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Peat swamp forests are forest ecosystems developed on un-decomposed and partly decomposed organic matter that has raised the peaty soil into a dome of which the top is above the normal flood level. The accumulation of un-decomposed organic matter can be attributed to water-logging and high acidity that restricts the presence and activity of decomposer organisms. Peat swamp forests are also characterized by poor soil nutrients due to continued leeching through surface run-off.

This forest ecosystem is believed to be an important carbon sink because of the huge amount of carbon sequestered from un-decomposed organic matter that the ecosystem has accumulated over so many years.

Peat swamp forests are the second most dominant forest type in the country covering around 90,884 hectares or 15.6% of the total land area. The most extensive peat swamp forest is found in the Belait district and continuous with the peat swamps of the Baram basin over the border in Sarawak.

Transitional Mangrove-Peat Swamp Forests

This forest is the distinctive  zone between the mangroves and the peat swamp forests marked by the limits of inundation by tidal saline water.  This is clearly defined with mangrove or semi-mangrove species being replaced by species that are intolerant of brackish water. Old trees of 'bakau minyak' (Rhizophora apiculata) and 'lingadai' (Bruguiera gymnorhiza) occur close to the boundary but at the extreme limits of inundation, transition species, such as Myrsine umbellata, 'manyam' (Glochidion littorale), 'tukil-tukil' (Allophylus cobbe), 'ipil-ipil' ( Instia bijuga) and 'rasau' (Pandanus affinis) are more common. Palms such as 'melawaring' (Cyrtostachys renda) and 'silad' (Licuala spinosa) are also abundant with occasional clumps of 'asam paya' (Eleiodoxa conferta). 

'Ramin-Pulaie' Association

 This is a type of peat swamp forest where 'ramin' (Gonystylus bancanus) and 'pulaie' (Alstonia pneumatophora) are predominant species.

'Jongkong' Forests

This type of peat swamp forest can be found in patches along the Tutong river. It is characterized by gregarious stands of 'jongkong' (Dactylocladus stenostachys) trees.


'Terentang' Forests

'Terentang' forest represents the early development stage of peat swamp forests. This forest is commonly dominated by 'terantang' (Campnosperma coriaceum)  and be seen in fragmented areas in Tutong and Brunei-Muara districts along the shallow peat of low tree density.


'Alan' Forests

'Alan' forest, dominated by 'alan' or 'seringawan' (Shorea albida) can be found in vast areas along the Belait drainage. The dominant and gregarious 'alan' trees have been designated an endangered species by the World Conservation Monitoring Centre. Observed from above, the even canopy of the 'alan' forest gives the appearance of a field of cauliflory with only a few species of understorey trees.


'Alan Bunga' Forests

'Alan bunga' forests are characterized by dense high canopy Shorea albida trees comparef to 'alan' community described above.  'Alan' trees in this forest have a medium-sized crown and can be observed to be growing with other species such as 'keruntum' (Combretocarpus rotundatus) and 'ramin' (Gonystylus bancanus). Other species like 'meranti kerukup' (Shorea pachyphlla), 'bintangor' (Calophyllum spp.) and 'nyatoh ketiau' (Madhuca curtisii) can be observed.


'Padang Alan' Forests

This is an 'alan' forest characterized by denser canopy and relatively smaller trees compared with 'alan' forests described above.


 'Padang' Mix Forests

An 'alan' forest mixed with patches of non-arboreal 'padang'. This mixed 'padang'community can only be found in the Baram/Belait peat swamp complex. Dominant species group represent a Shorea albida-Litsea-Parastemon association with other typical 'padang' species such as 'keruntum' (Combretocarpus rotundatus), 'bintagor kuning' (Calophyllum ardens), 'medang' (Litsea crassifolia), 'rengas' (Gluta macrocarpa) and 'geronggang putih' (Cratoxylum glaucum).


'Padang Keruntum' Forests

'Padang keruntum' forests can be observed in the extreme headwaters of Sungai Pandaruan, as well as in Sungai Mendaram. The forest is almost entirely open with the swamp surface exposed. All species, both arboreal and non-arboreal are stunted. Small 'keruntum' (Combretocarpus rotundatus) trees occur frequently, and 'jongkong' (Datycladus stenostachys) and 'kandis' (Garcinia cuneifolia) trees are abundant.