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Southeastern Asia: Western part of the island of Sumatra in Indonesia

The Sumatra Montane Rain Forests [IM0159] are home to a wide variety of species and are one of the most outstanding examples of montane rain forests west of Wallace's Line. The Sumatran montane rain forests are home to both Amorphophallus titanum, which grows on a stalk that measures more than 2 m tall, and the parasitic Rafflesia arnoldii, which produces the largest flower in the world (up to 1 m wide). Sumatra's montane forests are also home to some of the most endangered species in the Indo-Pacific region. The Sumatran rhinoceros, tiger, and Sumatran rabbit all inhabit these forests.

  • Scientific Code
    (IM0159)
  • Ecoregion Category
    Indo-Malayan
  • Size
    28,100 square miles
  • Status
    Relatively Stable/Intact
  • Habitats

Description
Location and General Description
This ecoregion represents the montane forests (>1,000 m) along the Barisan Mountain Range of Sumatra. The geologic history of Sumatra provides insights into the origins and amount of Sumatra's biodiversity. About 150 million years ago Borneo, Sumatra, and western Sulawesi split off from the Gondwanaland and drifted north. About 70 million years ago India slammed into the Asian landmass, forming the Himalayas, and an associated thrust formed Sumatra's Barisan Mountains, which run the length of Sumatra. As the Barisan Range buckled upward, it formed a deep water channel to the west of Sumatra. Today, to the east of the Barisan Range, low hills and plains exist as a result of tectonic and volcanic events. Continued mountain building, volcanic activity, and sedimentation in the lowland have occurred over the past 25 million years. Podzolic soils associated with altosols or litosols are the predominant soils found in this ecoregion. Large limestone areas occur in northern Sumatra, and they are associated with brown podzolic and renzina soils (Whitten et al. 2000).

Based on the Köppen climate zone system, Sumatra falls in the tropical wet climate zone (National Geographic Society 1999). The montane rain forests of the Barisan Range receive more rainfall on their western slopes than their eastern slopes, which are in a rainshadow. However, most of Sumatra experiences less than three consecutive months of dry weather (less than 100 mm rainfall/month), and rainfall in the montane rain forests averages more than 2,500 mm/year (Whitten et al. 2000).

Sumatra's montane rain forests can be separated into three major forest zones: lower montane forest, upper montane forest, and sub-alpine forest. Temperature and cloud level are the major factors determining these forest zones. The lower montane zone forests are similar to lowland rain forests but begin to get smaller. The canopy height typically is no more than 35 m high. Emergents may extend to 45 m, but buttresses are rare. Lianas usually are absent, and epiphytes such as orchid begin to increase in abundance. The upper montane zone sharply changes from lowland rain forests. The canopy becomes even and rarely exceeds 20 m. Emergents may extend to 25 m, but buttresses usually are absent. Trees rarely have compound leaves or lianas. Orchids and other epiphytes such as moss, lichen, and liverworts are very common. Beyond this forest lies the sub-alpine forest, a complex of grass, heath, and bog areas. Small, stunted trees may reach 10 m high, orchids become very rare, but moss, lichen, and liverworts are very abundant (Whitten et al. 2000).

The montane flora of Sumatra originates from two sources: local sources (autochthonous) and areas that have a center of origin outside of Sumatra (allochthonous). The local source can be divided into two categories: species that are characteristic of lowland rain forest, such as Dipterocarpaceae, Bombacaceae, and the genus Ficus (figs), and those that have a large global latitudinal distribution such as pines, Cruciferae (e.g., mustard), Theaceae (e.g., tea), and tree ferns. The allochthonous flora belong to genera whose species are found only in cold climates, not near equatorial rain forests. These species in the tropics are never found below 1,000 m and usually dominate the sub-alpine flora. Genera include Rhododendron, the pretty herbs Gentiana, and grass Deschampsia. Most of these species dispersed from Asia or Australia during cooler glacial periods when the Sunda region was a single landmass. Forest zones were all 350-400 m lower than their present height, providing numerous stepping stones (Steenis 1950).

The characteristic vegetation in lower montane forests changes from Dipterocarpaceae, the dominant lowland family, to Fagaceae (oaks) and Lauraceae (laurels). Lithocarpus, Quercus, and Castanea are common genera in the Fagaceae family, and Cinnamomum burmansea, Persea americana, and Litsea spp. are common Lauraceae species. Other families common to the lower montane region include Cunoniaceae, Monimiaceae, Magnoliaceae, and Hamamelidaceae (FAO 1981; Whitten et al. 2000). Tree ferns in the genus Cyathea are also common in the lower montane forests. The upper montane forest is characterized by conifers (pines and related trees), particularly by the Ericaceae (Rhododendron, Vaccinium) and Myrtaceae (Eucalyptus, Melaleuca) families. Dacrycarpus imbricatus and Leptospermum flavescens are also abundant in these forests, which because of their smaller stature are called elfin forests. Lichens are common to the drier parts of this zone, whereas mosses and liverworts are common in the moister parts of this zone that coincide with where clouds form and are commonly called cloud or moss forests. The sub-alpine zone is characterized by smaller specimens of the montane forest. There is also an increased abundance of grasses (Agrostis and Festuca), rushes and sedges (Juncus, Carex, Scirpus, and Cyperus), and small, colorful herbs (Whitten et al. 2000).

Five of the sixteen species of the parasitic Rafflesia plant are found in Sumatra and have been recorded as high as 1,800 m on Mt. Lembuh, Aceh. Rafflesia arnoldii, which produces the largest flower in the world, is found in this ecoregion. Its large brown-orange and white flowers can reach 1 m in diameter. Rafflesia have no leaves, instead deriving all their energy from the tissues of its host, the ground vine Tetrastigma. Large buds emerge from the vine and have five large, flowery petals surrounding spikes, which smell like rotting meat and attract pollinating insects (Whitten et al. 2000; MacKinnon 1986).

Biodiversity Features
Sumatra's montane forests contain far higher levels of mammal and bird endemism than the lowland forests, in part because of their longer periods of isolation and distinctive forest types. Seven mammal and eight bird species are endemic (table 1, table 2), whereas the lowland Sumatran Lowland Rain Forests [IM0158] have only one mammal and one endemic bird species. The mammal species includes Thomas's leaf-monkey (Presbytis thomasi), one of Sumatra's four leaf-monkeys, Sumatran rabbit (Nesolagus netscheri), and Sumatran shrew-mouse (Mus crociduroides). This ecoregion also contains two near-endemic mammal species and twenty-one near-endemic bird species.

Table 1. Endemic and Near-Endemic Mammal Species.

  Family Species
Sorcidae Crocidura baluensis
Cercopithecidae Presbytis thomasi*
Sciuridae Hylopetes winstoni*
Muridae Mus crociduroides*
Muridae Rattus korinchi*
Muridae Rattus hoogerwerfi
Muridae Maxomys hylomyoides*
Muridae Maxomys inflatus*
Leporidae Nesolagus netscheri*

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

Table 2. Endemic and Near-Endemic Bird Species.

  Family Common Name Species
Phasianidae Red-billed partridge Arborophila rubrirostris*
Phasianidae Sumatran pheasant Lophura hoogerwerfi*
Phasianidae Salvadori's pheasant Lophura inornata*
Phasianidae Bronze-tailed peacock-pheasant Polyplectron chalcurum
Columbidae Green-spectacled pigeon Treron oxyura
Columbidae Pink-headed fruit-dove Ptilinopus porphyreus
Cuculidae Sumatran ground-cuckoo Carpococcyx viridis*
Strigidae Rajah scops-owl Otus brookii
Apodidae Waterfall swift Hydrochous gigas
Trogonidae Blue-tailed trogon Harpactes reinwardtii
Pittidae Schneider's pitta Pitta schneideri*
Pittidae Black-crowned pitta Pitta venusta
Dicruridae Sumatran drongo Dicrurus sumatranus*
Campephagidae Sunda minivet Pericrocotus miniatus
Irenidae Blue-masked leafbird Chloropsis venusta
Turdidae Shiny whistling-thrush Myiophonus melanurus
Muscicapidae Rufous-vented niltava Niltava sumatrana
Muscicapidae Sunda robin Cinclidium diana
Muscicapidae Sumatran cochoa Cochoa beccarii*
Pycnonotidae Cream-striped bulbul Pycnonotus leucogrammicus
Pycnonotidae Spot-necked bulbul Pycnonotus tympanistrigus
Pycnonotidae Sunda bulbul Hypsipetes virescens
Zosteropidae Black-capped white-eye Zosterops atricapillus
Sylviidae Sunda warbler Seicercus grammiceps
Timaliidae Sunda laughingthrush Garrulax palliatus
Timaliidae Vanderbilt's babbler Malacocincla vanderbilti*
Timaliidae Rusty-breasted wren-babbler Napothera rufipectus
Timaliidae Marbled wren-babbler Napothera marmorata
Fringillidae Mountain serin Serinus estherae

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

This ecoregion overlaps with a portion of the Sumatra and peninsular Malaysia EBA (158) (Stattersfield et al. 1998). Thirty-five restricted-range bird species are found in this ecoregion and include two threatened species that are also endemic to this ecoregion: the Sumatran cochoa (Cochoa beccarii) and Sumatran ground-cuckoo (Carpococcyx viridis).

Another distinctive feature of Sumatra's fauna is that it can be split into two regions, one to the north of Lake Toba and the other to the south. Lake Toba formed 75,000 years ago as part of a volcanic eruption that had a devastating impact on Sumatra (Stone 1994). Seventeen bird species are found only north of Lake Toba, and ten are found only to the south. The white-handed gibbon (Hylobates lar) occurs only north of Lake Toba, and the dark-handed gibbon (Hylobates agilis) is found to the south. The tarsier (Tarsius bancansus), banded leaf-monkey (Presbytis melalophus), and endangered Malayan tapir (Tapirus indicus) are all found to the south of Lake Toba (Whitten et al. 2000). The Malayan tapir is the largest of the four living tapir species and the only Old World representative. The Sumatran population of the Malayan tapir is close to extinction, with no more than fifty animals left in the wild, mostly in the lowland forests (McClung 1997).

The Sumatran tiger (Panthera tigris), Indonesia's largest terrestrial predator, lives in lowland and montane rain forest, as well as freshwater swamp forests throughout Sumatra. There are an estimated 500 Sumatran tigers remaining in Sumatra, with approximately 100 found in Gunung Leuser National Park in northern Sumatra (Franklin et al. 1999). There are three Level I TCUs in Sumatra that overlap this ecoregion (Dinerstein et al. 1997). The two-horned Sumatran rhinoceros (Didermocerus sumatrensis) once ranged through much of southeast Asia. Today the entire population numbers about 500 individuals scattered in several populations in Sumatra, Borneo, and peninsular Malaysia (McClung 1997). Another distinctive species of Sumatra's montane forests is the serow (Capricornis sumatraensis). The serow lives from 200 m to the vegetated summits of Sumatra's highest peaks. It can also be found on forested limestone hills (Whitten et al. 2000).

Several other mammal species are found in this ecoregion, including numerous primate species such as several leaf-monkeys, siamang (Hylobates syndactylus), the region's largest gibbon, wild dog (Cuon alpinus), sun bear (Ursus malayanus), and clouded leopard (Pardofelis nebulosa) (Whitten et al. 2000; Stone 1994).

Current Status
Despite Sumatra's dense human population, this montane ecoregion contains several large blocks of intact forest, stretching along the Barisan Mountain Range, running the length of the island. Numerous protected areas are scattered along the range and cover 40 percent of the ecoregion's area (table 3). The large Gunung Leuser national park extends into the northern part of the ecoregion. In the middle, the Kerinci-Seblat National Park-the largest reserve in Sumatra-protects the watersheds of two of Sumatra's most important rivers: the Musi and Batang Hari (IUCN 1991). To the south, Bukit Barisan Selatan, another of Sumatra's large reserves, also extends into this ecoregion, covering more than 2,000 km2.

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

  Protected Area Area (km2) IUCN Category
Lingga Isaq [IM1001] 790 VI
Gunung Leuser [IM0158] 6,000 II
Dolok Sembelin [IM0158] 110 VI
Dolok Surungan 320 IV
Dolok Sipirok 70 I
Malampah Alahan Panjang [IM0158] 250 PRO
Lembah Harau 220 VI
Maninjau 230 VI
Gunung Sago/Malintang/Karas 80 VI
Gunung Singgalang 240 VI
Gunung Merapi 100 VIII
Kerinci Seblat [IM1001] 7,960 II
Punguk Bingin 100 VI
Bukit Dingin/Gunung Dempo 530 VI
Bukit Hitam 790 VIII
Bukit Balal [IM0158] 150 VI
Bukit Raja Mandara [IM0158] 100 VIII
Gumai Pasemah [IM0158] 320 IV
Isau-Isau Pasemah [IM0158] 100 IV
Gunung Patah/Bepagut/Muara Duakisim 580 VI
Bukit Balai Rejang 710 VIII
Bukit Barisan Selatan [IM0158] 1,620 II
Bukit Nantiogan Hulu/Nanti Komerung Hulu [IM0158] 720 VI
Gunung Raya 750 IV
Tanggamus 50 VI
Gunung Betung 60 VI
Total 22,950  

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

Types and Severity of Threats
At the current rate of deforestation, Sumatra's remaining lowland rain forest will be completely gone within the next ten years unless drastic actions are taken to halt the rampant logging (Holmes, Essay 1). Given this ominous prediction, the only remaining natural forests in Sumatra will be the hill and montane forests of this ecoregion. This ecoregion is extremely fragile and sensitive to disturbance, especially in the upper montane and sub-alpine zones (Whitten et al. 2000). They probably will be targeted for intense logging activities, especially in light of the rampant illegal logging currently taking place throughout Indonesia. From 1985 to 1997, 15,000 km2 of montane forest was destroyed, more than 1,000 km2/year. Since 1997 this annual rate of forest loss has increased gradually, and after the fall of the Suharto government and economic collapse of 1998, even the large protected areas such as Gunung Leuser, Kerinci Seblat, and Bukit Barisan Selatan national parks are threatened by encroachment and poaching.

One example of illegal logging inside Indonesia's protected areas is the unabated illegal logging that has occurred in and around the biologically valuable Gunung Leuser National Park since April 1999. The destruction includes prime habitat for the orangutan, siamang, and white-handed gibbon. In Gunung Kerinci Seblat National Park, Indonesia's first fully gazetted national park, illegal logging is increasing, and more than 400 families are staking claims along the road bordering the park. Kerinci-Seblat National Park is an important site for one of the last remaining populations of the Sumatran rhinoceros and Sumatran rabbit, as well as siamang and agile gibbon (Colijn, http://www.bart.nl/~edcolijn/).

Poaching is another threat to the great diversity of life in these forests. From 1990 to 1996 the number of Sumatran rhinoceros in Kerinci-Seblat National Park fell from 300 to about 30 (Mittermeier et al. 1999).

Justification of Ecoregion Delineation
We recognized six ecoregions on Sumatra. MacKinnon (1997) placed all the biomes in Sumatra within two subunits (21a and 21b), with the subunit division based on a faunal break to the south of Lake Toba. We used the 1,000-m contour from a DEM (USGS 1996) to delineate the montane forests along the Bukit Barisan-Gunung Leuser Mountain Range as the Sumatran Montane Rain Forests [IM0159]. However, these montane forests include extensive forest areas over limestone. Whitmore (1984a) shows small, scattered patches of limestone forests in his vegetation map of Malesia. The FAO map (1974) shows no limestone. Much of the uncertainty probably arises because these limestone substrates do not show extensive outcropping (Whitmore 1984a). Based on recommendations by Tony Whitten (pers. comm., 1999), we treated these limestone forests as a distinct habitat type within the more broadly distributed montane moist forests rather than placing them in a separate ecoregion.

MacKinnon's biounit 21 largely corresponds to Udvardy's Sumatra biogeographic province. However, Udvardy did not include the Nicobar Islands. Eight ecoregions overlap Udvardy's Sumatra biogeographic province: Sumatran Lowland Rain Forests [IM0158], Sumatran Montane Rain Forests [IM0159], Mentawai Islands Rain Forests [IM0127], Sumatran Peat Swamp Forests [IM0160], Sumatran Freshwater Swamp Forests [IM0157], Sundaland Heath Forests [IM0161], Sumatran Tropical Pine Forests [IM0304], and Sunda Shelf Mangroves [IM1405].

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 and Tony Whitten
Reviewed by:

 

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