Mangroves and sediment dynamics along the coasts of southern Thailand

Mangroves are a specific type of evergreen forest that is found along the coastlines of tropical and subtropical regions, particularly along deltas and bays where rivers discharge freshwater and sediment to the sea. These mangroves provide important ecological and socio-economic functions to coastal dwellers and societies. For example, they are natural spawning and living ground for many species of fish and crustaceans. Their timber benefits local people as construction material, firewood and charcoals and their marvelous root systems contribute to sediment deposition, mud flat formation and substrate stabilization. Thus, these forests also function as a shelter belt during storms, cyclones and tsunamis. This function was evident when the mangroves in Southern Thailand helped to mitigate the recent 2004 tsunami's devastation on lives and properties of the villages situated behind them.</>

The present status of these valuable resources is critical, particularly in Southeast Asia, where the mangrove forest area is estimated to have declined by more than 50 percent over the past 35 years. The major causes of this loss were encroachment due to population expansion, conversion to aquaculture ponds, coastal erosion, lack of awareness and obscure or poorly enforced regulations, In Thailand, for example, recent satellite images show ample evidence of established and widespread aquaculture ponds along the coast of the Gulf of Thailand due to lack of restriction. More recently, government and public awareness of the importance of mangroves has risen, which has translated into several restoration and afforestation projects. Restoration of degraded mangrove areas was found to require much effort and encountered land-right problems. Therefore, many projects had switched to afforest on newly formed mudflats. Such projects, however, have met with variable success or failed to achieve the stated goals. This was probably because planting was carried out straightaway without carefully determining site-species suitability, appropriate planting technique as well as self-recovery or self-colonization capabilities from nearby stands.

To obtain useful information for supporting sound mangrove and coastal zone management, this dissertation aims to gain a better understanding of mangrove (re)-colonization and factors effecting colonization success as well as factors underlying coastal changes. Firstly, the impact of sedimentation and water turbulence on seedling survival and growth of three common SE Asian mangrove taxa: Avicennia, Rhizophora and Sonneratia, were examined experimentally at the bay scale. Then, this research was broadened to assess the coastal dynamics of Southern Thailand by synthesizing data from coastal surveys over the period 1961-2000 along with field studies. Finally, simple demographic models were developed to simulate the mangrove colonization process.

The two experiments reveal that after successful establishment on the mudflat, seedlings were susceptible to mortality due to sudden high sediment burial and water turbulence. Mortality was found to increase with increasing sediment accretion and none of Avicennia seedlings was able to survive under the highest experimental burial of32 cm while those of Rhizophora and Sonneratia still showed substantial survival (30% and 60%, respectively). Among the three taxa, Sonneratia was the least affected due to burial and exhibited a rapid growth rate. Seedlings of Rhizophora survived less m highly exposed plots (low neighboring plant density) than in sheltered ones. In contrast, survival of Avicennia and Sonneratia seedlings were higher at more exposed plots. This finding confirms the behavior of Avicennia and Sonneratia as pioneer species that may colonize unoccupied mudflat and Rhizophora as a successor. It a!so suggests a higher success of the first two species in re-colonization. However, in areas where sudden high sediment loads are possible, Sonneratia might be better able to cope with burial than Avicennia.

The assessment of changes along Southern Thai coastlines revealed that the coast of the Gulf of Thailand had undergone more change than the western side of the peninsula. Ongoing erosion occurred irregularly at the high energy coastal segments and was observed along 29% and 11% of the total coastal length of the east and the west coasts, respectively. Subsequent total area loss accounted for 116 ha annually (91 and 25 ha for the east and the west coasts, respectively). Factors responsible for coastal erosion were found to be mangrove area loss, increase of shrimp farming area, less sediment delivery to coastal area due to upstream river damming and exposure to fetch from the monsoonal wind. Erosion was less in mangrove-dominated coastlines, and progression occurred mainly in sheltered and mangrove-dominated coastal segments. Areal progress accounted for 37 ha per annum on the east and 5 ha on the west coasts. Thus, summed over the whole of Southern Thailand, a net erosion of 74 ha y-1 was observed over the past three decades. These results also reveal that the existence and progression of the mangroves as well as sufficient sediment supply to coastal area contributed significantly to coastal stability.

Thirty-year runs of the demographic simulation model revealed that herbivory and water turbulence were the main factors influencing the success of colonization in a mangrove-dominated bay while gradual sedimentation and salinity had little effect. Avicennia was the most successful taxon in colonizing open mudflats followed by Sonneratia while Rhizophora exhibited less success. However, colonization success may vary with changes in environmental conditions as revealed by our predictive scenarios. For example, accelerated sea level rise will reduce the success in colonization of all three taxa and lack of freshwater discharge due to river damming may enhance the colonization success of Avicennia but adversely affect Sonneratia.

Overall, this study suggests that success in re-colonization of mangroves depends on both ecological and hydrological factors. Seedling herbivory and water turbulence are important factors that may seriously hinder colonization success. Also, the three taxa studied were found to differ critically in colonization capacity. Gradual sedimentation has little effect on mangrove colonization but positive sedimentation provides habitats for mangrove to colonize, particularly, in sheltered coastal segments. Less sediment delivery due to upstream river damming associated with conversion of mangrove and beach forest to aquaculture ponds will probably intensify coastal erosion in vulnerable areas. Therefore, managers should pay more attention to the balance between ecological and socio-economic demands for sustainable development of the coastal zone.