Himalayan subtropical broadleaf forests
The Himalayan Subtropical Broadleaf Forests ecoregion includes several forest types along its length as it traverses an east to west moisture gradient. The forest types include Dodonea scrub, subtropical dry evergreen forests of Olea cuspidata, northern dry mixed deciduous forests, dry Siwalik sal (Shorea robusta) forests, moist mixed deciduous forests, subtropical broadleaf wet hill forests, northern tropical semi-evergreen forests, and northern tropical wet evergreen forests.
The ecoregion also forms a critical link in the chain of interconnected Himalayan ecosystems that extend from the Terai and Duar grasslands along the foothills to the high alpine meadows at the top of the world's highest mountain range. For instance, several Himalayan birds and mammals exhibit seasonal altitudinal migrations and depend on contiguous habitat to permit these movements. Therefore, conservation actions in the Himalayas must pay due attention to habitat connectivity because degradation or loss of a habitat type along this chain will disrupt these important ecological processes.
Location and General Description
This ecoregion represents the east-west-directed band of Himalayan subtropical broadleaf forests along the Siwaliks or Outer Himalayan Range, lying between 500 and 1,000 meters (m). The ecoregion achieves its greatest coverage in the middle hills of central Nepal, but the long, narrow ecoregion extends through Darjeeling into Bhutan and also into the Indian State of Uttar Pradesh. The Kali Gandaki River, which has gouged the world's deepest river valley through the Himalayan Range, bisects the ecoregion.
The Himalayas rose from beneath the ancient Tethys Sea when the Deccan Plateau collided with the Eurasian continent about 50 million years ago and forced the latter upward to form the highest mountain range in the world. The Himalayas now consist of three east-west-directed parallel zones: the southernmost outer Himalayas, which represent the Siwaliks; the Middle Himalayas, representing a series of ridges and valleys that rise to about 5,000 m; and the Inner Himalayas, which include the tallest mountain peaks in the world. The Siwalik Hills, where this ecoregion lies, are composed of alluvium deposited over the ages by the rivers that drain this young mountain range.
Rainfall varies from east to west, but annual rainfall can be as much as 2,000 millimeters (mm). The Himalayas capture moisture from the monsoons that sweep in from the Bay of Bengal, and most of this rainfall is expended in the eastern Himalayas. Therefore, the western Himalayas are drier, a trend reflected in the timberline that declines from 4,000 m in the east to about 3,500 m in the west.
The forests in this ecoregion are very rich in biodiversity. The forest types are varied because of the subtropical climate, complex topography, rich alluvial soils, moisture gradient, and intermingling of taxa from the Indo-Malayan and Palearctic regions. These forest types consist of Dodonea scrub, subtropical dry evergreen forests of Olea cuspidata, northern dry mixed deciduous forests, dry Siwalik sal forests, moist mixed deciduous forests, subtropical broadleaf wet hill forests, northern tropical semi-evergreen forests, and northern tropical wet evergreen forests.
The forests generally reach 30 m, although in favorable areas the canopy can reach as high as 50 m. The top canopy is less dense than the tropical evergreen forests, and a mid-canopy and shrubby undergrowth are recognizable. Grasses are absent, but there is a well-developed herb cover. Climbers and epiphytes are common.
The diversity and richness of woody species decrease from east to west. In the western foothills of Himachal Pradesh and Uttar Pradesh, characteristic tree communities are represented by Shorea robusta, Terminalia tomentosa, Anogeissus latifolia, Mallotus philippinensis, Olea cuspidata, Bauhinia restusa, and Bauhinia variegata. In the eastern foothills, the characteristic species include Schima wallichii, Castanopsis tribuloides, C. indica, Terminalia crenulata, Terminalia bellerica, Engelhardtia spicata, Betula spp., and Anogeissus spp. In eastern Nepal, Engelhardtia spicata, Erythrina spp., and Albizia spp. are important components of the subtropical forest associations. Alnus nepalensis is an early-successional species that invades landslide areas and forms monospecific stands. Many of the trees in these broadleaf forests such as Gnetum montanus, Cycas pectinata, Cyathea spinulosa (tree ferns), Rauwolfia serpentina, Pandanus nepalensis, Calamus lalifolius, C. leptospadix, Phoenix humilis, and Phoenix sylvestris have become very rare in Nepal.
This ecoregion is a critical link in the Himalayan ecosystem, where altitudinal connectivity between the habitat types (each represented by a different ecoregion) is important for ecosystem function. In addition to its importance in maintaining ecosystem dynamics, the ecoregion also harbors several threatened species that warrant conservation attention. The large areas of intact habitat have been included within two high-priority (Level I) Tiger Conservation Units (TCUs). They extend across adjacent ecoregions representing the broadleaf and subtropical conifer forests and the savanna grasslands along the foothills.
|Table 1. Endemic and Near-Endemic Mammal Species.|
The mammal fauna consists of ninety-seven species including one that is endemic to this ecoregion (Table 1).
This charismatic, endemic golden langur (Semnopithecus geei) has a small range distribution, being limited to the broadleaf forests north of the Brahmaputra River. It is shared between this and the adjacent Eastern Himalayan broadleaf forests.
Several of the ecoregion's mammals are threatened species. These include the tiger (Panthera tigris), Asian elephant (Elephas maximus), golden langur, smooth-coated otter (Lutrogale perspicillata), clouded leopard (Pardofelis nebulosa), gaur (Bos gaurus), serow (Capricornis sumatraensis), Irrawaddy squirrel (Callosciurus pygerythrus), and particoloured squirrel (Hylopetes alboniger).
|Table 2. Endemic and Near-Endemic Bird Species.|
|Phasianidae||Chestnut-breasted partridge||Arborophila mandellii|
The chestnut-breasted partridge (Arborophila mandellii) is shared with several of the other eastern Himalayan ecoregions (Eastern Himalayan broadleaf forests, Eastern Himalayan subalpine conifer forests, and Himalayan subtropical pine forests) but has a very limited range within these ecoregions. The globally threatened white-winged wood duck (Carina scutulata) and five hornbill species are found here. The latter in particular need mature forests for nesting and are good indicators of habitat quality. BirdLife International's Endemic Bird Area (EBA), Eastern Himalayas (130), overlaps with this ecoregion.
More than 70 percent of the natural forests in the ecoregion have been cleared or degraded. Cultivation is especially extensive in the fertile valleys of large rivers such as the Karnali, Babai, and Rapti and in the flat lands between the Trisuli River and Kali Gandaki. But most of the hill forests above 1,000 m still remain uncut because the shallow, erosion-prone soils are unsuitable for cultivation.
The eight protected areas that extend into this ecoregion (Table 3) cover a little over 2,700 km2, representing about 7 percent of the ecoregion's area. Several of these protected areas—especially Royal Manas, Royal Chitwan, Royal Bardia, and Valmikigar—are important for the large vertebrates that can be considered umbrella species (the tiger, Asian elephant, clouded leopard, and hornbills are candidates) for overall biodiversity.
|Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.|
|Protected Area||Area (km2)||IUCN Category|
|Royal Bardia National Park||510||II|
|Parsa Wildlife Reserve||400||IV|
|Royal Chitwan National Park||560||II|
Although almost all the protected areas are smaller than 500 km2 within the ecoregion (the exception is Royal Chitwan National Park); Royal Bardia National Park and Royal Manas National Park extend into the adjacent ecoregions. Both parks are more than 800 km2. Three of the other protected areas also overlap across adjacent ecoregions (Table 3).
Types and Severity of Threats
The primary threats to the ecoregion's natural habitat stem from fuelwood collection, intensive livestock grazing, and annual burning by pastoralists to encourage the growth of new shoots for livestock. Heavy grazing even within intact forests has destroyed the undergrowth, including the saplings that should eventually replace the mature canopy trees. Therefore, the long-term viability of these forests is compromised. However, farmers have also begun to plant and maintain fodder trees on their land to feed livestock, and these agroforestry practices have begun to ameliorate the degradation of natural habitats.
Justification of Ecoregion Delineation
In a previous conservation assessment, MacKinnon identified four biounits along the Himalayas. But these units were based on longitudinal boundaries, and each included the range of habitat types represented from the lowlands to the alpine habitats. In our analysis, we sought to represent distinct ecosystems of regional extent in separate ecoregions. Therefore, we used MacKinnon's digital map of the distribution of original vegetation to delineate the boundaries of the subtropical forests that run the length of the eastern and central Himalayas, flanked by the Terai and Duar savanna grasslands and the temperate broadleaf forests. These subtropical forests were then defined as the Himalayan Subtropical Broadleaf Forests. All the Himalayan ecoregions are part of Udvardy's Himalayan highlands biogeographic province.
Additional information on this ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
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