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Sundaland heath forests

The Sundaland Heath Forests [IM0161], known in Indonesia as kerangas (or land too poor for rice growing once cleared, in the Iban language) foster the growth of specialist plants such as the carnivorous pitcher plant Nepenthes, sundews Drosera, and bladderwort Utricularia. Like the rest of Borneo and Sumatra's lowland forest types, the heath forests are not immune to intentionally set fires for commercial logging and agriculture. This quest for land is exploiting the nutritionally impoverished soils of the heath forests.

  • Scientific Code
    (IM0161)
  • Ecoregion Category
    Indo-Malayan
  • Size
    29,600 square miles
  • Status
    Vulnerable
  • Habitats

Description
Location and General Description
This ecoregion is made up of heath forests scattered throughout Borneo on raised beaches, sandstone plateaus, and ridges. Heath forest is found on well-drained acidic soils (pH lass than 4) with a low clay content, derived from siliceous rocks under ever-wet conditions. These soils are commonly called white-sand soils. These soils usually originate from old, eroded sandstone beaches that isolated during the mid-Pleistocene (Burnham 1984). A layer of peat or humus often covers these soils but is lost once the natural vegetation is cleared. If the soils become waterlogged (lose their drainage capabilities), they develop into kerapah forests. These forests still remain heath forests but are more swampy in character (Whitmore 1984). Based on the Köppen climate zone system, this ecoregion falls in the tropical wet climate zone (National Geographic Society 1999).

Heath forest soils degrade very quickly to bleached sand once the forest cover is removed, making this type of forest extremely fragile (Whitmore 1989). Periodic water stress and lack of available nutrients may be important in the formation of this forest, which is notoriously poor for agriculture (Whitmore 1989). Heath forests are vastly different from lowland dipterocarp forests in structure, texture, and color. Heath forests have a low, uniform single-layered canopy. Leaf size is smaller, and trees often are densely packed and difficult to penetrate. Trees reach up to 20 m in height. Large trees are rare, buttresses are smaller, and epiphytes are common (Whitmore 1984). Under favorable conditions heath forests contain many plant species found in lowland evergreen rain forest. Dipterocarps are prominent in the canopy, and palms are common. Under the worst conditions, no dipterocarps may exist, and palms may be rare. Species of the Australian Myrtaceae and Casuarinaceae families predominate, and conifers such as Agathis, Podocarpus, and Dacrydium (WWF and IUCN 1995) are abundant. In Kalimantan, the dominant trees are Dipterocarpaceae (Shorea and Hopea spp.), Myrtaceae, Gonystlus spp., Agathis spp., Dacriydium elatum, Styphelia spp., and Bachea spp. (FAO 1981). It is estimated that the heath forests of Sarawak and Brunei contain 849 tree species, and, along with the Nabawan heath forests of Sabah, these forests are richer in plant species and endemics than elsewhere in the ecoregion (WWF and IUCN 1995).

Heath forests generally are less species-rich than comparable dipterocarp forests. They share many features in common with moss forests in the upper montane zones, such as a dense undergrowth, abundant bryophytes, presence of conifers, and the presence of Casuarina nobilis (a nitrogen-fixing plant) (Richards 1936). They share at least 146 tree species with freshwater and peat swamp forests (Brünig 1973), including the dipterocarps Shorea albida, S. pachyphylla, and S. scabrida.

Ground vegetation in heath forests generally is sparse, primarily composed of mosses and liverworts, with a host of insectivorous plants. The presence of insectivorous plants may be an evolutionary response to growing in nitrogen-poor conditions. The sundews Drosera have leaves covered with long red hairs that entrap insects. A bladderwort Utricularia has a hollow bag on the end of a stalk, with the entrance guarded by hairs. If an insect touches these hairs, a rush of water is released, dragging the insect inside to be digested (Mabberley 1987). Six species of pitcher plants are common in heath forests, including Nepenthes bicalcarata, found exclusively in heath and peat swamp forests (Smythies 1965). In other cases, a symbiotic relationship exists between plants and insects. This is the case with Myrmecodia and Hydnophytum species. Myrmecodia harbors ants in its thickened stem, and the ants in turn provide the plant with much-needed nutrients in the form of dead insects and other food they bring into their colony (Payne et al. 1994).

Biodiversity Features
Animals in heath forests are confronted with many of the same problems as those in peat swamp forests. Poor soils cause low productivity, and plants defend themselves from predators with toxic or unpalatable compounds (Brünig 1974). Because of these unfavorable conditions, heath forests are less species-rich, and animal communities are reduced in diversity and abundance.

Orangutans (Pongo pygmaeus) may frequent this forest types, but less often than other forest types. Heath forests have no turtles and less than one-half of the number of frog, lizard, and snake species found in other Bornean forests (Lloyd et al. 1968). There is a noticeable lack of small vertebrates, helping explain why heath forests contain only one-third of the snakes found in dipterocarp forests. With a lack of prey, snakes become less diverse, and this effect cascades up through the food chain. The area supports only a single near-endemic mammal and one near-endemic bird species (table 1, table 2).

Table 1. Endemic and Near-Endemic Mammal Species.

Family Species
Cercopithecidae Presbytis hosei

An asterisk signifies that the species' range is limited to this ecoregion.

Table 2. Endemic and Near-Endemic Bird Species.

Family Common Name Species
Zosteropidae Pygmy white-eye Oculocincta squamifrons

An asterisk signifies that the species' range is limited to this ecoregion.

Current Status
The Sundaland Heath Forests [IM0161] ecoregion includes two blocks of intact habitat larger than 5,000 km2, and the most intact part is in south Kalimantan. Although more than half of the ecoregion has been cleared, heath forests cannot sustain agriculture, probably accounting for the good condition of this vegetation type. There are seven protected areas that cover 4,440 km2 (6 percent) of the ecoregion (table 3). Tanjung Puting is the largest of the protected areas, Kutai, which extends into the ecoregion and is also greater than 1,000 km2.

Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.

Protected Area Area (km2) IUCN Category
Tanjung Putting [IM0104], 89, 110] 1,780 II
Pararawen Baru [IM0102] 300 PRO
Pantai Samarinda 180 V
Bukit Soeharto 840 V
Kutai [IM0153], [IM0143] 610 II
Kutai (extension) [IM0104], 89, 97] 610 PRO
Muara Kaman Sedulang [IM0104], [IM0153] 120 I
Total 4,440  

Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of Threats
Heath forests are easily degraded by logging or burning activities. Once degraded, they develop into an open savanna of shrubs and trees over sparse grass and sedge. This formation is called a padang. Heath forest can recover from a padang, but this takes a very long time to do. The replanting or reestablishment of native vegetation has proved ineffective (Mitchell 1963). Extensive areas of this ecoregion were severely damaged in the great forest fire of 1982-1983, which consumed a total area of 33,000 km2, mostly in East Kalimantan (IUCN 1991), and more recently during the 1997-1998 fires.

Justification of Ecoregion Delineation
The large island of Borneo was divided into nine ecoregions. Most of the island's lowland and submontane forests are dominated by dipterocarp species (MacKinnon et al. 1996). MacKinnon and MacKinnon (1986) divided the island's lowland forests into six subunits, with a central subunit representing the montane forests. MacKinnon (1997) revised the boundaries of these seven subunits but retained the same general configuration. These authors used the major rivers, the Kapuas and Barito, to represent zoogeographic barriers to a few mammal species and based subunits largely on these barriers but also used climatic regimes for the drier eastern biounits (MacKinnon and MacKinnon 1986; MacKinnon 1997).

Because ecoregions are based on biomes, we first isolated the central montane ecoregion-the Borneo Montane Rain Forests [IM0103]-above the 1,000-m elevation contour using the DEM (USGS 1996). We then assigned the large patches of peat forests, heath forests, freshwater swamp forests, and mangroves, in the lowlands and along the periphery of the island, into their own ecoregions: the Borneo Peat Swamp Forests [IM0104], Sundaland Heath Forests [IM0161] (which also includes Belitung Island and the heath forests in Bangka island), Southern Borneo Freshwater Swamp Forests [IM0153], and Sunda Shelf Mangroves [IM1405], respectively. The alpine habitats of the Kinabalu Mountain Range were represented by the Kinabalu Montane Alpine Meadows [IM1001].

References
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List

Prepared by: Colby Loucks
Reviewed by:

 

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