Thailand: Mangrove Restoration at Klong Ngao in the Ranong Biosphere Reserve

Project Stage:
Planning / Design

Start Date:
2007-05-22

End Date:
2007-05-22

Primary Causes of Degradation

Degradation Description

The Ranong mangroves include blocks of former concession forest, disused tin mining areas and abandoned shrimp ponds.

Project Goals

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Monitoring

The project does not have a monitoring plan.

Description of Project Activities:
Nine mangrove sites at four locations with different management histories were identified along Klong Ngao. At each study site a 100 m2 vegetation plot (10 m_10 m) was marked out. A second replicate 100 m2 quadrat was made for each mangrove species at location 3. Within each 100 m2 quadrat, the total number of tree species and the number of individuals per species were determined. The diameter of each tree was recorded using callipers and from this the cross-sectional area, or basal area (BA), was calculated for each tree to give an indication of growth (English et al., 1997). Tree height was recorded using an extendable measuring pole. The abundance of seedlings and undergrowth species was recorded in 10_1 m2 quadrats within each 100 m2 vegetation plot. Soil samples were collected in each 100 m2 vegetation plot at low tide from a depth of 30-50 cm using a soil corer. The soil samples were put into labelled, airtight, plastic bags and taken to the laboratory for analysis. The samples were weighed before and after oven drying (on aluminium foil at 80 _C for 24 h) to determine the water loss by evaporation. Soil moisture content was calculated as a percentage of wet mass. Salinity and pH were measured in surface water and in porewater at a depth of 50 cm at each study site. Salinity was measured using an optical refractometer (Atago, Japan). A portable pH meter (Portamess 911 pH, Knick, Berlin) and a Gel-Plast electrode (Hamilton, Switzerland), which was calibrated with pH 4 and 7 standard buffers, was used to measure pH. The temperature of the soil at 50 cm depth was recorded in situ at low tide using a _20 _C to +60 _C soil thermometer (Gundlach, Denmark). The total numbers of green (freshly fallen), yellow (senescent) and brown (decaying) leaves were counted in 10_1 m2 quadrats. A shore profile was prepared for each study site by surveying at low tide with a total station; shore heights were referenced to a known tidal level above chart datum. Tidal inundation was then calculated for each site (number of days per year reached by high tides) using published tide tables for Ranong Harbour and the methods of English et al.(1997).

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
This present study recorded significant differences in the community structure of intertidal crabs and molluscs between the rehabilitated sites and the mature mangrove forest at site 1a. Overall, the species diversity indices were high for location 1, but faunal density and biomass were lower in the mature forest compared to the mangrove plantations. There is greater habitat heterogeneity in the mature forest, with six common mangrove tree species and a well developed soil structure. In this respect, one important feature of site 1a which was not quantified in the sampling programme, is the presence of many large earthen mounds created by the mudlobster Thalassina anomala. This animal was not evident in the plantation sites, except for some old mounds covering a small part of location 3. Mud lobster mounds provide niche opportunities for other crustaceans, especially sesarmid crabs (Macnae, 1968). The 10-year-old Rhizophora plantation at the former tin mining site still has a reduced macrofauna diversity compared to the 4 and 5-year-old plantations at the degraded former concession forest area, but this almost certainly reflects the much greater habitat disturbance associated previously with tin mining. The mixed Rhizophora plantation in the former shrimp pond is most similar to the mature forest both in terms of tree species number and basal area and mollusc species composition, abundance, diversity and biomass. Although the earth dyke around the pond remains intact, the mangroves planted within the former pond experience a nearly normal tidal regime. This seems to be the principal reason why the survival and growth of the trees has been very high at location 4.

Factors limiting recovery of the ecosystem:
Mangrove vegetation is well known to respond to, and moderate the environmental conditions prevailing in different intertidal zones and habitats (e.g. Kathiresan & Bingham, 2001). In the present study vegetation community structure was not correlated with the environmental data collected. This is not surprising because the mangroves in sites 2-4 were planted artificially, mainly as monocultures irrespective of the local environmental conditions. Tree development and natural colonization by other species is just beginning to modify habitat conditions in the plantations. The importance of mangrove species selection for rehabilitation clearly merits further investigation, as relatively few types of mangrove have been used to date, Rhizophora being the preferred choice (Field, 1996). The mangrove forest at the Klong Ngao tin mine site remains sparse and the Rhizophora trees have developed huge root systems, suggesting that the environmental conditions at the site are still very stressful for mangroves. The tin mining site was almost completely barren in 1985 before it was replanted (Macintosh pers. obs.). Ten years later, with mangrove vegetation now well established as described in the present study, the macrofauna has increased to seven molluscan species (including oysters) and eight crab species. However, the former tin mine site is still dominated by the crab Metaplax elegans, which prefers soft silty sediment and lower shores (e.g. Tesch, 1918; Jones, 1986). As a result of the removal of sediments for tin extraction and corresponding disruption to the habitat, site 2 is still at an earlier stage of ecological recovery compared to the other plantation sites.

Socio-Economic & Community Outcomes Achieved

Key Lessons Learned

From an environmental management perspective, the results from Klong Ngao suggest that some groups of the macrofauna may be useful as biological indicators to assess the ecological impact of mangrove rehabilitation. A high dominance of one species of macrofauna (as in the case of Metaplax elegans at the former tin mining site) may indicate a stressful environment, whereas a higher diversity (e.g. of sesarmid species) may be indicative of a more stable, mature forest (as in site 1a).

The concept of ecological indicator species has been used effectively in temperate regions and could be developed similarly for mangrove systems. The structure of mangrove crustacean and molluscan communities could be a useful tool for habitat assessment, but further research on the ecological role of nominated indicator/key species is needed.

Other Resources

Sonjai Havanond
Mangrove and Swamp Research and Development Group – Royal Forestry Department
Email: sonjai_h@hotmail.com

UNESCO MAB Biosphere Reserve Site:
http://www.unesco.org/mabdb/br/brdir/directory/biores.asp?mode=all&code=THA+04

Primary Contact

Organizational Contact