Tropical Rainforests and Tropical Seasonal Forests

Tropical rainforests are hot, wet, evergreen forests found near the equator that receive more than about 2,000 mm of rain per year and have the greatest biodiversity of any land biome, while tropical seasonal forests grow in the tropics where the year has distinct wet and dry seasons and many trees shed their leaves in the dry season. In brief: rainforests have average temperatures around 25–28 °C all year, four vertical layers (emergent, canopy, understory and forest floor) and fast nutrient cycling in thin, poor soils. The largest rainforests are in the Amazon basin, the Congo basin and Southeast Asia.

Tropical forests regulate climate, store large amounts of carbon and shelter a large share of the world’s species. This guide explains the climate and structure of rainforests, how plants and animals adapt, what makes seasonal forests different, compares both in a table, works through calculations, outlines the history of exploration and corrects common mistakes.

What is the climate of tropical rainforests?

Rainforests lie roughly between 10° north and south of the equator. The Sun is high all year, so there is little seasonal change in temperature, but the difference between day and night can exceed the difference between months. Moist air rises, cools and condenses, so heavy convectional rain falls, often in afternoon storms. Humidity stays high, usually above 80%. Much of the moisture is recycled: trees release water vapor by transpiration, which forms clouds and falls again as rain.

What are the layers of a rainforest?

• Emergent layer: a few giant trees rise 40 to 60 m or more above the canopy, exposed to sun and wind.
• Canopy: a dense roof of branches at about 25 to 40 m that contains most of the leaves and animal life, such as monkeys, birds and insects.
• Understory: a dim layer of young trees, shrubs and large-leaved plants adapted to little light.
• Forest floor: dark and humid, with only a small fraction (often about 1–2%) of the sunlight reaching it, covered with litter that decays quickly.

Because competition for light is intense, trees grow tall and fast, and gaps made by fallen trees allow new growth.

How do plants and animals adapt?

Many trees have buttress roots that support them in shallow soil, and leaves with drip tips and waxy surfaces that shed water quickly. Epiphytes such as orchids and bromeliads grow on branches to reach the light, and lianas (woody vines) climb up trunks. Animals show adaptations such as the prehensile tails of monkeys, bright colors in poison dart frogs, camouflage in insects and sloths, and strong beaks in toucans. The number of species is enormous: a single hectare can contain hundreds of tree species in some forests, though most are rare and individuals are far apart.

How do nutrients and soils work?

Rainforest soils are often deep but poor in nutrients. Heavy rain washes out (leaches) minerals, leaving red soils rich in iron and aluminum oxides. Nearly all the nutrients are held in the living plants and the litter. Warmth and moisture mean that dead leaves are decomposed within weeks, and the nutrients are taken up by shallow roots and fungi at once. This is a closed, fast cycle. When the forest is cleared and burned, the nutrients in the ash support crops for only a few years before the soil is exhausted and washed away, which is why cleared forest land is often abandoned.

What are tropical seasonal forests?

Tropical seasonal forests (also called monsoon or tropical deciduous forests) occur farther from the equator, in parts of India, Southeast Asia, Africa, northern Australia and South America. They have a wet season, which may last several months, and a dry season, when trees such as teak shed their leaves to save water. The canopy is lower and more open than in rainforest, with fewer layers, more light on the ground and a denser understory. They have lower biodiversity than rainforests, but still large numbers of species. Seasonal forests are often cleared more easily because dry-season fires and logging are easier, so they are among the most threatened tropical ecosystems.

Rainforest and seasonal forest compared

Worked example: light, rain and carbon

1. Light on the floor: if 1,000 units of sunlight fall on the canopy and 2% reach the ground, the floor receives 1,000 × 0.02 = 20 units, a fiftieth of the amount.
2. Rainfall: a rainforest with 2,600 mm of rain per year has an average of 2,600 ÷ 365 = about 7.1 mm per day. One millimeter of rain over 1 m² is 1 liter, so 1 hectare (10,000 m²) receives 2,600 × 10,000 = 26,000,000 liters, or 26,000 m³, per year.
3. Recycling of water: if trees return 50% of the rain to the air by transpiration and evaporation, 26,000 × 0.5 = 13,000 m³ per hectare per year goes back into the atmosphere.
4. Carbon (hypothetical example): if a hectare of forest stores 150 tonnes of carbon in its biomass, clearing 1,000 ha releases up to 150 × 1,000 = 150,000 tonnes of carbon, or about 150,000 × 3.67 = 550,000 tonnes of carbon dioxide (since one tonne of carbon corresponds to 44 ÷ 12 = 3.67 tonnes of CO₂).
5. Area change: a forest of 200,000 ha that loses 1.5% a year loses 3,000 ha in the first year. After 10 years, at a constant 1.5%, 200,000 × 0.985¹⁰ = about 172,000 ha remain, a loss of about 14%.

How did we come to know these forests?

European naturalists such as Alexander von Humboldt and Alfred Russel Wallace explored tropical forests in the 19th century, collecting thousands of species that helped develop the theory of evolution. In the 20th century, scientists discovered how the nutrient cycle works in thin soils and the importance of the canopy, which was first reached by ropes, towers and walkways. Satellite images since the 1970s have tracked forest loss. Today researchers study how rainforests influence rainfall and global carbon, while many species remain undescribed.

Which mistakes are common?

Mistake 1: Thinking that rainforest soil is rich

The trees are lush, but the soil is poor. The nutrients are stored mainly in the biomass.

Mistake 2: Believing that the forest floor is a dense jungle

The undergrowth is usually thin because little light reaches the ground. A dense tangle appears mainly along rivers and in gaps.

Mistake 3: Assuming that all tropical forests are the same

Seasonal forests have a dry season, shed their leaves and have a more open canopy than rainforests.

Mistake 4: Treating the rainforest as the “lungs of the world”

Mature forests use most of the oxygen they produce in respiration and decay. Their key roles are carbon storage, water cycling and biodiversity.