Tropical evergreen and semi-evergreen forests

Detail

What these forests are

• Tropical evergreen forest: a dense forest in hot, very wet places. The trees do not all lose their leaves at the same time, so the forest stays green all year.
• Tropical semi-evergreen forest: a forest in slightly less rainy parts of the same regions. It has evergreen trees mixed with moist deciduous trees (trees that shed their leaves for a few weeks in the dry season).
• In the Champion and Seth classification used by Indian forest departments, these are two separate groups: Tropical Wet Evergreen Forests and Tropical Semi-Evergreen Forests [3].

Where they grow

Region	Location	Why it is wet
Western Ghats (western slopes)	Windward (rain-facing) 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

• In 1988 the states with tropical wet evergreen forest were Arunachal Pradesh, Assam, Karnataka, Kerala, Manipur, Nagaland, Tamil Nadu, Goa and the A&N Islands [3].
• In 1988 tropical semi-evergreen forest was found in Assam, Gujarat, Karnataka, Kerala, Maharashtra, Goa, Nagaland, Odisha, Tamil Nadu and the A&N Islands [3].

Why they grow there (climate)

• 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 (June–September)
• it 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 (western) side

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

• Rain-shadow effect: the eastern (leeward, wind-sheltered) side of the Ghats gets much less rain. There the evergreen forest gives way to semi-evergreen forest and then to moist deciduous forest.

• Why the forest is 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, and the canopy (the leafy top layer of a forest) never goes bare

Structure: forest in layers

• 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

• The canopy is very thick, so little sunlight reaches the floor. That is why the lower layers are full of shade-loving creepers.

Main species

• Evergreen: rosewood, mahogany, aini, ebony.
• Semi-evergreen: white cedar, hollock, kail.

Why they are used less than deciduous forests

• Three reasons commercial use is limited:
• Hardwood: the timber is very hard, so it is hard 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 leaves in the dry season) have softer wood in purer stands, such as teak and sal, so they are used much more.

Semi-evergreen forests in detail

• They grow in the less rainy parts of the same three regions: the Western Ghats, the north-east hills and the A&N Islands.
• Mix of trees: evergreen trees plus moist deciduous trees.
• Why they still look green: creepers and climbers in the undergrowth (low plants under the trees) stay green even when the deciduous trees drop their leaves.

Area (old baseline)

Forest type group	Area (1988)	Share of forest area (1988)
Tropical wet evergreen	51,249 sq km	8.0%
Tropical semi-evergreen	26,794 sq km	4.1%
Together	78,043 sq km	12.1%

• Source of these figures: National Wasteland Development Board (1988), using the Champion and Seth types [3]. For the current area, check the latest biennial India State of Forest Report (ISFR) published by the Forest Survey of India.

Conservation importance

• Biodiversity hotspot: a region that has a very large number of species found nowhere else and has also lost much of its natural habitat. The Western Ghats evergreen belt is part of the Western Ghats–Sri Lanka hotspot.
• UNESCO World Heritage Site (2012): the Western Ghats were inscribed in 2012 under natural criteria (ix) and (x) [2].
• The site is a serial site (one heritage site made up of many separate parts): 39 protected areas in 7 clusters, spread over 1,250 km [2].
• UNESCO calls its forests some of the best non-equatorial tropical evergreen forests anywhere [2].
• It is home to at least 325 globally threatened species of plants, animals, birds, amphibians, reptiles and fish [2].
• UNESCO calls it one of the world's eight "hottest hotspots" of biodiversity [2].

• Its high mountain forests affect the Indian monsoon and make it one of the best examples of a monsoon system on Earth [2].

• Silent Valley (Palakkad district, Kerala): the classic example of tropical evergreen forest in India.

• Endemism means a species lives naturally in only one place. It is covered in the biodiversity-conservation note.

Prelims Hooks

• Climate limits of tropical evergreen forest: >200 cm rain and >22°C mean annual temperature.
• They grow on the western (windward) slopes of the Western Ghats, not the eastern slopes, which lie in the rain shadow.
• Three regions: Western Ghats, north-east hills, Andaman and Nicobar Islands.
• Tallest trees reach 60 m or more, and the forest grows in clear layers.
• Evergreen species: rosewood, mahogany, aini, ebony. Semi-evergreen species: white cedar, hollock, kail. Trap: teak and sal are deciduous species, not evergreen.
• Semi-evergreen forest = evergreen trees + moist deciduous trees, kept green by undergrowth climbers.
• Western Ghats World Heritage Site: 2012, criteria (ix) and (x), 39 protected areas in 7 clusters [2].
• Silent Valley is in Kerala (Palakkad district).
• In 1988, wet evergreen forest (8.0%) covered more area than semi-evergreen forest (4.1%) [3].

Mains Points

• Distribution follows rainfall and relief (GS-I): the monsoon 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 is a clear case of how climate controls vegetation.
• Low commercial value but high ecological value (GS-III): hardwood, mixed stands and poor access limit their use for timber. But these forests hold some of India's richest biodiversity, help regulate the monsoon, and protect the sources of the peninsular rivers [2].
• Conservation responses (GS-III): the Save Silent Valley movement, the Western Ghats World Heritage listing (2012), and the protected-area network of 39 sites [2]. These belong in answers on forest policy (National Forest Policy 1988) and on how to balance development with protection in the Ghats.
• Threats: plantations, mining, dams, roads and shifting cultivation in the north-east break up these forests. Because the forests are so diverse and slow to grow back, the losses are hard to reverse.