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Review of Globally Important Ecosystems and Their Biodiversity of the Lower Mekong Basin


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Lowland River Channels: The Mekong and its Largest Low Gradient Tributaries


Lowland river channels include the main channels of the Mekong and its major tributaries, the Nam Ngum, Songkram, Pak Mun, Xe Kong, Se San, and Srepok. These are permanent rivers which in the dry season are relatively slow-flowing forming a series of deep pools interspersed by rapids and islands. This is most pronounced on the Mekong at Siphandon (Lao meaning 4,000 islands) in southern Laos where the best example of a braided river in SE Asia can be found, and at the Ramsar site in Stoeng Treng and in Kratie Province in Cambodia. The main habitats associated with these systems are detailed below:

      1. Main river channels


Water height in the main river channels varies by up to 10 metres between the wet and dry seasons. These channels are vitally important for the seasonal longitudinal migration of white fish species (see 3.3.3), including the commercially important cyprinids, Henicorhynchus lobatus (locally called ‘trey reil’) and H. siamensis, which migrate over 800 km from Lake Tonle Sap into Laos. Other highly migratory groups are catfish species, including the endangered Giant Catfish Pangasianodon gigas, Pangasius sanitwongsei, P. sutchi, P. larnaudi and Jullien’s Barb Probarbus jullieni. Although the main channels support a rich assemblage of aquatic species, they exhibit little endemism.
The Mekong River and its wider low gradient tributaries are important for a distinctive guild of riverine bird species. The Globally Vulnerable Indian Skimmer Rynchops albicollis may have already disappeared from this habitat in Cambodia, Laos and Vietnam, and the Globally Vulnerable Black-bellied Tern Sterna acuticauda and Little Tern Sterna albifrons appear to be on the verge of doing so. Other significant species in this habitat include the Globally Near-Threatened Grey-headed Fish Eagle, and Great Thick-knee Esacus recurvirostris, River Lapwing Vanellus duvaucelii, and Small Pratincole Glareola lactea.
      1. Deep pools


Deep pools have formed in fast flowing channels of larger rivers. These pools are maintained by the scouring of the high river flows during the wet season. In the dry season, these areas are connected by shallower stretches of the main rivers. They are very common on the Mekong in southern Laos and northern Cambodia, and on some major tributaries including the Se San, Srepok and Xe Kong.
Deep pools are a vital dry season refuge for both resident and migratory fish species and undoubtedly contain a diverse assemblage of undescribed invertebrates. These habitats are a vital dry season refuge for the Globally Critically Endangered Irrawaddy Dolphin Orcaella brevirostris, which prefers areas with a water depth greater than 8 metres.

      1. Small islands and riverine sand-bars


Small islands and riverine sand-bars are common on stretches of the Mekong and its tributaries, including the Se San, Srepok and Xe Kong. They are formed by natural deposition during seasonal high river flow. The plant Anogeissus rivularis dominates the pioneer community of low-lying vegetated sandbars and rock outcrops. The older, larger sandbars, built up into islands over many years, have more substantial vegetation similar to the neighbouring gallery forest.
The smaller sand bars and islands provide safe breeding sites for many species of waterbirds, some of which are globally rare and endangered. The only recent confirmation in Indochina for the breeding of the Globally Vulnerable Black-bellied Tern was recorded on such islands. They also support the largest numbers of Great Thick-knee, River Lapwing, and Small Pratincole in South East Asia.

      1. Seasonally-inundated riverine forest


A unique type of riverine inundated forest occurs on the Mekong River in the Stoeng Treng Ramsar site (between Stoeng Treng town and the Lao border). It is found on sandy islands and on isolated rocky outcrops and comprises tree species identified tentatively Barringtonia sp., Eugenia sp., and Arcacia sp.. Strangely, the trunks of the trees are often bent almost horizontal in the direction of the river flow with branches and leaves swept in the same direction (giving the appearance of being swept away). In some cases these trees are supported by large buttress roots (1-2m. high) protruding downstream to support the trunks against the current. Many of the trees have reduced, narrow leaves reminiscent of rheophyllic trees and there are many epiphytes or parasites particularly of the fig Ficus sp..
The biodiversity values of this habitat are not known. The inundated forest may be important for fish breeding and shelter during peak flow, and some fish species are known to feed on the fruit of the trees. The associated rocky substrate and rapids are definitely important for suitably adapted fish species. The forest may be important for monkeys and gibbons.


    1. Permanent and Seasonally-inundated Floodplain Wetlands


A number of permanent and seasonal lotic and lentic wetlands are found in the Lower Mekong Basin. The high wet season water levels of the Mekong inundate many of these wetlands seasonally; groundwater and seasonal monsoonal rains maintain others. These provide some of the most productive habitats in the Lower Mekong Basin and include reed and sedge beds, swamps, lotus ponds, inundated grasslands and inundated forest.
The seasonal changes in water level of the Mekong inundated plain drives a seasonal migration of large waterbirds between wetlands. In the dry season, many species move to permanent wetlands and grassy plains around Lake Tonle Sap and the Delta, while in the wet season they retreat to higher seasonal wetlands in northern Cambodia and southern Laos.

      1. Seasonally-inundated riparian forest


One of the most important wetland habitats of the Lower Mekong Basin is the seasonally-inundated riparian forest found on the gently-sloping plains adjacent to lakes, rivers and tributaries and submerged by the seasonal 8-10 metre flood levels of the wet season.
The composition of this forest type shares little with swamp forests and mangrove systems. Over 200 species of plants have been found in these inundated forests. Major communities include Barringtonia acutangula, Elaeocarpus madopetalus and Diospyros cambodiana; floating and emergent herbs including Brachiaria mutica, Eichornia crassipes, Polygonium barbatum, P. tomentosum and Sesbania javanica, and a diverse mixed scrubland containing over 60 species. The woody species of this forest are often laden with fruits and seeds at the time of inundation, providing food for the 34 species of fruit-eating fish of the Lower Mekong Basin. Over 200 species of fish use this habitat as a feeding, breeding, and nursery ground and it is vitally important for breeding colonies of large waterbirds. The forest of Lake Tonle Sap is the best known and most productive example of this habitat.
        1. Seasonally inundated riparian forest of Lake Tonle Sap


The seasonally inundated forest of Lake Tonle Sap is legendary. Acting as part of the Mekong floodplain, the flood pulse from the Mekong River reverses the flow of the Tonle Sap River to inundate the forest surrounding the Lake which expands in area from around 300,000 hectares to 1.2 million hectares at peak flood height, inundating over 500,000 hectares of forest. As the water level of the Mekong River drops, the Tonle Sap River resumes its normal direction of flow and discharges the lake into the Mekong Delta.
Over 170 species of plants have been identified from the shores around Lake Tonle Sap. Many species are deciduous, shedding leaves with the rising water. Over 140 species of fish have been recorded in the Lake. The lake’s productivity is prolific, making a substantial contribution to the estimated 400,000 tonnes annual fish production in Cambodia.
Lake Tonle Sap’s inundated forest is one of the most important breeding sites for large waterbirds in Asia. Species breeding in the forest include the Globally Endangered Greater Adjutant Leptoptilos dubius and White-winged Duck Cairina scutulata; Globally Vulnerable Spot-billed Pelican Pelecanus philippensis, Milky Stork Mycteria cinerea, Lesser Adjutant Leptoptilos javanicus; and Globally Near-threatened Oriental Darter Anhinger melanogaster, Painted Stork Mycteria leucocephala. These species are believed to migrate to other wetlands in the Lower Mekong Basin during periods of high water level on Lake Tonle Sap.

      1. Marshes, small pools and seasonal wetlands in the lowland plain


Extensive seasonal and permanent marshes, small lakes, and other palustrine wetlands occur throughout the Lower Mekong Basin. They are usually shallow, filled by seasonal rainfall and typically are connected to river systems which in the wet season form the inundated plain of the Lower Mekong Basin. They contain a mosaic of wetland habitats including reeds, sedge, lotus beds and open water. Submerged communities are dominated by Ceratophyllum demersum and Utricularia aurea.
In the dry season, these wetlands are vital in maintaining breeding stocks of floodplain fish, including air-breathing species (e.g. gouramies, walking catfish), while in the wet season they function as breeding and nursery grounds for many fish species such as Oryzias mekongrnsis, Rasbora spilocerca, Indostomus sp., Boraras micros, and Amblypharyngodon chulabornae. These wetlands are important for almost all waterbirds in the Lower Mekong Basin, particularly cormorants, Oriental Darter, Spot-billed Pelican, Greater and Lesser Adjutants, Milky Stork, Woolly-necked Stork Ciconia episcopus, Black-necked Stork Ephippiorhynchus asiaticus, Painted Stork, The Globally Endangered White-shouldered Ibis Plegadis davisoni, Glossy Ibis P. falcinellus, Black-headed Ibis Threskiornis melanocephalus, White-winged Duck, Pallas’s Fish Eagle Haliaeetus leucoryphus, Grey-headed Fish Eagle, the Globally Vulnerable Masked Finfoot Heliopais personata, and the Globally Near-threatened Sarus Crane Grus antigone.
        1. Treapangs and nongs


Of particular note in this grouping are treapangs and nongs (Khmer and Lao names), which are small wetlands or wet grassy glades, often less than 100 ha in area, within mosaics of deciduous dipterocarp and semi-evergreen forest. Many are seasonal, while others maintain water perhaps from groundwater, throughout the dry season. Found mainly in southern Laos and northern Cambodia, these wetlands are of vital importance for biodiversity conservation.
In the dry season, they provide some of the few water sources for large mammals (elephants, gaur, deer, rhinoceros and wild cattle species; see Section 3) while in the wet season they form feeding and breeding habitats for species such as the Sarus Crane. In particular,

  • The Globally Critically Endangered Giant Ibis Pseudibis gigantea was re-discovered in this habitat in southern Laos in 1993, and it is also critical habitat for White-shouldered Ibis, Greater and Lesser Adjutants, and White-winged Duck. The highest populations of the Globally Vulnerable Green Peafowl are probably concentrated in these areas.

  • One of the few remaining viable populations of the Globally Critically Endangered Siamese Crocodile is found in these wetlands.


      1. Inundated grasslands


Seasonally inundated grasslands are common on the floodplains of the Lower Mekong Basin. Close to the water edge, floating or emergent vegetation forms dense mats or stands up to 3 metres tall. As water levels rise, dense mats may dislodge and float, propelled by currents or the wind. The main species include Achyranthes aquatica, Brachiaria mutica, Eichornia crassipes, Polygonium barbatum and Sesbania javanica. Other plant species found on the upper reaches of the inundated plain include several grasses, including Echinochloa stagina, sedges including Cyperus pilosis, Rhynchospora sp., and dicotyledons such as Aeschynomene indica, Impatiens sp., Ludwigia hyssopifolia and Nelumbo nucifera (lotus).
As in the Delta, these areas and support Sarus Crane, White-shouldered Ibis and Greater and Lesser Adjutants. Although, in the Lower Mekong Basin, these areas are greatly disturbed, they do hold more substantial grasslands that other parts of S.E. Asia and thus are a priority for conservation. They are of crucial importance for the Globally Endangered Bengal Florican Houbaropsis bengalensis.

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