Location and General Description
The ecoregion represents the swath of semi-evergreen forests along the upper Brahmaputra River plains. Most of the ecoregion lies within the eastern Indian state of Assam, but small sections extend into the neighboring states of Arunachal Pradesh and Nagaland and also into the southern lowlands of Bhutan.
This northeastern region represents the area where the northward-migrating Deccan Peninsula first made contact with the Eurasian continent during the early Tertiary period and represents a gateway for species exchanges between the typically Indian and Malayan' faunas (Rodgers and Panwar 1988). The wide Brahmaputra River is also a biogeographic barrier for several species. For instance, the golden langur (Semnopithecus geei), hispid hare (Caprolagus hispidus), and pygmy hog (Sus salvanius) are limited to the north bank of the river, whereas the hoolock gibbon (Hylobates hoolock) and stump-tailed macaque (Macaca arctoides) are limited to the south bank (Rodgers and Panwar 1988).
The June to September southwest monsoon is funneled through the Gangetic River plains, flanked by the Himalayas to the north and the Mizo Hills to the south, deluging the ecoregion with 1,500-3,000 mm of rainfall, depending on the topographic variation. The substrate consists of deep alluvial deposits, washed down over the centuries by the Brahmaputra and other rivers such as the Manas and Subansiri, which drain southern slopes of the Eastern Himalaya. The ecoregion's vegetation therefore is thus influenced by the rich alluvial soils and the monsoon rains.
Champion and Seth (1968) recognize the following forest types in this ecoregion: Assam Valley semi-evergreen forest, Assam alluvial plains semi-evergreen forest, eastern submontane semi-evergreen forest, sub-Himalayan light alluvial semi-evergreen forest, eastern alluvial secondary semi-evergreen forest, sub-Himalayan secondary wet mixed forest, and Cachar semi-evergreen forest. But most of the ecoregion's original semi-evergreen forests have been converted to grasslands by centuries of fire and other human influences. Only small patches of forests now remain, scattered along the Indo-Bhutan border and along the border of Assam and Meghalaya. Many of these forest patches are confined to protected areas.
According to Champion and Seth (1968), the typical evergreen tree species in these forests are Syzygium, Cinnamomum, Artocarpus, and Magnoliacea, and the common deciduous species include Terminalia myriocarpa, Terminalia citrina, Terminalia tomentosa, Tetrameles spp., and Stereospermum spp. Shorea robusta is present in disturbed habitats, especially in areas that have been subjected to fire, and represents a sub-climax community. Other Dipterocarpus species are considered to be indicative of a forest in retrogression from the tropical evergreen or as a preclimax stage. Typically, the canopy trees are 20-30 m high.
The understory is of Lauraceae (mostly Phoebe spp., Machilus spp., and Actinodaphne spp.), Anonaceae (Polyalthia spp.), Meliaceae (Aphanamixis spp.), Mesua ferrea, Tetrameles spp., Stereospermum spp., and species of Meliaceae, Anacardiaceae, Myristicaceae, Lauraceae, and Magnoliaceae, with several bamboos such as Bambusa arundinaria, Dendrocalamus hamilitonii, and Melocanna bambusoides (Champion and Seth 1968; Puri et al. 1989).
The riparian areas along the Brahmaputra River that have been cleared are characterized by wet grasslands with similar communities and dynamics as described under the Terai-Duar Savanna and Grasslands [IM0701] description. Species such as the hispid hare and pygmy hog are found in these grasslands, especially in the soft soils and muddy areas along the river courses.
This ecoregion harbors India's largest elephant population (Rodgers and Panwar 1988), the world's largest population of the greater one-horned rhinoceros, tigers (Panthera tigris), and wild water buffalo (Bubalus arnee) (WII 1997). The ecoregion overlaps with a high-priority (Level I) TCU that extends north to include the subtropical and temperate forests of the Himalayan midhills (Wikramanayake et al. 1999).
The known mammal fauna consists of 122 species, including 2 near-endemic species (table 1). Of these, the pygmy hog and the hispid hare are confined to the grassland habitats.
Table 1. Endemic and Near-Endemic Mammal Species.
Cercopithecidae Semnopithecus geei
Bovidae Caprolagus hispidus
An asterisk signifies that the species' range is limited to this ecoregion.
There are several threatened mammal species in this ecoregion, including the tiger, Asian elephant, greater one-horned rhinoceros, swamp deer (Cervus duvaucelii), gaur (Bos gaurus), clouded leopard (Pardofelis nebulosa), hispid hare, pygmy hog, capped leaf monkey (Semnopithecus pileatus), Asiatic black bear (Ursus thibetanus), and sloth bear (Melursus ursinus) (IUCN 2000). Although it is not strictly a part of the terrestrial ecoregion, the Brahmaputra River also harbors the freshwater dolphin (Platanista gangetica), which is also a threatened species (IUCN 2000).
The bird fauna is richer, with more than 370 species and 2 species that are near endemic (table 2). The ecoregion overlaps with BirdLife International's EBA, Assam Plains (131) (Stattersfield et al. 1998), which contains three restricted-range bird species.
Table 2. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Phasianidae Manipur bush-quail Perdicula manipurensis
Timaliidae Marsh babbler Pellorneum palustre
An asterisk signifies that the species' range is limited to this ecoregion.
The Bengal florican is one of the most endangered species here and is largely limited to the protected areas. Conversion of grasslands and other human disturbances can make this species locally extinct.
Nearly three-quarters of the habitat in this ecoregion has been cleared or degraded; however, some large blocks of intact habitat-including sub-regional-scale swampland, grasslands, and fringing woodland habitats that are critical for the waterbirds and the large herbivores-still remain. The larger forest blocks are in central Assam.
At present, twelve protected areas cover about 2,500 km2 of intact habitat, or 5 percent of the ecoregion (table 3). Of these, Manas, Dibru-Saikowa, Kaziranga, and Mehao are the larger and more important reserves. Mehao extends over two other ecoregions and is only partially within this ecoregion (table 3). Kaziranga has the world's largest population of the greater one-horned rhinoceros, estimated at 1,100 individuals (Foose and van Strien 1997). Because of the large number of wide-ranging large vertebrates in this ecoregion, additional protection is urgently needed. Specifically, habitat connectivity should be provided within the Buxa-Manas complex and the Barail-Intanki-Kaziranga complex to allow elephants to disperse and migrate (Rodgers and Panwar 1988).
Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Mehao [IM0401], [IM0301] 190 IV
Manas 560 IV
Barnadi 90 IV
Kaziranga 320 II
Orang 110 IV
Laokhowa 170 IV
Pobitora 80 IV
Sonai Rupai 160 IV
Nameri 90 IV
Dibru-Saikowa 490 PRO
D'Ering Memorial 190 IV
Pabha 110 IV
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
Types and Severity of Threats
The overarching threats to the ecoregion's natural habitat stem from forest clearing and livestock grazing. But the settlements and agriculture go back thousands of years and have already taken a heavy toll on the natural habitat and biodiversity. Vast areas of original habitat were taken over by large tea plantations.
The Naxalite insurgency prevents effective government administration of conservation areas (protected areas and reserve forests). The movement is also funded to some extent by poaching and the wildlife and timer trade.
Justification of Ecoregion Delineation
In previous analyses of conservation units in the region, Rodgers and Panwar (1988) and MacKinnon identified biounits-Brahmaputra Valley 8A and Brahmaputra valley (I9a), respectively-that cover the semi-evergreen forests along the Brahmaputra River plains. We retained the boundary of their units as the Brahmaputra Valley Semi-Evergreen Rain Forests [IM0105]. This ecoregion overlaps with two of Udvardy's biogeographic provinces, the Himalayan highlands and Burma monsoon forest.
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Gopal S. Rawat, Ajay Desai, Hema Somanathan, and Eric D. Wikramanayake