Tropical Rainforests: Life in the Hot, Wet Forest

Where rainforests are and their climate

Tropical rainforests are found in a belt roughly between the Tropic of Cancer and the Tropic of Capricorn, mainly close to the equator: the Amazon basin in South America, the Congo basin in central Africa, and South-east Asia. Smaller areas include the Western Ghats and north-east India.

• Temperature: 25–28 °C every month. The sun is high in the sky all year.
• Rainfall: usually 2,000 mm or more a year. Hot air rises in the morning, cools, forms tall clouds and gives heavy afternoon storms (convectional rain).
• Humidity: very high, often above 80%.
• Seasons: almost none, so plants grow all year.

A climate graph for a rainforest shows a nearly flat temperature line and tall rainfall bars in every month.

Structure, nutrient cycle and soils

The forest has layers:

• Emergent layer: a few giant trees up to 50 m, in full sun and wind.
• Canopy: a continuous roof of leaves about 30 m up. Most animals live here.
• Under-canopy and shrub layer: young trees and shrubs in shade.
• Forest floor: dark (only about 2% of sunlight), damp and covered in leaf litter.

Nutrient cycle: leaves fall all year, and heat plus moisture let fungi and insects break them down in weeks. Plant roots take the nutrients back quickly. So most nutrients are stored in the living plants (the biomass), and the soil (often red, called latosol) is thin and poor. Remove the trees and the nutrients are lost.

Biodiversity: rainforests cover a small part of the land but hold more than half of all land plant and animal species.

Plant and animal adaptations

Plants:

• Buttress roots: tall wall-like roots hold up huge trees in shallow soil.
• Drip tips: waxy, pointed leaves let heavy rain run off, so they do not rot or grow mould.
• Lianas: woody vines rooted in the ground that climb trees to reach the light.
• Epiphytes (orchids, ferns): grow on branches high up to get light; they take water from the air and rain.
• Thin, smooth bark: no need for protection from cold; stops climbers from gripping.

Animals:

• Sloth: moves slowly to save energy; algae in its fur act as camouflage.
• Toucan / hornbill: big beak to reach and break fruit.
• Spider monkey: long limbs and a gripping tail for life in the canopy.
• Poison-dart frog: bright colours warn predators that it is toxic.
• Flying lizards and frogs: glide between trees.

Causes and impacts of deforestation

Deforestation is the cutting down or burning of forest so the land can be used for something else.

Causes

• Commercial farming: soy, palm oil, sugar cane.
• Cattle ranching for beef (a major cause in the Amazon).
• Subsistence farming by growing populations.
• Logging for hardwoods such as mahogany and teak.
• Mining for gold, iron ore and bauxite.
• Roads, dams (hydroelectric power) and new settlements.

Impacts

• Environmental: soil erosion and landslides; rivers fill with silt; loss of biodiversity; less rain locally because fewer trees release water; carbon released when trees are burnt, adding to climate change.
• Economic: jobs and income from farming, mining and timber; money from exports; but long-term loss of forest products and tourism.
• Social: indigenous peoples lose land and their way of life; conflicts over land.

Sustainable management of rainforests

Sustainable management meets the needs of people today without harming the forest for future generations.

• Selective logging: cut only some mature trees; young trees grow on. Helicopters or animals can remove logs to avoid big roads.
• Replanting (afforestation and reforestation): plant new trees where forest was cut.
• Agroforestry: grow crops between trees, so the soil stays protected.
• Ecotourism: small-scale tourism that gives local people income for protecting the forest.
• Protected areas and laws: national parks, reserves and community forests.
• International agreements: trade rules on hardwood, and debt-for-nature swaps where part of a country's debt is cancelled in return for protecting forest.
• Certification and consumers: buying certified wood and deforestation-free products.

Try it

In the 3D free play, raise "forest cleared" step by step and watch the trees, species index and erosion risk. Decide where you would stop. At home: water two pots of soil from the same height, one covered by leaves and one bare. Which loses more soil in the tray below? That is soil erosion after deforestation.

Key formulas and definitions

• Climate: 25–28 °C all year; rainfall > 2,000 mm a year; no clear seasons
• Layers (top → bottom): emergent → canopy → under-canopy/shrub → forest floor
• Nutrients: stored mainly in biomass; soil thin and poor
• Deforestation causes: farming, cattle ranching, logging, mining, roads/dams, settlements
• Deforestation impacts: soil erosion, biodiversity loss, climate change, loss of indigenous land
• Sustainable management: selective logging, replanting, agroforestry, ecotourism, protected areas, international deals

Worked examples

Common mistakes

• Thinking rainforest soil is rich. It is thin and poor; the nutrients are in the plants.
• Mixing up the canopy (thick leaf roof) and the emergent layer (a few giant trees above it).
• Listing only environmental impacts. Exams also expect economic and social impacts, including benefits.
• Confusing lianas (rooted in the ground and climbing) with epiphytes (growing on branches).