Location and General Description
This ecoregion is not a homogeneous unit but contains forest patches having affinities with other ecoregions. Forest on the west of the Chao Phraya, in the drainage of the Khwae River system, grades into Tenasserim-South Thailand Semi-Evergreen Rain Forests [IM0163] in wetter areas and Central Indochina Dry Forests [IM0202] in the more seasonal or drier areas. As in most of northern and western Thailand, the gibbon present is the wide-ranging Hylobates lar.
The area to the east of the Chao Phraya has more affinities with the Southeastern Indochina Dry Evergreen Forests [IM0210], and is characterized by the presence of some mammals (e.g., Hylobates pileatus) and species or subspecies of birds that are not found west of the Thai Lower Central Plain.
Average annual rainfall is in the region of 1,000-1,100 mm per year in the west to 1,300 mm per year in the east, roughly 80 percent of which falls during the southwest monsoon, May to October. Mean maximum and minimum temperatures are around 34°C to 23°C (Meteorological Department 1987).
In the western area of the region, some areas contain fragments of a lush, moist rain forest formation that has more in common with Tenasserim-South Thailand Semi-Evergreen Rain Forests [IM0163] and supports a few characteristic Sundaic bird species. Such habitats may have been more widespread than formally realized but have been largely lost.
Huge areas have been converted to agricultural land, principally growing tapioca and sugar cane.
Limestone karst formations are found in the plains around Ratchaburi and Phetchaburi and still support some drier forest. One of the most significant such formations lies inside Khao Sam Roi Yot National Park, Prachuap Khiri Khan Province.
Biodiversity information that is known from this ecoregion has been gathered in a few key protected areas. For example, the Khao Ang Ru Nai Wildlife Sanctuary lies on the margins of this ecoregion. Larger mammals present include banteng and pileated gibbon. The area also supports a few freshwater crocodiles (Crocodylus siamensis). Some typical Indochinese birds found here, which are not found west of the Lower Central Plain, include Siamese fireback (Lophura diardi), Indochinese magpie (Cissa hypoleuca), and scaly-crowned babbler (Malacopteron cinereum). Most of these species were once found slightly further west, in the plains of Chonburi Province, before these areas were so completely deforested. A few larger birds are still present in Khao Ang Ru Nai, including, at least until the early 1990s, up to five pairs of woolly-necked stork (Ciconia episcopus, the last such remaining in Thailand), pompadour pigeon (Treron pompadora), and green imperial pigeon (Ducula aenea).
Khao Sam Roi Yot National Park supports a few serow (Capricornis sumatrensis). A Tenasserimese pheasant, previously placed as a race of Kalij pheasant (L. leucomelana crawfurdii) but treated by McGowan and Panchen as a race of silver pheasant (L. nycthemera), occurs here and in other protected areas west of the Chao Phraya plain.
Most of the larger wildlife species, such as Asian elephant and tiger, no longer survive in the wild in this ecoregion. This is because of the heavy human influence on the forests. A single endemic bat is found in this ecoregion (table 1).
Table 1. Endemic and Near-Endemic Mammal Species.
Vespertilionidae* Myotis rosseti*
An asterisk signifies that the species' range is limited to this ecoregion.
Forest in the western area, though largely degraded, is still contiguous with the 17,000 km2 contiguous block of protected habitat in Thailand's western forest complex. The eastern portion is contiguous, or nearly so, with Khao Ang Ru Nai and Khao Soi Dap wildlife sanctuaries, which constitute part of the Southeast Indochina Dry Evergreen Forests [IM0210] and Cardamom Mountains Rain Forests [IM0106], respectively. Though degraded, such fragments still remain and therefore have the potential to be recolonized by large mammals and some birds.
The four protected areas cover more than 1,400 km2 (7 percent) of the ecoregion, but none are greater than 1,000 km2 (table 2).
Table 2. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Khao Laem [IM0119], [IM0163] 840 II
Sai Yok [IM0163] 440 II
Khao Sam Roi Yot 90 II
Chalerm Ratanakosin National Park 60 ?
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
Types and Severity of Threats
Very little of the original forest cover remains, and most of what is left has been selectively logged and degraded by fire. New roads and the spread of industry and new housing from Bangkok, including proposed new towns and a new Bangkok International Airport, will place pressure on remaining land, especially through land speculation. Poor villagers continue to produce charcoal from forest trees in degraded forest areas, leading to further degradation of forest cover.
The Thai government recently completed a gas pipeline from the gulf of Marteban, which has created a new corridor through previously intact forest between this zone and the Tenasserim South Thailand rain forest zone.
The Industrial Estates Authority of Thailand is aggressively promoting industrial development throughout the more rural areas of this ecoregion.
Justification of Ecoregion Delineation
MacKinnon's subunit 05d also extends into the Kayah Karen and Tenasserim mountains, and we included these montane forests in the Kayah-Karen Montane Rain Forests [IM0119] and Tenasserim-South Thailand Semi-Evergreen Rain Forests [IM0163], respectively. The lowland moist deciduous forests along the lower reaches of the Chao Phraya River were placed in the Chao Phraya Lowland Moist Deciduous Forests [IM0108].
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Sompoad Srikosmatara, Philip D. Round, and Eric Wikramanayake