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The tropical heath forest is locally known as kerangas, which means "land which cannot grow rice". Heath forests occur on acidic sandy soils produced from the area's siliceous parent rocks. The sandy soil of the kerangas forest is often lacking in nutrients; generally lacking of nitrogen which impedes plant growth in these forests.
In Brunei, this forest type is found mainly on Pleistocene and Quaternary terraces and on the tip slope of the cuesta land from of the Belait formation. The vegetation composition is widely variable, and often contains species that are rare in other habitats. Ant-plants or myrmecophytes are common and mossy carpets form on the forest floor. The forest is very dense and unstratified.
There are about 3,000 hectares of heath forests in Brunei Darussalam. They can be found mainly along the coastline, on the ancient white-sand terraces such as Brunei-Muara and Tutong Districts, on sandstone ridges of the hills farther inland, and also on the thick sand deposits associated with peat swamp complexes.
Belait Pleistocene Forests
The most extensive areas of kerangas forest in the Belait district are located on Pleistocene river terraces. The terraces are relatively flat, or with micro-knoll relief, with extensive depressions where peat has accumulated. 'Alan' (Shorea albida) is the dominant species combined with other species such as 'kapur paya' (Dryobalanops rappa) , 'keruntum' (Combretocarpus rotundatus) , 'jelungan' (Aglaia rubiginosa) , and 'semundu' (Parishia maingayi) , which are also typical peat swamp forests tree species. Other tree species present are 'keruing sindor' (Dipterocarpus borneensis), 'rengas' (Gluta beccarii), and 'geronggang putih' (Cratoxylum glaucum).
'Tulong' (Belait Quaternary) Forests
Concentrated communities of 'tulong' (Agathis borneensis) can be found at Bukit Puan, Badas and Sungai Mau areas. Other prominent trees are 'kerung sindor' (Dipterocarpus borneensis) and 'resak durian' (Cotylelobium burckii).
'Kapur Paya' Forests
'Kapur paya' forest represents the transition between the peat swamp forest and the kerangas forest. A common example is the Anduki Forest Reserve, wherein 'kapur paya' (Dryobalanops rappa) is predominant.
Mountain or upland kerangas can be found on the hills and ridges of Bukit Peradayan, Bukit Patoi, Bukit Biang, Bukit Telugong, Bukit Gelagas, and Gunong Pagon priok.
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