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Vegetation of any place is modified by the environmental factors; climate, geology and biotic factors.
The great area of Indian subcontinent has wide range of climate and corresponding diversity in the vegetation.
India has been divided into the following botanical zones by D. Chatterjee (1962) Fig. 11.4:
(1) Western Himalayas,
(2) Eastern Himalayas,
(3) Indus plain,
(4) Gangetic plain,
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(5) Central India,
(6) Deccan,
(7) Western coasts of Malabar,
(8) Assam, and
(9) Bay Islands of Andaman and Nicobar.
1. Western Himalayas:
The northern part of our country is bounded by highest ranges of Himalayas and is one of the important botanical regions of the world with climate and vegetation ranging from truly tropical near the low altitudes to temperate arctic types at the high altitudes. The northern mountain division can phytogeographically be divided into western, central and eastern zones.
Western Himalayas consist of north Kashmir, south Kashmir, a part of Punjab, H.P., Garhwal and Kumaon. This zone is wet in outer southern ranges and slightly dry in inner northern zone. The average annual rainfall in this region is from 100 to 200 cm. Snowfall occurs in this region during winter season. The region may be divided into three subzones (Fig. 11.5).
(i) Submontane zone or lower region or tropical and subtropical belts (up to about 1500 metres altitude from the sea level).
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(ii) Temperate zone (from 1500 metres to 3500 metres altitude),
(iii) Alpine zone (above 3500 metres and up to the line of perpetual snow).
(i) Submontane or lower region or tropical and subtropical belts:
It includes outer Himalayas, particularly region of Siwaliks and adjoining areas where annual average rainfall is over 100 cm. This zone ranges between 300 and 1500 metres above sea level. In this zone, forests dominated by timber trees of Shorea robusta are common. Other important tree species are Salmalia malabaricum, Butea monosperma. Acacia catachu and Zizyphus species.
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In the swampy areas, Dalbergia sisso (Shisham), Ficus glomerata, Eugenia jambolana are of common occurrence. In west dry regions sal trees are replaced by xeric plants particularly Zizyphus, Carissa, Acacia, and thorny Euphorbias. At higher elevation, around 1000 to 1500 metre altitude, cheer (pine) forests are also found at certain places. The common species of pine are Pinus longifolia and Pinus roxburghii. Ground vegetation is scanty.
(ii) Temperate zone:
It commonly ranges at the altitudes from 1500 to 3500 metres above the sea level. Oaks are dominant along with Populus, Rhododendron, Betula and Pyrus. Pinus excelsa, Cedrus deodara, Picea, Abies, Cupressus and Taxus baccata are found in the heavy rainfall region (between 1600 and 1800 m). Herbs are also common in this region. Common herbs are Ranunculus hirtila, Polygonum, Pedicularia, Potentilla argyrophylla. Primula, Delphinium, Clematis, crucifers and many members of asteraceae.
In cultivated drylands of Punjab, wheat and barley are main crops. In Kashmir, Betula (birch), Salix (cane), Populus (poplar) are of common occurrence. Besides these, Quercus semicarpifolia, Q. dilatata, Aesculus indica (chestnut) and many conifers are commonly met within this region. In west Kashmir rice cultivation is common Sar or saffron (Crocus sativus), apples, peaches, walnut, almonds and other fruits are important economic plants of Kashmir region.
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(iii) Alpine zone:
Above the altitude of 3500 metres and up to snowline (about 5000 m) is alpine zone. The vegetation consists of evergreen conifers and some low and broad leaved trees. The vegetation of this region is characterized by cushion habit, dwarf nature and gregarious habit. In lower alpine region, shrubby forests are common which may be (a) Birch—fir forest which is fairly dense and is mixed with evergreen shrubby Rhododendron at higher level and (b) Birch— Rhododendron forests in which silver fir, Betula, Rhododendron and Juniperus are common. In the upper alpine region, prominent herbaceous plants are the species of Primula, Polygonum, Gentiana, Cassiope, Meconopsis, Saxifraga, Potentilla, Geranium, Aster, Astragalus etc. which form alpine meadows. At about 5000 metre altitude and above snow perpetuates round the year and plant growth is almost nil. This altitude is called snow line or ice line.
Populations of Draba, Braya, Cortia, Leontopodium go on increasing with the increase in altitude. Species of Ephedra, Juniperus, Berberis are also found scattered. Poa, Stipa and Fectuca are common grasses of alpine zone.
2. Eastern Himalayas:
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Eastern Himalayas extend from Sikkim to upper Assam, Darjeeling and NEFA. Vegetation of this region differs from that of western Himalayas. The chief differences are due to changed environmental factors as heavy monsoon rainfall, less snowfall and high temperature and humidity.
This region can also be divided into:
(i) Tropical submontane zone
(ii) Temperate or Montane zone, and
(iii) Alpine zone (Fig. 11.5).
(i) Tropical or Submontane Zone:
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The tropical subzone characterized by warm and humid conditions extends from plain up to the altitude of about 1800 m. In this zone mostly sal forests, and mixed deciduous forests consisting of important plants, such as Sterculia, Terminalia Anthocephalus cadamba and Bauhinia are common. In the savannah forests, common plants are Albezzia procera, Bischofia, Salmelia, Dendrocalamus. Evergreen forests of Dillenia indica, Michelia champaca, Echinocarpus, Cinnamon, etc. are common.
(ii) Temperate or Montane Zone:
It may be further divided into upper and lower zones Lower temperate zone is the region between 1800 and 3000 metre altitudes. In the lower temperate zone, Oaks (Quercus). Michelia, Pyrus, Cedrela, Eugenia, Echinocarpus are common plants. In upper temperate zone (3000-4000 metre altitude), conifers and Rhododendrons are common. Important conifers of this region are Picea spinulosa, Abies, Larix, Juniperus, Tsuga griffithi, Tsuga brunoniana, etc.
(iii) Alpine Zone (from 4000 metres up to snow line):
Climate is humid and extremely cold. The vegetation in the alpine zone is characterised by complete absence of trees and predominance of shrubs and meadows. Important plants of this zone are Rhododendron and Juniperus. Eastern Himalayan vegetation is considered to be one of the richest vegetational units in the world and consists of several species of plants which are native of foreign countries, such as, China, Japan, Burma, Malaya and European countries.
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3. Indus Plains:
It includes part of Punjab, Rajasthan, Cutch, Delhi, a part of Gujarat. Some part of this plain is now in Pakistan. The climate of this zone is characterised by dry hot summer, and dry cold winter. Rainfall is usually less than 70 cms, but in certain regions it is as low as 10-15 cms. The soil of a wide area except cultivated land, is saline. Much of the land has become desert due to excessive dryness.
Vegetation is mainly bushy and thorny Acacia arabica, Prosopis spicigera, Salvadora Capparis decidua are very common plants of this region. Salsola phoetida and Lunakh grass are found mostly in saline soils. Other plants of this botanic province are Anageissus, Eugenia, Mango, Dalbergia sisso, Albizzia lebbek, Zizyphus nummularia, etc.
Historical evidences indicate that the area was covered by dense forest some 2000 years ago, but gradual destruction of vegetation cover either by biotic agencies or by any other agency led to the development of desert in this plain. Saccharum munja, Cenchrus ciliaris, Prosopis spicigera. Acacia leucophloea, A. Senegal are the important plant species which are grown for checking the spread of desert.
4. Gangetic Plains:
This is one of the richest vegetational zones in India. This zone covers flat land of a part of Delhi, whole of U.P., Bihar, and West Bengal and also a part of Orissa. Rainfall in this zone is from 50 cm to 150 cm. A great part of the land is under cultivation. The common crop plants are wheat, barley maize. Sorghum (jowar), Bajra, urad, Moong (Phaseolus mungo), Cajanus cajan, til (Sesamum indicum), sugarcane. Pea (Pisum sp.), gram (Cicer arietinum), potato, Brassica, rice.
In western part of U.P. annual rainfall is from 50 cm to 110 cm. Dry deciduous and shrubby forests are common in this part. Important plants of south-western part of U.P. are Capparis, Saccharum munja, Acacia arabica. In the north-western part of U.P. near Himalayas foothills Dalbergia sisso. Acacia arabica are most common plants.
In eastern gangetic plain, the conditions are cold and wet (annual rainfall, 150 cm in West Bengal). In this part evergreen forests are common. In central part, the annual rainfall is about 100 cm to 150 cm. The vegetation consists mainly of deciduous trees. Sal trees are dominant. Other common trees are Terminalia tomentosa, T. belerica. Acacia species, Bauhinia, Diospiros (Biri Ka patta or tendu) Eugenia sp., neem trees (margosa), Madhuca indica (Mahua), Cordia myxa (Lasora), Tamarindus, Mango (Mangifera indica). Ficus etc.
In Bihar and Orissa hills, Rubus, Potentilla, Fragaria (Rosaceae), Pyrus etc. are common. Mangrove vegetation is common in tidal regions in West Bengal near Sunder-ban, and Orissa. Rhizophora mucronata, R. conjugata, Sonneratia, Ceriops roxburghiana and Acanthus ilicifolius, Kandelia rheedii, Bruguiera gymnorhiza are common mangrove plants in those regions.
5. Central India:
Central India covers Madhya Pradesh, part of Orissa (now Odisha), Gujarat and Vindhya. The areas are hilly. The average rainfall per annum may be 100-170 cm. Some places are at the altitudes of 500-700 m from the sea level. Biotic disturbances are very common in this botanical province which have led to the development of the thorny vegetation in open areas. In this region teak (Tectona grandis) and sal (shorea robusta) forests are very common. Other trees are Terminalia tomentosa, Bauhinia, Mango, Phyllanthus, Ficus glomerata, etc. Among common shrubs are Mimosa rubricaulis, Desmodium, Acacia sp., Zizyphus rotundifolia and other.
Entire forest vegetation of central India may be divided into:
(i) Sal forests
(ii) Mixed deciduous forests
(iii) Thorny forests.
At Sarguja (M.P.) many species have been reported to occur. Some of them are Pyrus, Barberis asiatica, Rubus, elipticus, etc.
6. Deccan:
This region comprises whole of the southern peninsular India including Satpura and southern part of Godawari River. Average annual rainfall in this region is about 100 cm.
It may be divided into the following two subdivisions:
(i) Deccan plateau
(ii) Coromandel coast.
In Deccan plateau teak forests containing Diospiros, Acacia, Prosopis spicigera. Santalum a hum (chandan tree) and Cedrda toona are common. On rocks, Capparis, Euphorbias, Phyllunthus are common. Teak, Pterocarpus, Borassus, Foenix silvestris are also common in this area In Chhota Nagpur plateau, important species are Clematis natans, Barberis, Thallictrum and also many members of Annonaceae, Rosaceae, Compositae, Araliaceae, Apocynaceae, Lauraceae, Amaranthaceae, Orchidaceae. Some ferns also common.
In Coromandel coast vegetation consists largely of some halophytic species.
7. Western Coast of Malabar:
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This is small botanical province covering Cape Comorin to Gujarat and Western Ghats .This is a region of heavy rainfall.
In this zone, four types of forests are common:
(i) Tropical forests (occur at 700 m altitude).
(ii) Mixed deciduous forests (found at the altitude up to 1600 m).
(iii) Temperate evergreen forests (occur above 1200 m altitude), and
(iv) Mangrove vegetation.
In tropical evergreen forest the trees are tall and they have root buttresses. Important species are Cedrela toona Dipterocarpus. Mangifera indica, Sterculia alata, Artocarpus hirsuta. In the mixed deciduous forests, important plants are Terminalia tomentosa, Terminalia peniculata Tectona grandis, Dalbergia, Lagerstroemia lanceolata and bamboo species, particularly Dendrocalamus and Bamboosa arundinacea. On the Nilgiri hills sub-tropic and temperate conditions exist. Important plants of Nilgiri vegetation are Rubus, Rhododendron arboreum, Barberis, Thallictrum Ranunculus, Fragaria, Potentilla. Many other herbs along with many grasses are also common.
Temperate forests commonly called as “sholas” contain Gardenia obtusa, Michelia nilgirica Eugenia species are also common. In Malabar, plants belonging to family Dipterocarpaceae’ Tihaceae, Anacardiaceae, Meliaceae, Myrtaceae, Piperaceae, Orchidaceae and many ferns are common. The west coast of Malabar region receives very high rainfall. In the coastal region mangrove plants grow luxuriantly.
8. Assam:
This botanical province is very rich in vegetation and covers valley of Brahmaputra, Naga hills and Manipur. This is the region of heaviest rainfall. Cherapunji is one of the rainiest place in the world where annual rainfall often exceeds 1000 cm. Excessive wetness and high temperature in this zone are responsible for the development of dense forests. Broad leaved, tall evergreen angiosperms and some conifers are very common in the forests.
Common plants occurring in this region are Ficus, Artocarpus, Michelia champaca, Sterculia alata. Morus species. Besides these bamboos canes, climbers, and green bushes are also common. Prominent plants in the northern forests of this zone are Alnus nepalensis, Betula. Rhododendron arboreum. Magnolia, Michelia and Prunus. Sal also occurs at Garo hills. Orchids and fern species are very rich in this zone.
9. Bay Islands of Andaman and Nicobar (India):
Islands:
These are represented by the Andaman and Nicobar islands in the east and Lakshadweep islands in the west. The Andaman and Nicobar islands are a group of more than 300 islands, which support many characteristic plants and animals. The forests range from tropical evergreen to moist deciduous and even mangroves. The Lakshadweep group of islands comprise 36 major Islands, which together from an area of 32 sq km. Many varied marine fauna are present here that include turtles, crabs, molluscs and fishes. Beautiful coral reefs are also present in this part of India.
These bay islands represent elevated portions of submarine mountains. Climate is humid in the coastal region. In Andaman, beech forests, evergreen forests, semi-evergreen forests deciduous forests and mangrove vegetation are of common occurrence. Rhizophora Mimusops, Calophyllum, etc. are common plants in mangrove vegetation. In the interior evergreen forests tall trees are common. Important species of trees are Calophyllum, Dipterocarpus, Lagerstroermia and Terminalia etc. Some part is under cultivation. The important crops are paddy and sugarcane.
