Borneo montane rainforests
The Borneo montane rainforests can be likened to montane islands in a sea of lowland dipterocarp forests. This isolation has produced a unique and diverse set of montane species. Of Borneo's endemic bird species, twenty-three (73 percent) are montane. There are more than 150 mammal species in montane forests, making this ecoregion globally outstanding for mammal richness, and it is the most speciose montane rain forest found in the Indo-Pacific region. Despite this wealth of diversity, large tracts of Borneo's montane forests have not been explored to catalog the flora and fauna.
Location and general description
This ecoregion represents the montane forests in the central region of the island of Borneo and falls within the boundaries of all three nations with territory in Borneo: Malaysia, Indonesia, and Brunei. Montane forests are much cooler and moister than lowland forests. For every 1000 meters (m) above sea level there is an average 5°C drop in temperature. This partly explains why montane regions contain plants normally found in temperate regions. Rainfall is higher than in the lowlands, and many forests also derive moisture from clouds that bathe the region. The geology of the Borneo montane rainforests is primarily old volcanic rocks and melange (rock fragments in clay) that is continental in origin. Montane soils also change with altitude, generally becoming more acidic and nutrient-poor. Montane soils are primarily inceptisols and ultisols. Based on the Köppen Climate Classification System, this ecoregion falls in the highland climate zone.
The montane flora of Borneo is derived from both Asian and Australian families, making it one of the most diverse montane habitats on Earth. Araucariaceae, Clethraceae, Ericaceae, Fagaceae, Lauraceae, Myrtaceae, Podocarpaceae, Symplocaceae, and Theaceae are all families commonly found in montane forests. In the lower elevations of montane forest (about 1,000-1,200 meters) the dominance of dipterocarp species ends. These are replaced with highly speciose oak (Quercus and Lithocarpus spp.) and chestnut (Castanopsis spp.) forests. Myrtaceae are also important in the lower montane forests. Above 1,500 meters this forest grades into a montane ericaceous belt followed by an alpine meadow on the very highest peaks. Three major and parallel changes occur in montane forests with increasing altitude. First, there is a decrease in forest height. Montane forests do not have giant emergent trees, and their overall height is much lower. The canopy typically is 10-20 meters high. Second, the size and shape of the leaves change. Trees with buttresses usually are absent. Lowland forests are dominated by tree species with medium-large (mesophyll) and billowing canopy trees. Montane forests are dominated by slender trees, small leaves (microphyll), and a flattish crown surface. The third difference is the increased presence of epiphytes. Orchids, ferns, moss, lichen, and liverworts are more abundant in montane forests than in lowland rainforests. Upper montane forests share many common species and features of structure and appearance with heath forests.
Pitcher plants, rhododendron, and orchids are especially diverse in Borneo's montane habitats. More than one-half of Borneo's thirty pitcher plant (Nepenthes) species are found in this ecoregion's montane habitats. Rhododendrons are characteristic of upper montane flora, and more than twenty Vireya species are found in this ecoregion. Rhododendrons are found on acidic and peat soils and have adapted to the harsh upper montane environments. Orchids are found at all levels of the forest but are common epiphytes in the upper montane forests. For growth, orchids need light and moisture as well as a mycorrhiza relationship with a tree or other plant to derive their nutrients. The well-lit and moist conditions of the moss forest in the upper montane zone provide ideal growing conditions for many orchid species.
Although in southeast Asia most limestone occurs in the lowlands, Borneo has important limestone forests in the montane zone. Gunung Api is highly diverse botanically, with fourteen of Borneo's fifteen Monophyllea species, disrupted altitudinal zonation compared with forest over other substrates, and montane birds occurring at atypically low altitudes. The few high-altitude swamp forests (which help regulate water supply to downstream areas) are found in the Ulu Meligan/Ulu Long Pasia montane area and the Usun Apau Plateau.
|Table 1. Endemic and Near-Endemic Mammal Species.|
The noticeable difference in vegetation structure and species composition also affect faunal communities found in montane forests. Animals in montane regions must face adverse climate, lack of shelter, and food shortages. For example, on Gunung Mulu in Sarawak, of the 171 bird species found on its lowland slopes, the range of most species does not exceed 900 meters. By the boundary of the upper montane region at 1,300 meters, only twelve species are still found. Similarly, mammal richness tends to decrease with altitude. Most primate species prefer lowland habitats, and orangutans (Pongo pygmaeus), gibbons, and langurs all show significant decrease in density between 500 meters and 1,500 meters in altitude. The macaque species density shows no large change between lowland and montane regions, perhaps being attributed to a greater dietary versatility. Smaller mammals such as civets, tree shrews, squirrels, and rats dominate the montane region. Indeed, all near-endemic species fall into one of these categories (Table 1). The vast tracts of montane forest still remain undisturbed and therefore still support some of the larger megafauna such as orangutan and Sumatran rhinoceros (Dicerorhinus sumatrensis).
More than 250 bird species are attributed to this ecoregion. This ecoregion overlaps with a large portion of the Bornean Mountains. Many of the mountains of Borneo are ornithologically unexplored and poorly known, so the habitat needs and distributions of many species are incomplete. Twenty-one near-endemic and two endemic bird species are found in these forests (Table 2).
|Table 2. Endemic and Near-Endemic Bird Species.|
|Accipitridae||Mountain serpent-eagle||Spilornis kinabaluensis|
|Phasianidae||Red-breasted partridge||Arborophila hyperythra|
|Phasianidae||Crimson-headed partridge||Haematortyx sanguiniceps|
|Podargidae||Dulit frogmouth||Batrachostomus harterti|
|Trogonidae||Whitehead's trogon||Harpactes whiteheadi|
|Capitonidae||Mountain barbet||Megalaima monticola|
|Capitonidae||Golden-naped barbet||Megalaima pulcherrima|
|Eurylaimidae||Hose's broadbill*||Calyptomena hosii*|
|Eurylaimidae||Whitehead's broadbill||Calyptomena whiteheadi|
|Pachycephalidae||Bornean whistler||Pachycephala hypoxantha|
|Oriolidae||Black oriole*||Oriolus hosii*|
|Turdidae||Everett's thrush||Zoothera everetti|
|Muscicapidae||Eyebrowed jungle-flycatcher||Rhinomyias gularis|
|Zosteropidae||Pygmy white-eye||Oculocincta squamifrons|
|Zosteropidae||Mountain black-eye||Chlorocharis emiliae|
|Sylviidae||Bornean stubtail||Urosphena whiteheadi|
|Sylviidae||Friendly bush-warbler||Bradypterus accentor|
|Timaliidae||Bare-headed laughingthrush||Garrulax calvus|
|Timaliidae||Mountain wren-babbler||Napothera crassa|
|Timaliidae||Chestnut-crested yuhina||Yuhina everetti|
|Dicaeidae||Black-sided flowerpecker||Dicaeum monticolum|
|Nectariniidae||Whitehead's spiderhunter||Arachnothera juliae|
|An asterisk signifies that the species' range is limited to this ecoregion.|
|Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.|
|Protected Area||Area (km2)||IUCN Category|
|S. Kayan S. Mentarang||5,290||?|
|SAR (Sanctuary Reserve)||670||PRO|
|SAR (Sanctuary Reserve)||530||II|
|SAR (Sanctuary Reserve)||190||PRO|
|SAR (Sanctuary Reserve)||350||PRO|
|SAR (Sanctuary Reserve)||920||PRO|
|Bukit Batikap I, II, III||3,100||PRO|
|Bukit Baka-Bukit Raya||1,060||II|
|Gunung Penrisen/Gunung Niut||770||IV|
The ecoregion is largely intact, and only about 8 percent of the area has been cleared or converted. There are seventeen protected areas that cover 26,380 km2 (about 25 percent) of the ecoregion (Table 3). Several very large (more than 5,000 km2) reserves account for most of this protected area system. The ecoregion includes the largest protected block of rainforest in Borneo, the Kayan Mentarang National Park, which covers 140,000 km2 of lowland dipterocarp forest as well as mountain forests; this park and Gunung Bentuang both exceed 5,000 km2. The Kayan Mentarang National Park and surrounding area is inhabited by several thousand indigenous people who depend on forest resources. However, commercial logging activities, road building, and intensive extraction of commercially valuable non-timber forest products now threaten the natural integrity of the reserve and the livelihoods of local people. The montane forests of Borneo have largely escaped the fires of 1997-1998, which were intentionally set throughout most of the lowland forests in Borneo. With the rapid pace of habitat loss in the lowland forests, Borneo's montane forests may serve as a final refuge for many of Borneo's species.
Types and severity of threats
Although little of the ecoregion has been cleared or degraded, significant threats from planned mining operations, large dams, and conversion to agriculture and high-altitude timber plantations are now increasing. Illegal collection of species for the commercial trade and shifting cultivation are also increasing, threatening the integrity of Borneo's highly distinctive montane biodiversity.
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 and MacKinnon divided the island's lowland forests into six subunits, with a central subunit representing the montane forests. MacKinnon 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. Because ecoregions are based on biomes, we first isolated the central montane ecoregion—the Borneo Montane Rainforests—above the 1000-metre elevation contour using the DEM.
- Malaysia Geography Collection
- Forests of Borneo
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