Natural Vegetation and Habitats

• Tropical deciduous forests are also called 'Monsoon forests'. 

The natural vegetation can be broadly divided into five different types. They are:

The biome occurs in equatorial or sub equatorial regions where both rainfall and warmth are abundant Rainfall is above 140 cm/yr usually between 200-500 cm/yr. It may be upto 1000 cm. Rain occurs through major parts of the year.

Matching rainfall patterns to vegetation: Moderate/heavy year-round supports evergreen forests (3); 7-8 months supports deciduous forests (4); winter rain supports grasslands (2); summer rain supports scrub (1).

Halophytes are specialized plants that naturally thrive in saline environments, such as salt marshes and mangrove swamps, where high salt concentrations prevail.

• Forest is also called the Lung of Earth because they absorb a large amount of carbon dioxide from the atmosphere and release oxygen into the atmosphere.
• The Amazon rainforest functions as a giant air machine that absorbs a large amount of carbon dioxide, and produces oxygen. More than 20 per cent of the world oxygen is produced in the Amazon Rainforest. 
  
• Hence Forest is also called a Lung of Earth.
• So, the correct answer is 'Lung of Earth'.

The equatorial region has hot wet and rainy weather.

Rain forests include more than two-thirds of the world's plant species. These plants live in a warm humid environment. It allows enormous variation. There are so many flowering plants. Aerial plants are also found in rain forests. The humidity of rain forest encourages adaptations. Mahogany, ebony, green heart, etc. are the main trees found in rain forests.

Tropical rainforests have four layers: Emergent Layer, Canopy Layer, Understory Layer and Forest Floor each with different plants and animals adapted for life in that particular area. 

Tropical rainforests or Natural forest have four layers:

Emergent Layer: These giant trees thrust above the dense canopy layer and have huge mushroom-shaped crowns.

Canopy Layer: The broad, irregular crowns of these trees form a tight, continuous canopy 60 to 90 feet above the ground.

Understory: Receiving only 2-15% of the sunlight that falls on the canopy, the understory is a dark place. It is relatively open and contains young trees and leafy herbaceous plants that tolerate low light. Many popular house plants come from this layer.

 Forest Floor: The forest floor receives less than 2% of the sunlight and consequently, little grows here except plants adapted to very low light. 

Amazon River is the largest river of the world. The basin made by this river is known as Amazon basin. It is the largest lowland in Latin America. It has a total area of 7 million square kilometers. Amazon basin is famous for dense forest cover. More than two-third of this basin is covered by immense forests. About half of the Earth's rain forests is found in Amazon basin.

The tropics are a region of the Earth surrounding the Equator. They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at 23.5°) N and the Tropic of Capricorn in the Southern Hemisphere at 23.5°). The tropics are also referred to as the tropical zone and the torrid zone. 

In equatorial region the temperature is high throughout the year, the annual average temperature is about 27 degree calcious. Rainfall is heavy and distributed throughout the year. The annual average rainfall is about 250cm. The constantly high temperature and all the year round rainfall helps to produce the most luxuriant evergreen vegetation in the region. So the forests of equatorial region are also known as evergreen. 

In forestry and ecology, understory comprises plant life growing beneath the forest canopy without penetrating it to any great extent, but above the forest floor. Only a small percentage of light penetrates the canopy so understory vegetation is generally shade tolerant.