Sikkim is a small mountainous State in the Eastern Himalayan region extending approximately 114 km from North to South and 64 km. from East to West, having a total geographical area 7096 sq. km. The State is situated between 880 00’ 58” and 880 55’ 25” East longitudes and 270 00’ 46" and 280 07’ 48” North latitudes.
The State receives an annual Rainfall of 2000mm to 4000mm. Tista & Rangit, which originates respectively, from Cholamu Lake and Rathong Glacier, are the two major rivers of the State.

The State is bestowed with abundant natural resources. Covering just 0.2% of the geographical area of the country, it has tremendous biodiversity and has been identified as one of the Hot Spot for biodiversity. The Sikkim Himalayas that spread over Sikkim and the hill region of Darjeeling harbors more than 26 % of the flowering plants reported in the country and known to be an important phytogeographical reserve of the country. Species wise, it has approx. 5000 Flowering plants, 515 Orchids, 36 Rhododendrons, 16 Conifers, 23 Bamboos, 362 Ferns and Ferns allies, 8 Tree Ferns, 60 Primulas, 11 Oaks, over 424 medicinal plants, 150 Mammals, 552 Birds, 48 Fishes and over 690 nos. of Butterflies and also 28 nos. of Mountains/Peaks, 21 Glaciers, 227 High altitude lakes & wetlands and over 104 rivers and streams. The Sikkim Himalayas has excellent scope for value added eco-tourism and traditions.

Physiographically, Sikkim can be said to have its feet in the ocean and its head in the sky. The altitudes vary from 300 meters to 8500 meters above mean sea level. The entire state is a young mountain system with highly folded and faulted rock strata at many places. It encompasses the lesser Himalayas, Central Himalayas and the Tethys Himalayas. Great Mountain ranging from 3000 meters to 8500 meters in height separates the state from surroundings. In fact, it has no flat piece of good size land any where. Major portion is covered by the pre-Cambrian rock and is much younger in age. The rock type consists of phyllites and schists and therefore, the slopes are highly susceptible to weathering and prone to erosion and landslides. The trend of the mountain system is in general east-west direction. The mountains rise in elevation northward. The northern portion of the state is deeply cut into steep escarpments, and except in the Lachen and Lachung valleys, is not populated. Southern Sikkim is lower, more open, and fairly well cultivated. This configuration of the state is partly due to the direction of the main drainage which is southern. The Rangeet and the Teesta which form the main channels of drainage, run nearly North-South. The valleys cut by these rivers and their chief feeders are very deep. The valleys are rather open towards the top, but usually attain a steep gorge like structure as we approach the bed of the rivers. There are 227 perennial lakes /wetlands at different altitudes. Many hot water springs i.e. Phur sachu, Ralang Sachu, Yumthang, Momay are also found in the State. The perpetual snow line in Sikkim may be approx. at 16,000 ft.

Soil of the region being the nutrient medium, is indispensable in vegetations. Soil moisture, mostly depending upon the soil thickness has an explicit impact on forest type and coverage in an area. The entire state primarily consists of gneissose rocks and half-schistose rocks. The soil developed from the gneissic group of rocks is brown clay, generally shallow and poor. They are typically coarse, often with ferric concentrations, neutral to acidic with poor organic/mineral nutrients. They tend to carry most of the evergreen and deciduous forests. The high intensity of rain fall in the state often causes extensive soil erosion and heavy losses of nutrients of land by leaching.

Source: Agriculture Department (Soil), Government of Sikkim.

Climatically, Sikkim experiences variable temperature with summer in the foothills and freezing winter on the high mountains. The climate of the state has been divided roughly into the tropical, temperate and alpine zones. The general trend of decrease in temperature with increase in altitudes holds good every where. For most of the period in a year, the climate is cold and humid as rainfall occurs in each month. The area experiences a heavy rainfall due to its proximity to the Bay of Bengal. The state receives an average annual rainfall of 500 cm. which is the highest in the Eastern Himalayas. The high density of rainfall causes extensive soil erosion and frequent landslides. The pre-monsoon rain occurs in April-May and the monsoon occurs normally from the month of May and continues up to early October. The temperature varies with altitudes and slope. While in lower altitudinal zones the mean temperature varies between 4.50C to 9.50 C. The maximum temperature is recorded usually during the month of July & August and minimum during December & January. During the period from May to September fog becomes a common feature in this area. Also during winter snowfall is common in high altitude places. The mean temperature in the lower altitudinal zones varies between 4.5 degree centigrade to 18.5 degree centigrade, whereas at higher altitudinal zones, it varies from 1.5 degree centigrade to 9.5 degree centigrade. Temperature varies with altitude and slope. Biting cold is experienced at high altitude places in the winter months and snowfall is also not uncommon during this period.

An examination of available rainfall data shows that the mean annual rainfall is minimum at Thangu (82 mm/)in north Sikkim and maximum at Gangtok (3494 mm/) in east Sikkim. An isohyetal analysis of this data reveals that there are two maximum rainfall areas: (i) South- East quadrant, including Mangan, Singhik, Dikchu,Gangtok and Rongli (ii) South- West quadrant, including Hilley. In between these two regions, there is a low rainfall region (e.g. Namchi). Rainfall in this area is about half of that in the former areas. There is an area in north - west Sikkim which gets very little rainfall (even less than 4.9 mm/). This area is mainly snow-covered mountains. Rain-fall is heavy and well distributed during the months of May to early October. July is the wettest month in most places. The intensity of rainfall during south - west monsoon season decreases from south to North, while the distribution of winter rainfall is in the opposite order. The highest annual rainfall for individual stations may exceed 5000 mm/ and average number of rainy days (days with rain of 2.5 mm/ of more) range from 100 at Thangu to 184 at Gangtok.

The forest of the state exhibit diversified variety of flora and fauna. The myriad life forms of both plants and animals that characterize this bio-geographical region are unique in India and perhaps in the whole world. Sikkim being a high rain fall region encompasses within its narrow belt a luxuriant floristic composition ranging from tropical screw pines to alpine Primulas. The state is renowned for its Rhododendron and Orchids as also high altitude Primulas, Meconopsis and blue puppies. The flora of Sikkim is also esteemed for several medicinal plants. Sikkim is also rich in faunal wealth and Himalayan animals such as Snow Leopard, Musk Deer, Blue sheep, Himalayan Tahr and rare pheasants are found in several parts of the pristine spots and virgin forests of the state. The lakes fed by melting snows are the habitat of a number of resident and migratory water fowls and birds.

Most of the lower southern valleys fall into this category. This mainly constitutes of deciduous plants like Sal Shorea robuta, Khair Acacia catechu and many other species like Lannaea grandis, Garuga pilmata, Terminalia belerica, Macaranga sp., etc. Secondary growth consists of various species of Strobilanthes, Barleria, Cissampelus, Tridax, Polygonum, etc. Also species like Bananas, Pandanus and various species of palms and canes occur in inner valleys.

Vegetation between 900-1,800 m (Tropical Moist forest or Broad-leaved Forests). This region includes mainly various broad-leaved species like Engelhardia, Schima, Castanopsis,. Acer, Litsea, etc. The secondary growth consists of Girardinia, Boehmeria, Maesa, Ardisia, Melastoma, Edgeworthia etc. It also includes various climbers like Pathos, Vilis, Rhaphidophora, Smilax, Dioscorea etc. Some species of bamboos are also found. 
 
Vegetation between 1,800-2,800 m( Temperate Broad -leaved Forests). This region is predominated by Rhododendron and Michelia. Other species found in this region are Mahonia, Castanopsis, Quercus, Schima, Ilex, Acer, Magnolia, Cinnamomum, Betula, etc The shrubs mainly consists of Debregeasia, Urtica,Gynura, Hypericum,Viburnum. Vaccinium, Piptanthus, Mahonia, Berberis etc. These are mostly temperate type of plants.
A maling bamboo is very common in east Sikkim around Gnathang to Zuluk.
 
Vegetation between 2,800-3,800 m (Temperate Coniferous and Broad leaved Forests).
Mostly consists of Rhododendron and Tsuga type of species. In some regions like Tsoka, Bakkim and Lachen area the coniferous species like Aibes densa, Thuja sp. also occur. Secondary growth mainly consists of Silene, Astragalus, Fragaria Impatiens,Geranium, smaller species of Rhododendrons, Gualtheria, Cynoglossum, Jasmine etc.  
Vegetation between 3,800-4,500 m (Sub-Alpine Vegetation). Mostly coniferous trees and smaller shrubs occur in the area. The tree line in the west  Sikkim is up to 4,000 m, while in the east it ceases at a much lower altitude due to  steepness of the terrain and deforestation. In north Sikkim in some parts like Tholung and Zema Valleys it is almost up to 4,500 m. Plants like Junipers, dwarf species of Rhododendrons, azaleas and many spring flowers like Potenti Primulas, Ligularia, Pedicularis, Senecio, Aster etc. become very common.  
Vegetation between 4,500-5,500 m (High Altitude Desert),
This region has typical high altitude desert plants which come out only in spring.    These mainly include various species of Meconopsis, Sedum, Waldh: Phlomis, Pedicularis, Bistorta, Potentilla, Saxifraga, Saussurea etc. The vegetation become sparse as one goes to North Sikkim towards the Tibet border plants grow flat across the land because of strong breeze,

Out of the total of about 1400 butter flies recorded from the Indian sub-continent almost 50% of the butterflies are recorded from Sikkim. Regarding birds, in no part of the world, of an equal area, are birds more profusely represented in species than in Sikkim. Some important species of animals and avifauna are as under:

Source: Sikkim at a Glance 2001