Biodiversity of Sikkim's Four Eco Regions
Sikkim is a land of vast variation in altitude within very short distances ranging from around 300m to 8585m. Elevation plays a prime role in fashioning the eco-regions of the state. This is evident from the presence of Sal forests in the Rangeet Valley in the south to the temperate fir forests in the north, beyond which lie the trans-Himalayas and cold desert of the Tibetan plateau. Broadly speaking there are four zones of vegetation according to altitude variations, but in some stray areas, altitude alone may not define a zone as exposure to other physical properties of the terrain can result otherwise.

Tropical
This zone lies between the 300 m. low river valleys to the mid-hill heights of around 1200 m. The topographical features are deep valleys and gorges with well-drained slopes. Beneath canopies of tall evergreen and semi-deciduous trees, the dense undergrowth in this belt includes various species of orchids, Rhapidophora, wild banana, Pandanus, nettles and giant bamboo. The Rangeet Valley area has an abundance of sal forests (Shorea robusta), a magnificent timber tree remarkable for its robustness and longevity.

Lowland forests of Sikkim are home to several endangered species of birds like the Rufous-necked Hornbill Aceros nipalensis, Great Indian Hornbill Buceros bicornis homrai locally called ‘Hongraio’, Chestnut-breasted Partridge, Black-breasted Parrotbill, Grey-crowned Prinia and Ward’s Trogon. Other lowland fauna includes Assamese Macaque, Barking Deer, Porcupine, Python, Geckos, and, a host of butterflies and other invertebrates, river fish, frogs and toads. The Peafowl which was introduced back in the early 1970’s have a thriving population in the forests between Melli and Jorethang. Several species of migratory water birds use the river systems during transit. Lantana is a major weed in this region.

Sub Tropical
This region has a range from about 1800 m to 3000 m and the physical features associated with this belt are the upper portions of high hills. Rainfall is heaviest in this zone and conditions remain humid throughout the year.

The vegetation in the upper storey consists mainly of Castanopsis hystrix (chestnut), Machilus spp. (Kawla), Rhododendron spp. (Chimal), Symplocos spicata (Kholme), Symplocos theifolia (Kharane), Michelia excelsa (magnolia), Quercus lamellosa (Himalayan Oak), Quercus lineata (Phalant), Leucoseptrum canum (Ghurpis), Quercus pachyphylla (Sungure Katus), Betula alnoides (Saur), Nyssa javanica (Lekh Chilaune), Bucklandia populnea (Pipli). In the underwood, Engelhardtia spicata (Mahuwa), Eurya japonica (Jhingni), Rhododendron arboreum (Gurans) and Viburnum spp. (Asare), are the predominant species.

In the upper reaches, dense tall evergreen forests with oak and rhododendron predominate. The upper storey consists of Quercus lamellosa (Himalayan Oak), Q. lineata (Phalant), Machilus spp. (Kaula). The undergrowth consists of Arundinaria maling (dwarf bamboo), dwarf rhododendron, ferns, epiphytic moss and orchids.

This area is also in bird variety and includes the Rusty-bellied and Lesser Shortwings, Kalij and Tragopan pheasants, Red Jungle Fowl, and reptiles like pangolin, cobra, Krait and Himalayan Pit Viper; Himalayan Bullfrog as well as a host of butterfly species.

Plantations of Large Cardamom beneath canopies of tall trees in forest patches, terraced farmland, and a tea estate at Temi are the dominant features of the landscape as much as the well matured exotic Cryptomeria japonica trees that were introduced to the region around a century back.

Most of the human population of Sikkim resides in the sub-tropical and tropical zones in an agricultural setting where terrace cultivation of rice, ginger, orange, cardamom are commercially grown while guava, banana, squash and marigold are common along with vegetables and herbs in homestead gardens. Wild edibles like bamboo shoot, ferns and nettles are also collected during season. Stall fed livestock is another feature in villages.

Temperate & Alpine
The region extends from 3000m to 4000m with mixed coniferous forests of hemlock, spruce, pine, fir and juniper with shrubby undergrowth of Rhododendron and dwarf bamboo.

Dominant mammals include the Musk Deer, Himalayan Tahr, Blue Sheep, Red Panda, Common Langur and Himalayan Black Bear, Goral, Serow and Lesser cats. Some of the avian fauna found in this region are Blood Pheasant, Ibisbill, Monal Pheasant, Fire-tailed Sunbird, Blue Magpie as well as a few species of reptiles and amphibians. Brown Trout, Salmo trutta fario has been introduced in high altitude lake and river systems.

Subsistence farming of wheat, barley and maize is carried out while beans, peas, some apple, peach and pear are grown on homesteads. Potato and cabbage are grown as cash crops. Some amount of cattle and yak rearing is practiced with grazing in forest pasturelands. Seabuckthorn, Hippophae spp. occurs in the wild; some of which is collected for medicinal purposes and also for its use as a dye.

The Alpine forests and scrub extend up to 4,500 m with small crooked trees and large shrubs interspersed with fir and pine. The stunted forest is mainly of rhododendron of many species.

Several plants found in this region attract interest for use in traditional medicine. Dwarf rhododendron leaves are used for burning as incense. This region has a very small resident human population, mainly Bhutias and mostly pastoral, herding livestock such as yaks and dairy cattle.

Trans-Himalayan
This region lies between 4500 m and 5500m and is characterized by cold desert vegetation, a feature exclusively restricted to the north of Sikkim. This eco-region has not yet been included in the protected area network of the state and is perhaps the most threatened as it contains a host of endangered species. Dominant among these are Kiang, Nayan, Tibetan Gazelle, Snow Leopard, Tibetan Wolf, Tibetan Snowcock, Lammergeier, Raven, Golden Eagle and the Ruddy Shelduck.

The region has a short four-month growing season during which grass, flowering plants and herbs grow abundantly supporting a host of insect fauna as well as wild and domestic herbivores, larks and finches.

There are no permanent settlements. Human population consists of a small number of nomadic herders of Tibetan origin called ‘Drokpas’ (who herd yak, sheep and the Capra hircus species of goats), and a large number of army personnel as the area lies close to the international border with Tibet in China. Closure of the border for the last three decades has led to intense grazing pressure on the Sikkim-side by both the domestic and wild herbivores on the land.