Tropical Rainforests: Structure, Climate and Biodiversity
TRFs are the most biodiverse biome on Earth — driven by year-round warmth, moisture, and complex vertical structure.
Location and climate:
- Found within approximately 10° of the equator (Amazon Basin — South America; Congo Basin — central Africa; South-East Asian islands — Borneo, Sumatra, New Guinea)
- Rainfall: >2000 mm per year, distributed throughout the year (no true dry season). High rainfall driven by the ITCZ (Inter-Tropical Convergence Zone) where trade winds from both hemispheres converge
- Temperature: constant 25–30°C year-round — little seasonal variation (< 2°C range)
- Humidity: typically 80–90% relative humidity — maintained by transpiration from the dense vegetation
Vertical structure (4 layers):
- Emergent layer (40–70 m): tallest individual trees project above the canopy; exposed to full sunlight and strong winds; home to eagles, harpy eagles, large parrots and monkeys
- Canopy (30–40 m): dense, overlapping tree crowns form a continuous roof; intercepts ~80% of incoming light; most animal species live here (birds, primates, arboreal mammals, vast insect diversity)
- Understory (5–20 m): shorter trees and tall shrubs growing in deep shade; broad, flat leaves maximise light capture; epiphytes (orchids, bromeliads, ferns) grow on branches
- Forest floor: receives <2% of surface light; dominated by roots, leaf litter, fungi, and decomposers; few green plants (some ferns); nutrients released by decomposition immediately re-absorbed by roots
Why TRFs have high biodiversity (50% of all species, <6% of land):
- Year-round growing conditions: constant warmth and moisture mean no harsh season kills off sensitive species; plants and animals can reproduce year-round → more species can coexist
- High productivity: abundant sunlight + rainfall → enormous plant biomass → food and habitat for millions of species
- Complex vertical structure: 4 distinct layers create many different ecological niches — different heights, light intensities, humidity levels, and food sources support different specialist communities. One hectare of Amazon rainforest may contain more beetle species than all of Britain.
- Long evolutionary history: TRFs have existed for ~100 million years without major glaciation disruption → extraordinarily long time for speciation and community assembly
- Geographic isolation promotes speciation: rivers, mountain ridges, and canopy gaps isolate populations → genetic divergence → new species
Nutrient cycling — tight and rapid: TRF soils (laterite/oxisols) are thin and nutrient-poor:
- Warm, humid conditions → extremely rapid decomposition by bacteria and fungi
- Nutrients released are immediately re-absorbed by shallow tree roots
- Heavy tropical rainfall quickly leaches free nutrients from soil
- ~90% of nutrients are stored in living biomass (trees, plants), NOT in the soil
- Consequence: when forest is cleared, the nutrient cycle breaks down completely — thin soils become infertile within 2–5 years of agricultural use
- TRF: 0–10° latitude; >2000 mm/yr; 25–30°C constant; 4 layers: emergent, canopy, understory, forest floor.
- 50% of all species in <6% of land: year-round warmth/moisture, high productivity, complex structure (4 layers = many niches), long evolutionary history.
- Nutrient-poor laterite soils: nutrients in biomass not soil; rapid decomposition + immediate re-absorption + rain leaching.