In the more humid regions of the east the mountains are almost everywhere covered with a dense forest which reaches up to 12,000 or 13,000 ft. Many tropical types here ascend to 7000 ft. or more. To the west the upper limit of forest is somewhat lower, from 11,500 to 12,000 ft. and the tropical forms usually cease at 5000 ft.
In Sikkim the mountains are covered with dense forest of tall umbrageous trees, commonly accompanied by a luxuriant growth of under shrubs, and adorned with climbing and epiphytal plants in wonderful profusion. In the tropical zone large figs abound, Terminalia, Shorea (sál), laurels, many Leguminosae, Bombax, Artocarpus, bamboos and several palms, among which species of Calamus are remarkable, climbing over the largest trees; and this is the western limit of Cycas and Myristica (nutmeg). Plantains ascend to 7000 ft. Pandanus and tree-ferns abound. Other ferns, Scitamineae, orchids and climbing Aroideae are very numerous, the last named profusely adorning the forests with their splendid dark-green foliage. Various oaks descend within a few hundred feet of the sea-level, increasing in numbers at greater altitudes, and becoming very frequent at 4000 ft., at which elevation also appear Aucuba, Magnolia, cherries, Pyrus, maple, alder and birch, with many Araliaceae, Hollböllea, Skimmia, Daphne, Myrsine, Symplocos and Rubus. Rhododendrons begin at about 6000 ft. and become abundant at 8000 ft., from 10,000 to 14,000 ft. forming in many places the mass of the shrubby vegetation which extends some 2000 ft. above the forest. Epiphytal orchids are extremely numerous between 6000 and 8000 ft. Of the Coniferae, Podocarpus and Pinus longifolia alone descend to the tropical zone; Abies Brunoniana and Smithiana and the larch (a genus not seen in the western mountains) are found at 8000, and the yew and Picea Webbiana at 10,000 ft. Pinus excelsa, which occurs in Bhutan, is absent in the wetter climate of Sikkim.
On the drier and higher mountains of the interior of the chain, the forests become more open, and are spread less uniformly over the hill-sides, a luxuriant herbaceous vegetation appears, and the number of shrubby Leguminosae, such as Desmodium and Indigofera, increases, as well as Ranunculaceae, Rosaceae, Umbelliferae, Labiatae, Gramineae, Cyperaceae and other European genera.
Passing to the westward, and viewing the flora of Kumaon, which province holds a central position on the chain, on the 80th meridian, we find that the gradual decrease of moisture and increase of high summer heat are accompanied by a marked change of the vegetation. The tropical forest is characterized by the trees of the hotter and drier parts of southern India, combined with a few of European type. Ferns are more rare, and the tree-ferns have disappeared. The species of palm are also reduced to two or three, and bamboos, though abundant, are confined to a few species.
The outer ranges of mountains are mainly covered with forests of Pinus longifolia, rhododendron, oak and Pieris. At Naini Tal cypress is abundant. The shrubby vegetation comprises Rosa, Rubus, Indigofera, Desmodium, Berberis, Boehmeria, Viburnum, Clematis, with an Arundinaria. Of herbaceous plants species of Ranunculus, Potentilla, Geranium, Thalictrum, Primula, Gentiana and many other European forms are common. In the less exposed localities, on northern slopes and sheltered valleys, the European forms become more numerous, and we find species of alder, birch, ash, elm, maple, holly, hornbeam, Pyrus, &c. At greater elevations in the interior, besides the above are met Corylus, the common walnut, found wild throughout the range, horse chestnut, yew, also Picea Webbiana, Pinus excelsa, Abies Smithiana, Cedrus Deodara (which tree does not grow spontaneously east of Kumaon), and several junipers. The denser forests are commonly found on the northern faces of the higher ranges, or in the deeper valleys, between 8000 and 10,500 ft. The woods on the outer ranges from 3000 up to 7000 ft. are more open, and consist mainly of evergreen trees.
The herbaceous vegetation does not differ greatly, generically, from that of the east, and many species of Primulaceae, Ranunculaceae, Cruciferae, Labiatae and Scrophulariaceae occur; balsams abound, also beautiful forms of Campanulaceae, Gentiana, Meconopsis, Saxifraga and many others.
Cultivation hardly extends above 7000 ft., except in the valleys behind the great snowy peaks, where a few fields of buckwheat and Tibetan barley are sown up to 11,000 or 12,000 ft. At the lower elevations rice, maize and millets are common, wheat and barley at a somewhat higher level, and buckwheat and amaranth usually on the poorer lands, or those recently reclaimed from forest. Besides these, most of the ordinary vegetables of the plains are reared, and potatoes have been introduced in the neighbourhood of all the British stations.
As we pass to the west the species of rhododendron, oak and Magnolia are much reduced in number as compared to the eastern region, and both the Malayan and Japanese forms are much less common. The herbaceous tropical and semi-tropical vegetation likewise by degrees disappears, the Scitamineae, epiphytal and terrestrial Orchideae, Araceae, Cyrtandraceae and Begoniae only occur in small numbers in Kumaon, and scarcely extend west of the Sutlej. In like manner several of the western forms suited to drier climates find their eastern limit in Kumaon. In Kashmir the plane and Lombardy poplar flourish, though hardly seen farther east, the cherry is cultivated in orchards, and the vegetation presents an eminently European cast. The alpine flora is slower in changing its character as we pass from east to west, but in Kashmir the vegetation of the higher mountains hardly differs from that of the mountains of Afghanistan, Persia and Siberia, even in species.
The total number of flowering plants inhabiting the range amounts probably to 5000 or 6000 species, among which may be reckoned several hundred common English plants chiefly from the temperate and alpine regions; and the characteristic of the flora as a whole is that it contains a general and tolerably complete illustration of almost all the chief natural families of all parts of the world, and has comparatively few distinctive features of its own.
The timber trees of the Himalaya are very numerous, but few of them are known to be of much value. The “Sál” is one of the most valuable of the trees; with the “Toon” and “Sissoo,” it grows in the outer ranges most accessible from the plains. The “Deodar” is also much used, but the other pines produce timber that is not durable. Bamboos grow everywhere along the outer ranges, and rattans to the eastward, and are largely exported for use in the plains of India.