Many tree species occur over a wide range of altitude and ecological zones and they predominate in a given zone but may occur in zones above or below them. Classification based on ecological basis gives sufficient weightage to both vegetation and climate. Forest tree occur in very restricted areas have been omitted. The major forest types studied by J. K. Jackson (1994) are as under:
1. Tropical Forest (up to 1000 m)
i. Shorea robusta Forest
ii. Acacia catechu-Dalbergia sissoo Forest
iii. Other riverain Forest
iv. Grassland v. Terminalia-Anogeissus Deciduous Hill Forest
2. Sub-tropical Forest (1000 m to 1700 m)
i. Pinus roxburghii Forest
ii. Schima-castanopsis Forest
iii. Alnus nepalensis Forest
iv. Riverain forest with Toona and Albizia species
3. Temperate Forest (2000 m to 3100 m)
i. Lower temperate Forest (2000 m to 2700 m)
a. Quercus leucotricophora and Quercus lanata Forest (Oak)
b. Quercus floribunda Forest
c. Quercus lamellosa Forest
d. Lower temperate mixed broad leaved forest with abundant lauraceae
e. Pinus wallichiiana Forest (Lower type)
ii. Upper temperate Forest (2700 m to 3100 m)
a. Quercus semecarpifolia Forest
b. Upper temperate mixed broadleaved Forest
c. Rhododendron Forest
d. Upper temperate coniferous Forest
4. Sub-alpine Forest (3000 m to 4200 m)
i. Abies spectabilis Forest
ii. Betula utilis Forest
iii. Rhododendron Forest
iv. Juniperus indica steppe
v. Caragana steppe
5. Alpine Forest (up to 4500 m)
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