Location and General Description
Mangrove forests occur in coastal areas of regular flooding by tidal or brackish water and develop on saline gleysols. The extent of mangroves in coastal areas of Thailand, Cambodia, and Vietnam was once high, but much of this area has been destroyed. Extensive mangrove forests once occurred around Pattaya in Thailand and in the areas of Veal Renh and Kompong Som Bays in Cambodia. The absence of more extensive mangrove stands in Cambodia is strongly related to the rocky coastline and lack of major estuaries or river deltas. In Vietnam, the largest area of remaining mangroves is around Camou Point at the southern tip of Vietnam, with smaller areas in the Mekong delta region, in south central Vietnam around Cam Ranh Bay, and in northern Vietnam in the Red River delta area. The central coast of Vietnam is largely free of mangroves because of the exposed coastline, absence of major river deltas, and low tidal fluctuations in this area. Far more extensive stands of mangroves once occurred around the Red River delta in northern Vietnam. The extensive military use of defoliants and napalm during the Vietnam War (1962-1972) destroyed a major part of mangrove forests in southern Vietnam, but these areas are slowing recovering under active reforestation programs today.
Mangrove diversity in the Indochina Mangroves [IM1402] ecoregion is high, with the presence of approximately 60 percent of the mangrove species known from anywhere in south and southeast Asia and Indonesia. The most diverse mangrove communities occur in areas that are inundated at high tide but are otherwise influenced by freshwater flows. Mangrove forests in the Red River delta and associated estuaries and mud flats have lower diversity than mangrove habitats in the south. This low mangrove diversity in the Red River delta area is the result of a combination of cooler growing conditions and a longer and more intense period of human impact.
Mangrove forests typically exhibit strong patterns of zonation. The pioneer species along the open coastline is typically Avicennia alba. Next along a gradient of decreasing exposure and submergence by sea water are Rhizophora apiculata and Brugiera parviflora, which become established after five or six years and grow to replace Avicennia after about twenty years. Higher ground subject to conditions of brackish water rather than seawater is dominated by Avicennia officinalis, Sonneratia caseolaris, Nypa fruticans, and Phoenix paludosa.
There are no endemic mammals in the ecoregion, but many species are known to use mangroves, including the tiger (Panthera tigris), tapir (Tapirus indicus), and siamang (Hylobates syndactylus). This ecoregion overlaps with a Level I TCU (Dinerstein et al. 1997).
Numerous waterbirds use the remaining parts of these mangroves, and many of them are endangered. Included in this assemblage are the lesser adjutant (Leptoptilos javanicus), Storm's stork (Ciconia stormi), white-winged wood duck (Cairina scutulata), and spot-billed pelican (Pelicanus philippensis).
There are several reptile species of conservation significance in this ecoregion, including the monitor lizard (Varanus salvator), the false gavial (Tomistoma schlegeli), and the estuarine crocodile (Crocodylus porosus). The Mekong delta supports a valuable fishery, especially for shrimp (MacKinnon 1997).
This ecoregion is highly threatened in nearly every site where it occurs. About half of the mangroves in southern Vietnam were destroyed by Agent Orange, tank movements, and bombing during the war. Since then, however, the government has launched a large-scale reforestation program. Although protected areas have been created to conserve these mangroves-seven small protected areas (average size of only 117 km2) cover a mere 820 km2 (3 percent) of the ecoregion-the majority of the ecoregion is threatened by a multitude of human activities (IUCN 1991) (table 1).
Table 1. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Do Son 30 UA
Unnamed 60 ?
Xuan Chuy 40 ?
Con Dao 190 II
Botum-Sakor [IM0106] 250 II
Dong Peng [IM0106] 200 VIII
Peam Krasop [IM0106] 50 IV
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
Mangrove forests often are treated as wasteland to be cleared for development (Spalding et al. 1997; Cubitt and Stewart-Cox 1995). In Thailand, large areas of the ecoregion have been logged, primarily to produce charcoal to supply the domestic market and markets in Malaysia, Singapore, and Hong Kong (Spalding et al. 1997). Fifty percent of the mangrove habitat in Thailand was lost between 1975 and 1991. The species most heavily exploited for charcoal are Rhizophora apiculata, R. mucronata, Avicennia marina, and Xylocarpus spp. (FAO 1981). Thailand's mangroves are also severely affected by prawn farming.
Types and Severity of Threats
In addition to exploitation for the domestic and international commercial markets, trees are cut or lopped for domestic consumption as fuelwood (Spalding et al. 1997). In Vietnam and in Thailand, large areas are cleared for aquaculture, salt ponds, and agriculture (Spalding et al. 1997). Poaching and illegal trade of animal products are another important threat, especially to estuarine crocodiles and monitor lizards. Fishing with explosives and trawlers with drag-nets has also caused extensive damage to this sensitive ecosystem (IUCN 1991).
Justification of Ecoregion Delineation
The ecoregion was delineated on the basis of the distribution of mangrove forests in MacKinnon's (1997) biounit 05. Spalding et al. (1997) was used to validate and supplement the information in MacKinnon.
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Eric Wikramanayake, Ramesh Boonratana, Philip Rundel, and Nantiya Aggimarangsee