Tropical Rainforests
Characteristics
Tropical rainforests are found in a band along the equator, between the Tropics of Cancer and Capricorn (roughly 23.5°N to 23.5°S). Major areas include the Amazon Basin (South America), Congo Basin (Central Africa), and Southeast Asia (Indonesia, Malaysia).
Climate: Hot and wet all year round — average temperature 25–28°C with very little seasonal variation. Rainfall exceeds 2,000 mm per year, often over 3,000 mm, with convectional rainfall almost daily.
Biodiversity: Tropical rainforests cover only ~6% of the Earth's land surface but contain over 50% of all plant and animal species.
Structure of the Rainforest
| Layer | Height | Features |
|---|---|---|
| Emergent | 40–50 m | Tallest trees; exposed to wind and direct sunlight; home to eagles, butterflies, bats |
| Canopy | 25–35 m | Continuous dense layer; most biodiversity found here; monkeys, birds, insects, tree frogs |
| Under-canopy | 10–20 m | Younger trees waiting for a gap in the canopy; climbing plants (lianas) |
| Shrub layer | Up to 10 m | Shade-tolerant plants; ferns, mosses |
| Forest floor | Ground level | Very dark (~2% of sunlight reaches here); thin litter layer; decomposition extremely rapid |
Plant Adaptations
- Buttress roots — wide, flaring roots at the base of tall trees for stability in shallow soil
- Drip tips — pointed leaf tips that channel water off quickly to prevent rot and fungal growth
- Lianas — woody climbing plants that use trees for support to reach sunlight
- Epiphytes — plants that grow on other plants (not parasites) to access light (e.g. orchids, bromeliads)
- Thin, smooth bark — no need for protection against cold; helps water run off
Animal Adaptations
- Camouflage — many species blend with foliage to hide from predators (e.g. leaf-tailed gecko)
- Loud calls — dense vegetation makes visual communication difficult, so many animals are very vocal (e.g. howler monkeys can be heard 5 km away)
- Prehensile tails — for gripping branches (e.g. spider monkeys)
- Bright colours — warning predators of poison (e.g. poison dart frogs)
- Nocturnal behaviour — many animals are active at night to avoid daytime heat and competition
Nutrient Cycle
The rainforest nutrient cycle is rapid and efficient:
- The biomass store is very large (massive trees and dense vegetation)
- The litter store is very small — fallen leaves decompose within 6 weeks in the warm, humid conditions
- The soil store is surprisingly small — nutrients are quickly taken up by plant roots
- Most nutrients are locked in the living biomass, not the soil
- If the forest is cleared, the nutrient cycle breaks down — the thin, infertile soils are exposed to heavy rain, causing leaching and erosion
Case Study: The Amazon Rainforest
The Amazon is the world's largest tropical rainforest, covering approximately 5.5 million km² across nine countries (mainly Brazil).
Causes of Deforestation
- Cattle ranching — the single largest cause; Brazil is the world's largest beef exporter; ~80% of deforested Amazon land is used for cattle
- Soya farming — Brazil is a major soya producer; large areas cleared for plantations
- Logging — both legal and illegal; hardwoods (mahogany, teak) are highly valuable
- Mining — gold, iron ore, bauxite (e.g. Carajás mine — world's largest iron ore mine)
- Road building — Trans-Amazonian Highway opened up vast areas to settlement and development
- Hydroelectric power — dams flood large areas (e.g. Belo Monte dam flooded ~500 km²)
- Population pressure — migration into the Amazon for farming land
Impacts of Deforestation
- Loss of biodiversity — habitat destruction threatens thousands of species
- Contribution to climate change — trees store carbon; deforestation releases CO₂ (the Amazon stores an estimated 150–200 billion tonnes of carbon)
- Soil erosion — without tree roots and canopy protection, heavy rain washes away topsoil
- Disruption of water cycle — fewer trees means less transpiration, reducing local rainfall
- Loss of indigenous communities — an estimated 1 million indigenous people depend on the forest
- Economic development — generates income, jobs, and resources for Brazil's economy
Sustainable Management
- Selective logging — only felling mature trees; allowing the forest to regenerate
- Agroforestry — growing crops alongside trees rather than clearing forest
- Ecotourism — small-scale tourism that funds conservation (e.g. lodges in Manaus region)
- International agreements — REDD+ pays developing countries to reduce deforestation
- Protected areas and national parks — e.g. Tumucumaque National Park (38,000 km²)
- Education of local communities — promoting sustainable farming techniques
- Rubber tapping and Brazil nut harvesting — non-destructive ways to earn income from the forest