The Alpine vegetation in the western Himalayas is found only up to a height of 3000 metres, while in the eastern Himalayas it is found up to a height of 4000 metres,. The reason for this variation in the same mountain range is that

The principal control on the height of alpine vegetation (tree-line) is local climate — chiefly temperature moderated by latitude and maritime influence. The Eastern Himalaya is closer to the equator and the Bay of Bengal moisture conveyor, giving warmer, more humid conditions that push the alpine/tree-line to higher altitudes. By contrast the Western Himalaya records relatively lower temperatures and less rainfall, so alpine conditions occur at lower heights [2]. Texts on Himalayan zonation emphasize that each life zone is governed by its insolation, temperature and precipitation relationships, so a warmer, moister eastern sector supports vegetation higher up the slope than the drier, colder western sector [3].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 5: Natural Vegetation and National Parks > 2. The North-Western Himalayan Region > p. 3
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 10: Indian Forest > 1. 2. Himalayan Moist Temperate Forest > p. 163
3. [3] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > 2. Abiotic (Non-living) Components > p. 18