Southeastern Asia: Northeastern Thailand, extending into Laos

This small ecoregion along the northern reaches of the Mekong River represents a transition from the dry forests of the Khorat Plateau to the moister forests of the Annamite Mountains. Like most ecotone habitats, the ecoregion contains a mix of species from dry and mesic habitats, increasing the overall biodiversity. But again, like many productive habitats that lie along river plains, much of the natural habitat has been cleared for agriculture.

  • Scientific Code
  • Ecoregion Category
  • Size
    6,500 square miles
  • Status
  • Habitats

Location and General Description
The Northern Khorat Plateau Moist Deciduous Forests [IM0138] are located in the middle Mekong River Valley, along the border of Thailand and Laos. The average annual rainfall for Nong Khai Province for the period 1956-1985 was 1,629 mm, and mean minimum and maximum temperatures were 21.6 and 31.8, respectively (Meteorological Department 1987).

The floristic structure of mixed deciduous forests in the Mekong Valley of southern Laos and adjacent areas of northeastern Cambodia suggests an intermediate ecological habitat between more mesic semi-evergreen forests and more xeric deciduous dipterocarp forests. Lowlands in this area commonly receive 2,000-3,000 mm of precipitation annually, with five to six months of dry season.

In many respects these forests represent a deciduous form of semi-evergreen forest and therefore are similar to the Central Indochina Dry Forests [IM0202]. The most common canopy species are Lagerstroemia angustifolia, Afzelia xylocarpa, Xylia xylocarpa, Peltophorum dasyrrachis, and Pterocarpus macrocarpus. In some areas, Lagerstroemia may form almost single-species dominance. In northern Cambodia, there is a transition between mixed deciduous forest and forms of semi-evergreen forest where deciduous species exhibit a strong co-dominance with evergreen species. Any separation of such types along this transition is somewhat arbitrary. This separation is complicated by the fact that many taxa such as Xylia, Pterocarpus, Lagerstroemia, and Irvingia have a wide range of ecological tolerance and are found in deciduous dipterocarp woodlands and mixed deciduous and semi-evergreen forest communities.

Biodiversity Features
Within this ecoregion, dry evergreen forest contains much greater mammalian diversity than deciduous forest because no arboreal mammals are known to feed on the fruit or foliage of any dipterocarp except for Shorea platyclados (Lekagul and McNeely n.d.). Several endangered species live here, including the pileated gibbon (Hylobates pileatus) and Asian elephant (Elephas maximusas), and several threatened species such as wild dog (Cuon alpinus), Himalayan black bear (Ursus thibetanus), sun bear (Ursus malayanus), clouded leopard (Pardofelis nebulosa), and common leopard (Panthera pardus). Tiger (Panthera tigris), gaur (Bos gaurus), and banteng (Bos javanicus) probably have already disappeared from the Thai portions of this area (Srikosamatara and Suteethorn 1995). The ecoregion overlaps with a Level II TCU (Dinerstein et al. 1997).

There appear to be significant differences in the forest bird community on the Lao and Thai banks of the Mekong River. The Indochinese endemic red-collared woodpecker (Picus rabieri) has been found at Sangthong, Lao PDR (Duckworth 1996) but not as far on the Thai bank.

Some sections of the Mekong River in this ecoregion are outstanding examples of upper perennial riverine habitat, with exposed bedrock, sand, and shingle bars and rapids. Extensive areas are covered by Homonoia riparia scrub (Euphorbiaceae), which support what is probably the largest population of the scarce Jerdon's bushchat (Saxicola jerdoni) known anywhere in southeast Asia (Duckworth 1996). A few great thick-knees (Esacus recurvirostris) are also present, as are river lapwings (Vanellus duvaucelii) and plain martins (Riparia paludicola). All these species are nationally at risk in Thailand (Round 2000) and Laos (Thewlis et al. 1998).

Current Status
The five protected areas cover 1,965 km2 (12 percent) of the ecoregion (table 1). The IUCN category on three of the protected areas is unknown, and so is the extent to which the ecoregion's biodiversity is protected.

Table 1. WCMC (1997) Protected Areas That Overlap with the Ecoregion.

  Protected Area Area (km2) IUCN Category
Phu Phan National Park 645 ?
Huai Huat National Park 830 ?
Phu Sri Than WS 250 ?
Phu Wua 190 IV
Phu Kao-Phu Phan Kham [IM0202] 50 II
Total 1,965  

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

Types and Severity of Threats
Most larger bird and mammal species have been greatly reduced or extirpated. A few elephants remain inside Phu Wua Wildlife Sanctuary. Hunting by ethnic Lao people is even more ubiquitous than in northern Thailand, for example, so that even common, open country birds are scarcer here than in many other ecoregions. Use of fire by hunters and farmers is widespread and continues to degrade remaining forest. Human use of fisheries on the Mekong River is almost certainly unsustainable, and other forms of human use (e.g., recreation, ferry traffic) place added pressure on riparian habitats.

Justification of Ecoregion Delineation
The tropical moist deciduous forests to the west and northwest of the Luang Prabang Range in northwestern Thailand and Laos are an extension of the teak (Tectona grandis)-dominated deciduous forests (Rundel and Boonpragob 1995), but the vegetation community of the moist deciduous forests in the Meking plains, near Vientianne, are dominated by Fabaceae, Lythraceae, and Rubiaceae. Therefore, these forests were placed in separate ecoregions, the former in the Northern Thailand-Laos Moist Deciduous Forests [IM0139] and the latter in the Northern Khorat Plateau Moist Deciduous Forests [IM0138].

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

Prepared by: Philip D. Round, Sompoad Sriksomatara, Nantiya Aggimarangsee, and Eric Wikramanayake
Reviewed by: