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Southeastern Asia: Vietnam and Cambodia

This ecoregion comprises the seasonally inundated forests that surround southeast Asia's largest lake, the Tonle Sap. Although most of the ecoregion, including the lake, was declared a protected area recently, it was too little too late. The protected area is a paper park with no protection or management, and it was declared protected after most of the habitat had been cleared for agriculture. This is prime rice-growing habitat.

  • Scientific Code
    (IM0164)
  • Ecoregion Category
    Indo-Malayan
  • Size
    10,000 square miles
  • Status
    Vulnerable
  • Habitats

Description
Location and General Description
The swamp shrublands and forest of the Tonle Sap Freshwater Swamp Forests [IM0164] ecoregion include two forest associations that have been described for the extensive floodplain area of Tonle Sap, a short tree shrubland covering the majority of the area and a stunted swamp forest around the lake itself. Similar swamp forests are also present along floodplains of the Mekong and other major rivers in Cambodia. The structure and composition of woody vegetation on the floodplain appear to be largely a function of the microheterogeneity of soil moisture conditions and seasonal flood dynamics. Much of this ecoregion is flooded for at least a six-month period extending from August to January or February.

In general, the dominant woody species of the short tree shrubland form a nearly continuous canopy of deciduous species reaching no more than 4 m in height. The height reached by individual species appears to be related to soil moisture conditions, with the tallest individuals occurring closer to the permanent lake basin and smaller individuals present at the periphery of the floodplain area. Several characteristic species with shrubby growth forms in this community are capable of reaching tree size in swamp forest habitats. The flora of these short-tree shrublands is dominated by species of Euphorbiaceae, Fabaceae, and Combretaceae, together with Barringtonia acutangula. Terminalia cambodiana is an important local endemic.

A band of stunted swamp forest, 7-15 m in height, originally dominated the dry-season shoreline of Tonle Sap, covering about 10 percent of the floodplain, and a similar community once occurred as a gallery forest along the seasonal floodplains of the Mekong, Bassac, and other major rivers in southern Cambodia and Vietnam. This community is covered by water for six to eight months each year, at which time the majority of species lose their leaves. Rather than forming a continuous forest, this community is broken into a mosaic of stands of large trees and open areas with floating aquatic herbs typical of the lake itself. Two tree species, Barringtonia acutangula and Diospyros cambodiana, are the primary dominants of this community.

The giant mimosa (Mimosa pigra) presents a serious invasive species problem. This aggressive species becomes established in fallow fields and disturbed shrubland and swamp forest area after clearance or burning. Once established, giant mimosa forms dense, impenetrable thickets of spiny growth that choke out other native species and have little value as wildlife habitat.

The strong seasonal cycle of flooding around the floodplain of Tonle Sap has made the great majority of woody species deciduous. Rather than lose their leaves in the dry season, however, these species lose their leaves when submerged as the lake deepens and the plants are submerged. However, there are several woody species that remain evergreen despite submergence for six to eight months each year. With only a few exceptions, flowering and fruit production in the floodplain trees and shrubs are delayed for several months after the flush of new leaves. The majority of woody species are laden with fruits and seeds at the time of submergence, suggesting that fish may serve as important dispersal agents.

The shrublands and swamp forests of the Tonle Sap floodplain have been heavily affected by human activities, and this impact has accelerated over the past decade. Very little of the original forest cover remains in pristine condition today.

In addition to areas of woody vegetation, the Tonle Sap Freshwater Swamp Forests [IM0164] ecoregion includes extensive areas of seasonally inundated grasslands growing in a mosaic of scattered individuals of Barringtonia acutangula. These hydromorphic savannas, called veal in the French literature, are commonly saturated for at least six months of the year.

Biodiversity Features
Mammals of conservation significance include the endangered pileated gibbon (Hylobates pileatus), tiger (Panthera tigris), and several threatened species, including wild dog (Cuon alpinus), sun bear (Ursus malayanus), clouded leopard (Pardofelis nebulosa), common leopard (Panthera pardus), and banteng (Bos javanicus). There is one near-endemic bat species (table 1). This ecoregion overlaps with a Level I TCU (Dinerstein et al. 1997).

Table 1. Endemic and Near-Endemic Mammal Species.

Family Species
Rhinolophidae Hipposideros halophyllus

An asterisk signifies that the species' range is limited to this ecoregion.

Although unsuitable for agriculture, areas that have been degraded to reed beds nonetheless are still important sites for waterfowl, providing feeding grounds for the eastern sarus crane (Grus antigone), white-shouldered ibis (P. davisoni), and near-endemic giant ibis (Pseudibis gigantea) (IUCN 1991) (table 2).

Table 2. Endemic and Near-Endemic Bird Species.

Family Common Name Species
Threskiornithidae Giant ibis Pseudibis gigantea

An asterisk signifies that the species' range is limited to this ecoregion.

Current Status
This ecoregion once consisted primarily of permanent and seasonal freshwater swamp forests, but much of the natural habitat has been cleared. Excessive forest exploitation has reduced many areas to scrub or secondary forest invaded by nonindigenous species, and the natural regeneration of large species is very slow (FAO 1981). Most of the remaining habitat is found in northern Cambodia, around the Tonle Sap and Tonle rivers. The large reserve surrounding the Tonle Sap Lake in Cambodia accounts for most of the 5,490 km2 of protected areas in this ecoregion (table 3). However, none of the protected areas have effective management or protection.

Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.

Protected Area Area (km2) IUCN Category
Tam Nong 70 IV
Tonle Sap Great Lake 5,420 VIII
Total 5,490  

Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of Threats
The mangroves and Melaleuca forests of the Mekong delta were severely affected by military activities in the Vietnamese wars; however, these have partially recovered through replanting programs (MacKinnon 1997). The vast floodplains of the Mekong River and Tonle Sap are extensively cultivated during the dry season (IUCN 1991). Local fishing communities have greatly altered the Tonle Sap swamp forests (IUCN 1991). The area of the Mekong delta that lies in southern Vietnam has been severely affected by deforestation in water catchments in Laos, Thailand, and southern China. As a result, there are dramatic fluctuations in the water level of the Mekong river-with frequent floods and low water levels during the dry season-that create increasing problems for local agriculture (IUCN 1991).

Justification of Ecoregion Delineation
We assigned the mangroves, freshwater swamp forests, and peat swamps in MacKinnon's subunit 05a to the Indochina Mangroves [IM1402], Tonle Sap Freshwater Swamp Forests [IM0164], and Tonle Sap-Mekong Peat Swamp Forests [IM0165], respectively.

References
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List

Prepared by: Eric Wikramanayake and Philip Rundel
Reviewed by: