove forests and the replanted site.
S.K. Lee and W.H. Tan : The regeneration and establishment of mangroves in Singapore, the Proceedings of Symposium on Science and Management of Mangrove Ecosystem, pp.18-22, 1998.