Biologists recognize a number of altitudinal and regional zones that support distinctive flora and fauna. The eastern Himalaya is considerably wetter than the western Himalaya because as the yearly monsoon moves north-westward from the Bay of Bengal, the moisture it carries is rapidly dissipated. The Northern facing slopes of the Himalaya and the Tibetan plateau, sequestered in a rain shadow beyond the monsoon's reach, are dry and practically rainless. Thus, Bhutan's eastern stretch of the Himalayas supports diverse and prolific forests, flowering plants and wildlife. This stretch of the Eastern Himalaya supports lush tropical montane forests, while at the same elevations in the west are found subtropical thorn or sage scrub.

Vertical zoning results from the changes in temperature and moisture that result from increased altitude. Temperature decreases while moisture increases up to a point and then, at higher levels, decreases. Generally, the temperature drops approximately 3.5 degrees Fahrenheit for every thousand foot rise. The tree line appears to coincide with elevation levels that have a mean temperature of 50 degrees Fahrenheit for the warmest day of the year. Generally, the height in the eastern Himalaya is about 12,500 feet.

Equally dramatic is the effect of aspect. Steep north facing slopes receive far less sunshine than those facing southward. In Bhutan, it is not unusual to cross from hot, oak covered or bare slopes over a ridge into the snowbound cool of a north facing fir or birch forest. Even in the dead of winter, it is not unusual to see grasshoppers and lizards feeding or basking in the sun on one slope, while a few yards away, the ground is covered in deep snow and all animals are hibernating. The permanent snow line also determines plant and animal populations. Its height varies according to summer temperatures, amount of snowfall, and exposure, fluctuating greatly even within the same range. In Bhutan's Himalaya, the permanent snow line may be as much as 3,000 feet lower on a north slope.

The Aeolian Zone (snow line and above): In the Aeolian Zone, flowering plants are absent, and life is limited to bacteria, fungi, insects, and crustaceans that subsist upon airborne food particles blown up from below. Lichens encrust rocks, and spiders, springtails and glacier fleas are able to survive in protected micro-climates among the rocks and in the soil.

At 19,000 feet, temporary glacial pools are known to support large populations of fairy shrimp. Birds and mammals such as the snow leopard are transient visitors.

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