Tropical evergreen forest

Also called: Tropical rainforest · Topic: Natural Vegetation and Forests of India · NCERT: Class 11, Ch 5 "Natural Vegetation"

Meaning

A tropical evergreen forest is a dense forest that grows in layers in hot, very wet places with more than 200 cm of rain a year and a mean annual temperature above 22°C. Its trees have no fixed time to shed their leaves, so the forest stays green all year. In India, these forests grow on the western slopes of the Western Ghats, in the north-east hills and on the Andaman and Nicobar Islands. They hold some of India's richest biodiversity and are a classic example of how climate controls vegetation.

Explanation

How it forms: climate controls the forest

• Two climate limits:
• rainfall over 200 cm a year

• mean annual temperature above 22°C

• How the Western Ghats get their rain:

• The moist south-west monsoon wind blows in from the Arabian Sea from June to September.
• The wind hits the Western Ghats and is pushed up the slope.
• The rising air cools, its water vapour condenses, and heavy rain falls on the windward (rain-facing) western side.

• The result is a narrow, very wet belt of evergreen forest on the western slopes.

• Rain-shadow effect (the dry side behind a mountain):

• The eastern, leeward (wind-sheltered) side of the Ghats gets much less rain.
• Moving east, the evergreen forest gives way to semi-evergreen forest and then to moist deciduous forest.

Why it stays green all year

• There is no long cold season and no long dry season.
• So the trees have no fixed time to shed leaves, flower or fruit.
• Different trees do these things at different times.
• The canopy (the leafy top layer of a forest) never goes bare.

Structure and species

• Stratified forest: the plants grow in clear layers, one above the other.

Layer	What grows there
Ground layer	Shrubs and creepers (climbing plants)
Middle layer	Short trees
Top layer (canopy)	Tall trees 60 m or more high

• Thick canopy, dark floor:
• The canopy is so thick that little sunlight reaches the ground.

• So the lower layers are full of creepers that grow well in shade.

• Main species: rosewood, mahogany, aini, ebony.

• Champion and Seth classification: Indian forest departments use this system. In it, Tropical Wet Evergreen Forests and Tropical Semi-Evergreen Forests are two separate groups [3].

Why it is used less for timber

• Hardwood: the timber is very hard, so it is difficult to cut and work.
• Mixed stands: many species grow mixed together, so loggers cannot find large patches of one useful tree.
• Poor access: the forests are dense and grow on steep, wet hills, so roads and transport are difficult.
• Deciduous forests (monsoon forests that shed their leaves in the dry season) have softer wood in purer stands, such as teak and sal. So they are used much more.

Where it occurs

Region	Location	Why it is wet
Western Ghats (western slopes)	Windward side in Maharashtra, Goa, Karnataka, Kerala and Tamil Nadu	The south-west monsoon from the Arabian Sea hits the hills directly
North-east hills	Assam, Arunachal Pradesh, Meghalaya, Nagaland, Manipur and nearby hills	The Bay of Bengal branch of the monsoon is trapped and pushed up by the hills
Andaman and Nicobar Islands	Bay of Bengal, about 6°N–14°N	Close to the equator, with sea all around and rain from both monsoons

• States in 1988: tropical wet evergreen forest was found in Arunachal Pradesh, Assam, Karnataka, Kerala, Manipur, Nagaland, Tamil Nadu, Goa and the A&N Islands [3].
• Area in 1988: tropical wet evergreen forest covered 51,249 sq km, which was 8.0% of India's forest area [3]. For the current area, check the latest biennial India State of Forest Report (ISFR) from the Forest Survey of India.
• Silent Valley (Palakkad district, Kerala): the classic Indian example of tropical evergreen forest.
• Western Ghats World Heritage Site: UNESCO inscribed it in 2012 under natural criteria (ix) and (x) [2].
• It is a serial site (one heritage site made of many separate parts) of 39 protected areas in 7 clusters, spread over 1,250 km [2].

• UNESCO says its forests are among the best non-equatorial tropical evergreen forests anywhere [2].

• World: the same type of forest grows in the hot, wet equatorial belt. Examples are the Amazon basin in South America, the Congo basin in Africa and parts of South-East Asia.

Don't confuse with

• Tropical semi-evergreen forest: it grows in the less rainy parts of the same three regions. It mixes evergreen trees with moist deciduous trees, and creepers in the undergrowth keep it looking green. Its species are white cedar, hollock and kail. In 1988 it covered 4.1% of forest area, against 8.0% for wet evergreen forest [3].
• Tropical deciduous (monsoon) forest: the trees shed their leaves together in the dry season. It has softer wood in purer stands, such as teak and sal, so it is used far more for timber. Teak and sal are not evergreen species.
• Eastern (leeward) slopes of the Western Ghats: these lie in the rain shadow. They have semi-evergreen and deciduous forest, not evergreen forest.

Prelims Hooks

• Climate limits: >200 cm of rain and a mean annual temperature >22°C.
• Found on the western (windward) slopes of the Western Ghats, not the eastern slopes, which lie in the rain shadow.
• Three Indian regions: Western Ghats, north-east hills, Andaman and Nicobar Islands.
• Species trap: rosewood, mahogany, aini and ebony are evergreen. White cedar, hollock and kail are semi-evergreen. Teak and sal are deciduous.
• The tallest trees reach 60 m or more, and the forest grows in clear layers (stratified).
• Western Ghats World Heritage Site: 2012, criteria (ix) and (x), 39 protected areas in 7 clusters [2]. Silent Valley is in Kerala (Palakkad district).

Mains Points

• Distribution follows rainfall and relief (GS-I):
• The monsoon wind hits the Western Ghats and the north-east hills and rises over them.
• The windward slopes get more than 200 cm of rain and grow evergreen forest.

• The rain-shadow side gets less rain and grows semi-evergreen and then deciduous forest. This shows clearly how climate controls vegetation.

• Low timber value but high ecological value (GS-III):

• Hardwood, mixed stands and poor access limit their use for timber.

• But they are home to at least 325 globally threatened species. UNESCO calls the Western Ghats one of the world's eight "hottest hotspots" of biodiversity, and its mountain forests affect the Indian monsoon [2].

• Threats and responses (GS-III):

• Plantations, mining, dams, roads and shifting cultivation in the north-east break these forests into pieces. The forests are very diverse and slow to grow back, so the losses are hard to reverse.
• Responses include the Save Silent Valley movement, the Western Ghats World Heritage listing in 2012 [2] and the National Forest Policy 1988. Use these in answers on how to balance development and protection in the Ghats.

Related concepts

• Tropical semi-evergreen forest

Read more

• Tropical evergreen and semi-evergreen forests