Tropical Rainforest
Location
Tropical rainforests are found in a belt either side of the equator (10° N–10° S), where conditions of high temperature and high rainfall are sustained year-round. The three major blocks are:
- Amazon Basin (South America) — the world's largest, ~5.5 million km², spanning Brazil, Peru, Colombia.
- Congo Basin (Central Africa) — the second largest, ~2 million km².
- South-East Asia (Indonesia, Malaysia, Borneo, New Guinea) — older, on islands.
Climate
- Temperature: consistently 25–28°C year-round with very low diurnal range (no seasons).
- Rainfall: 2,000–3,000 mm annually, often >100 mm every month — convectional rainfall driven by ITCZ uplift.
- Humidity: ~80%; mornings clear, afternoons cloudy with thunderstorms.
Structure (stratification)
Rainforests are layered:
- Emergent layer (40–60 m): tallest trees (kapok, Brazil nut) above the main canopy.
- Canopy (30–40 m): continuous interlocking layer where ~90% of biodiversity lives; absorbs most light.
- Under-canopy (10–30 m): smaller trees and saplings competing for light through gaps.
- Shrub layer (1–5 m): ferns, palms; very low light.
- Forest floor: dim, sparsely vegetated, leaf litter rapidly decomposing.
Nutrient cycling
Tropical rainforest soils are surprisingly poor — most nutrients are stored in the biomass (the trees themselves), not the soil. The cycle:
- Dead leaves and animals fall to the forest floor.
- Warm, wet conditions cause rapid decomposition by bacteria and fungi.
- Nutrients are quickly absorbed by tree roots before rain leaches them away.
- This is why deforestation is so damaging — once trees are removed, the nutrient store is lost and the soil cannot sustain agriculture for long (typically 2–3 years).
Soils
Latosols (oxisols): deeply weathered, red/orange (iron oxides), heavily leached, acidic, low in nutrients except in the thin upper organic layer.
Biodiversity and adaptations
Rainforests cover ~6% of land but host >50% of terrestrial species. Adaptations:
- Drip-tip leaves — pointed tips shed heavy rainfall, preventing fungal growth.
- Buttress roots — large flared bases support shallow-rooted tall trees in shallow soils.
- Lianas (woody vines) — climb tree trunks to reach light without growing thick stems.
- Epiphytes (e.g. orchids, bromeliads) — grow on branches to access canopy light.
- Camouflage and mimicry — high predator-prey diversity favours specialised camouflage (e.g. leaf insects, sloths).
Threats
Deforestation: ~17% of Amazon already lost; drivers include cattle ranching (~80% of deforestation), soy farming, logging, mining, road-building.
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