Location and General Description
This ecoregion consists of the freshwater swamp forests in the lowland alluvial plains of the Chao Phraya River in central Thailand and extends north up the valleys of its major tributaries, the River Ping and River Nan. It is roughly 400 km north to south and, at its widest, about 180 km from east to west.
The area can be roughly divided into two parts. The Lower Central Plain, which extends north as far as the province of Ang Thong (ca. 15( north), represents an area of Quaternary deposits of silt, of 15-30 m depth, overtopping the soft marine clays laid down when the area was once a huge bay of the South China Sea, about 6,000 to 8,000 years b.p., when sea levels were approximately 4 m higher than at present. The area is flat and low lying. The Lower Central Plain has an average elevation of about 2 m above mean sea level. Above this, the Upper Central Plain extends north up the Chao Phraya River and lower parts of the valleys of the Ping and Nan rivers and lies at >20 m above sea level. This plain was never subject to significant tidal flooding (Sinsakul 1997).
The area has a moist monsoonal climate, receiving approximately 1,400 mm rainfall per year. Mean maximum and mean minimum temperatures are around 33°C and 24°C, respectively, for Bangkok (Meteorological Department 1987).
Four major rivers enter the Central Plain. These are the Me Klong (Kwae) River system on the west; the Chao Phraya and its major branch, the Tachin River, in roughly the center of the region; the Pasak River, a tributary of the Chao Phraya; and the Bang Pakong River, entering from the east.
As alluvium built up, brackish water swamps were gradually superseded by freshwater habitats in which extensive open swamps were occupied by permanent or seasonally inundated vegetation. Successional vegetation, including open water, mats of floating vegetation, Typha, Phragmites, and scrub alternated with stands of towering Dipterocarpus alatus trees, and other species on higher ground, such as along river banks; Pandanus was common in many swampy forest areas.
These forests and swamp vegetation formerly transitioned into mangroves toward the coast and along the lower reaches of major rivers.
Phragmites and other marsh grasses once were widespread but have largely disappeared and been replaced by Typha angustifolia. A remnant of Phragmites marsh is present at Khao Sam Roi Yot National Park (shared with ecoregion [IM0108]), which to some extent represents a microcosm of central plains swamp habitat.
The northern part of the area has long been settled and to some extent cultivated. It included the Kingdoms of Sukkhothai (ca. twelfth to thirteenth century) and Ayutthaya (mid-fourteenth century to mid-eighteenth century.) Sukkhothai was based on an older, less intensive Mon-Khmer style of cultivation, which was less intensive and allowed much wildlife to coexist.
Although Nakhon Pathom on the western margin of the lower central plain was a major center of civilization as early as the fifth century a.d., much of the lower Central Plain remained a little disturbed swampland until the Siamese kings moved their capital to what is now Bangkok in 1782. In the early nineteenth century, the area began to be canalized, the banks beng claimed for agriculture by the aristocracy. With Siam's increased integration into the world economy after the signing of the Bowring treaty in 1855, further canals were dug and the export of rice on a significant scale began. The emergence of a free peasant class and a merchant class helped speed the conversion of remaining swamp to paddy land so that the fate of the megafauna, including Schomburgk's deer, was already sealed by the beginning of the twentieth century. Older, broadcast rice systems in the more deeply flooded areas were superseded by transplanted rice systems as control over irrigation and drainage increased, especially with a slew of irrigation projects that were implemented in the first half of the twentieth century, culminating in the completion of the Greater Chao Phraya Project in 1962.
More than a quarter of all the threatened birds in Thailand, as well as many mammals, live in wetlands (IUCN 1991). Most of the remaining colonies of large waterbirds breeding in Thailand are situated in this ecoregion.
Some of the mammals of conservation significance found in these freshwater swamps until at least the mid-nineteenth century included tiger (Panthera tigris), elephant (Elephas maximus), and Javan rhinoceros (Bradley 1876). Small populations of some of these, such as elephants, were still present in northern parts of the region during the early twentieth century.
There are two ecoregional endemic mammals and one near-endemic mammal in this ecoregion (table 1). One further ecoregional endemic, Schomburgk's deer (Cervus schomburki), was extirpated by the early twentieth century. The area still supports nationally important populations of Lyle's flying-fox (Pteropus lylei).
Table 1. Endemic and Near-Endemic Mammal Species.
Muridae* Niviventer hinpoon*
Muridae* Leopoldamys neilli*
Rhinolophidae Hipposideros halophyllus
Cervidae Cervus schomburgki (extinct)
An asterisk signifies that the species' range is limited to this ecoregion.
Many larger birds probably disappeared simultaneously with the mammals, so there are no historical nest records of spot-billed pelicans or adjutant storks, although it is likely that the ecoregion once included the huge waterbird rookeries described by Oates (in Smythies 1986) for the Sittang plain in Burma.
At the time Madoc (1950) wrote, a colony of Oriental darters still nested on trees in the middle of the city. He also found what may have been the last nest of sarus crane (Grus antigone) for the ecoregion, near Kamphaengphet toward its northern boundary.
Some commensals of humans, such as vultures, were still present into the mid-1960s (B. King, pers. comm., 1999) but have since vanished. Wintering populations of black kite (Milvus migrans) have collapsed within the past decade, and the breeding population numbers only a few pairs.
The white-eyed river martin (Pseudochelidon sirintarae) may have been endemic to this ecoregion, but its life cycle and habitat needs remain unknown (table 2). However, it is likely that it nested on riverine sandbars on the northern part of the region and was already nearing extinction at the time of its discovery in 1968. There have been no records apart from ten birds collected in 1968 and a sight record of five or six in 1977 (King and Kanwanich 1977).
Table 2. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Hirundinidae White-eyed river-martin Pseudochelidon sirintarae
An asterisk signifies that the species' range is limited to this ecoregion.
The area continues to support possibly the largest known concentration of the globally near-threatened Asian openbill (Anastomus oscitans), currently scattered in four or five colonies in the ecoregion and thought to number more than 10,000 pairs. Other large waterbirds including painted stork (Mycteria leucocephala), spot-billed pelican (Pelecanus philippensis), and black-headed ibis (Threskiornis melanocephalus), which probably formerly bred, are annual visitors, probably entering the country from Cambodia. Rarely adjutants (Leptotilos spp.) also occur as nonbreeding visitors. The area supports significant concentrations of the globally near-threatened grey-headed lapwing (Vanellus cinereus) and significant wintering and breeding populations of waterbirds including egrets (Egretta spp.).
Almost all of the natural habitat has been cleared for agriculture and settlements. The Central Plain of the Chao Phraya River is now almost completely under rice cultivation (IUCN 1991), with smaller areas occupied by sugar cane, bananas, fruit orchards, and other crops. Creeping urbanization and land speculation have also greatly affected the area within the past couple of decades. Most large wildlife has disappeared, and only grassland, scrub, or commensal species remain (IUCN 1991). Few wetlands are represented in Thailand's protected area system, and this ecoregion receives no protection. Such protected areas exist as "nonhunting areas" and are too small and isolated to have any relevance to the larger conservation situation. Tyson Roberts (pers. comm., 1999) pointed out that a major Holocene extinction has already taken place among fishes in river deltas throughout Asia, including the Chao Phraya Basin, with the conversion of swamps to paddy land.
There are no protected areas in this ecoregion with an IUCN designation I-IV (WCMC and ABC 1997).
Types and Severity of Threats
Almost all the wetlands have been drained or severely disturbed (IUCN 1991). Rice paddies have replaced the original freshwater swamp and monsoon forest of this ecoregion, none of which remains intact (IUCN 1991). Rice paddies have considerable importance as a modified form of wetland habitat (sustaining populations of wintering and resident waterfowl and other species). However, the area of rice is declining gradually with a switch to more profitable and destructive agriculture (e.g., vegetables), which requires more intensive management and a lesser volume of water. Increased use of land for housing estates and factories takes place without reference to any zoning plan. The Industrial Estates Authority of Thailand is aggressively promoting industrialization throughout the lower Chao Phraya Basin without reference to any zoning plan that takes account of its environmental attributes. Other inappropriate forms of land use include prawn farming, in which brackish or saline water is trucked or piped into former freshwater areas and the wastewater is disposed of inappropriately, leading to gradual salinization.
There is a gradually increasing pesticide and herbicide load. Although rice farming per se entails less intensive herbicide use than other forms of agriculture, in some instances pesticides (including persistent organochlorines such as endosulfan) are dumped into rice fields by farmers to kill the introduced snail Pomacea, which is a pest of rice crops.
Justification of Ecoregion Delineation
The tropical moist forests in the Cardamom and Elephant mountain ranges in southwestern Cambodia and parts of Thailand are included within the Cardamom Mountains Rain Forests [IM0106]. MacKinnon included these rain forests in the large Cardamom Mountains subunit (05d), which also included the freshwater swamp forests and mangroves of the Chao Phraya river and estuary. We placed the latter biomes in the Chao Phraya Freshwater Swamp Forests [IM0107] and Indochina Mangroves [IM1402], respectively.
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Philip Round, Sompoad Srikosamatara, Nantiya Aggimarangsee, and Eric Wikramanayake