3. The tropical thorn forests, 4. The littoral or tidal forests, and 5. The mountain forests.
Tropical Evergreen Forests:
These forests are found mainly in those areas where the annual rainfall is more than 200 cm, with a short dry season. The average annual temperature should range between 25 °C and 27 °C. The relative humidity should be around 70 per cent. The general climatic conditions should be hot and humid.
The trees in these regions are evergreen and do not shed their leaves. These forests are very dense and composed of tall trees reaching upto the height of about 60 metres. The trees have a multi-storeyed structure with good canopy. They look like a green carpet when viewed from above. Due to dense growth of trees, the sunlight cannot reach the ground. Thus, the undergrowth mainly consists of canes, bamboos, ferns, climbers, etc.
This makes the passage difficult through the forests. In some areas the evergreen trees are found mixed with deciduous trees. These forests are less dense and can be easily exploited. The timber from the tropical evergreen and semi- evergreen a forest is hard, durable, fine-grained and of high economic value. The important trees of these forests are rosewood, ebony, mahogany, rubber, cinchona, bamboo, coconut, palms, canes, lianas, etc. The main species in the semi-evergreen forests are cedar, hollock, kail, etc.
The true evergreen forests are mostly found along the western side of the Western Ghats, in the northern states and in the Andaman and Nicobar Islands. The semi-evergreen forests which are more gregarious are found along the lower slopes of eastern Himalaya Mountains, Assam, Orissa coast, Western coast and neighbouring hills.
2. Tropical Deciduous Forests:
These are the typical monsoon forests and are found mainly in those areas where the average annual rainfall ranges between 70 cm and 200 cm, with mean annual temperature of about 27 °C.
The average annual relative humidity should be 60 to 75 per cent. The tropical deciduous forests are most widespread in India. On the basis of availability of water, these forests are further divided into moist and dry deciduous forests. The moist deciduous forests are found in areas receiving rainfall between 100 cm and 200 cm, while the dry deciduous forests are found in areas having 70 cm to 150 cm of average annual rainfall. The trees in the deciduous forests shed their leaves for about 6 to 8 weeks during the spring and early summer seasons (March-April). During this period sufficient moisture is not available for the leaves. However, these forests again become green with the onset of the rainy season. Enough light reaches the ground to permit the growth of grasses and climbers.
The tropical deciduous forests are commercially most important as they yield valuable timber and a variety of other forest products. The important trees in the moist deciduous forests are teak, sal, shisham, sandalwood, khair, kusum, arjun, mahua, mulberry, etc. The dry deciduous forests are more open stretches and also have pipal, neem, tendu, bamboo, apart from those in the moist deciduous forests.
The tropical deciduous forests are commercially most exploited. Large tracts of these forests have been cleared to provide more land for agricultural purposes. These forests have also suffered from severe biotic factors, such as over-cutting, overgrazing, fires, etc.
These forests urgently need scientific management and conservation. The moist deciduous forests are found mainly in the north-eastern states, along the foothills of the Himalaya Mountains, Jharkhand, west Orissa, Chhattisgarh and on the eastern slopes of the Western Ghats. The dry deciduous forests are found mainly in the rainier parts of the peninsular plateau and the plains of Bihar and Uttar Pradesh.
Tropical Thorn Forests:
These forests are found mainly in those areas where the average annual rainfall is less than 75 cm, with a long dry season. The average annual temperatureshould range between 25 °C and 30 °C and the relative humidity should also be low, i.e. less than 50 per cent. The natural vegetation is thorny trees and bushes.
There is not much scope for thick and tall forests due to the shortage of moisture. The trees are scattered and have long roots. The roots penetrate deep into the soil to get moisture. The leaves are mostly thick and small, which retards evaporation. The trees have thick bark and their wood is generally used as fuel. The main trees found in the tropical thorn forests are acacias, palms, euphorbias and cacti. Other important trees include khair, babul, neem, kheijra, kanju, palas, nirmali, dharman, khagri, etc. In some areas, tussocky grass grows upto a height of about 2 metres.
The tropical thorn forests are found in the northwestern parts of the country, including semi-arid areas of Gujarat, Rajasthan, Madhya Pradesh, Uttar Pradesh, southwestern Punjab and western Haryana. These forests also grow on the leeward side of Western Ghats, covering large areas in Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu.
4. The Littoral or Tidal Forests:
These forests are found in and around the deltas of the Ganga, Mahanadi, Godavari, Krishna and Kaveri rivers.
These areas are prone to tidal influences. They also occur in tidal creeks and coastal areas where mud, silt and saline water have accumulated. The trees can grow and survive in fresh as well as in brackish water. The dense mangrove forests occur all along the coastline’ in the sheltered estuaries, backwaters, salt marshes and mud flats, in the Ganga-Brahmaputra delta, the Sundarban is covered with sundari tree, which provides hard and durable timber for construction and boat-making.
The mangrove can attain the height of about 30 metres. The other types of tree6 found in the tidal forests are palm, agar, coconut, keora, nipa, bhendi, canes, etc. The epiphytes (plants growing on other plants) are predominant all over the region. India has about 7 per cent of the world’s total area under tidal or littoral forests. These forests need protection against encroachment by human beings.
5. The Mountain Forests:
The natural vegetation in the mountains is greatly influenced by the decrease in temperature with increase in height above sea level. The mountain forests can be broadly classified into two major categories the forests in the Himalayan ranges and the forests in the peninsular plateaus and hill ranges.
In the Himalaya Mountains, one can notice a succession of natural vegetation belts, as we see in the tropical to the tundra region. Between the height of 1000 m and 2000 m, the evergreen broad-leaf trees such as oak and chestnut predominate. Between the height of 1500 m and 3000 m, the coniferous trees, such as pine, deodar, silver fir, spruce and cedar are found. The coniferous forests cover the southern slopes of Himalayas and parts of northeast India.
At higher elevations (about 3600 m above sea level) temperate grasslands are common. At attitudes above 3600 m, coniferous forests and grasslands give way to the alpine vegetation. Silver firs, junipers, pines and birches are common varieties of trees. Ultimately these forests merge into alpine grasslands, through the shrubs and scrubs. The grasslands are extensively used by the nomadic tribes like the Gujjars and the Bakarwals for grazing livestock. The southern slopes of the Himalaya Mountains have denser forests than the north facing areas.
This is due to relatively higher precipitation. At higher altitudes, mosses and lichens form part of vegetation. In the peninsular India, the mountain forests are found in the three district areas—the Western Ghats, the Vindhyas and the Nilgiris. These areas are near the tropics and only about 1500 m above the mean sea level. In the Western Ghats, the vegetation is sub-tropical in the lower regions and temperate in the higher regions. In the Nilgiris, the temperate forests are locally called sholas. Such forests are also found in the Satpura and Maikala ranges.
The important trees in this region are magnolia, laurel, cinchona and wattle.