The Sabangau Forest is in the south of Borneo island, in the Indonesian province of Central Kalimantan. Centered on the blackwater Sabangau River, it is bounded by the Katingan River to the west, Kahayan River to the east, the Java Sea to the south and the main Palangkaraya-Sampit road to the north. The ecosystem covers an area of approximately 9000 km2 of tropical peatland and most (6300 km2) remains forested. These forests form part of a great swathe of tropical peatlands that cover almost the entire lowland river plains of southern Borneo. The Sabangau Forest is the largest area of lowland rainforest remaining in Borneo.
The main habitat of the area is tropical peat swamp forest. This is often decribed as a dual ecosystem, with a diverse tropical forest ecosystem standing atop a thick peat layer. Peat is formed in heavily-waterlogged, acidic consitions, which prevents the complete breakdown of plant material. Tropical peat is thus the partially-decayed remains of fallen leaves, branches and trees, and the peat layer in the Sabangau has slowly formed over 20,000 years and is up to 15m deep in places. Being made almost completely of plant matter, peat is a major store of carbon - and if disturbed begins to break down and oxidise, thus releasing carbon dioxide into the atmosphere. Preserving tropical peat ecosystems in their natural state is one of the most important ways of controlling global warming. Unfortunately logging, peat-drainage and fire all disturb the ecosystem's natural balance, and these all occur when peat-swamp forests are converted to oil palm plantation and other forms of agriculture. When CO2 emissions from degraded peatlands are included, Indonesia is the world's third biggest emitter of greenhouse gases after the USA and China.
The peat forms a dome over the landscape, with the shallowest peat found near the rivers where seasonal flooding brings in nutrients from the river. The rest of the peat dome only receives nutrients from rainwater, and can thus be described as an ombrogenous ecosystem. Peat depth and surface gradient affects the type of forest that grows on the surface and four major habitat types are recognised in the Sabangau Forest. Riverine forest occurs in the seasonally-flooded margins of the peat dome and is a tall, diverse forest-type. Inland from the rivers, on peat of 1-5m deep is the Mixed Swamp Forest habitat-type, the most common habitat in the ecosystem. This has a canopy of 30-35 m high, is flooded throughout the wet season and is high in biodiversity. Beacuse it is flooded, many of the trees have adaptations to living in a water-logged ecosystem, such as growing stilt roots, producing breathing roots, called pneumatophores and spaghetti roots, and growing on hummocks raised above the water level. In the centre of the biggest domes can be found Tall Interior Forest, on peat 10-15m deep. Natural drying of the peat in this area has allowed the forest to grow tall and big, and it supports the highest densities of all our primate species. Many species of bird in the Sabangau are only found here. This habitat-type appears to be unique to the Sabangau Forest, but is also the most inaccessible. A trip here is hard work, but well worth the effort! In between the mixed swamp and tall interior forest types is a large area of Low Pole Forest. Changes in surface gradient means that water draining from the centre of the dome collects here and forms a saturated swamp. In the wet season the water can be almost 2m deep - and this means that only a few tree species can survive, and hardly any grow tall. A low canopy allows lots of light to reach the forest floor, which benefits pandan, a species of shrub with razor-sharp serrated edges. Any visitor to the Sabangau quickly develops a hatred of pandan, and nowhere more so than in the low pole forest where it forms a 2m high carpet. The low pole is unsuprisingly one of the least diverse parts of the Sabangau, but scientifically it is very interesting, and ecologically very important.
The landscape is divided into two regions, the western catchment between the Sabangau and Katingan Rivers, and the eastern catchment between the Sabangau and Kahayan Rivers. There is no longer any contiguous forest cover connecting the two regions or places where orangutans may cross the Sabangau River. The western catchment is the largest (ca. 7200 km2) and remains largely forested, with the exceptions of a 1 – 2 km strip of grassland running the length of the Sabangau river; agricultural lands surrounding the southern transmigration communities of Paduran and Bantanan on the Sabangau River and Pegatan on the Katingan River; and around 30 areas varying in size from 2 to 50 km2 that burnt during the 1997-98 and 2002 fires. The coastal forest (extending up to 20 km inland) has been badly damaged by illegal logging and fire. Timber has been harvested throughout the area under the concession system. All logging concessions granted in the area have now ceased and most of the area has since been protected as either National Park or the Natural Laboratory Research Area, also known as the Laboratorium Alam Hutan Gambut or LAHG. This is a 500 km2 area, protected for the purposes of research, located on the southern bank of the upper Sabangau River about 20 km south of Palangkaraya.
There are about 2000 residents in the transmigration settlements on the southern Sabangau river, aside from these there is only one permanent community of 100 residents on the Sabangau River in the western catchment. 15 large villages of 200-2000 residents are found along the length of the Katingan River, with the large town of Kasongan marking the north-western boundary of the catchment. Several smaller villages occur along the river, together with the large transmigration settlement of Pegatan at the mouth of the river and temporary communities of up to 3000 people centred on timber sawmills, although these are starting to dwindle as migrant workers return home following the cessation of illegal logging in the region.
The western catchment is smaller and much more heavily degraded. The route of the trans-Kalimantan highway runs close to the Kahayan River, between Palangkaraya and Pulang Pisau, with many small settlements along its length. The northern boundary is marked by the provincial capital of Palangkaraya (ca. 100,000 residents); south of this is the large village of Kereng-Bangkerai (ca. 5,500 residents) on the northern bank of the Sabangau River. The entire area forms Block C of the failed Mega-Rice Project, a program that planned to convert this area of deep peatland into rice padi. A large network of wide, deep, canals completely traverses this area, ostensibly built to provide irrigation, but, in fact, a major reason for the project’s failure. The project was abandoned before any forest was cleared in this region, but the drained and dried out peat has burnt regularly since 1997 and much of the forest in this area has been destroyed. The canals have partitioned the remaining forest. The area is still officially designated for agriculture although no effort or investment has been put towards this since the cessation of the Mega-Rice Project.