Tropical Rainforests: Climate, Structure and Biodiversity

About 27°C (any figure in the range 26-28°C is acceptable).

Tropical rainforests are found in a belt within about 10° north and south of the Equator (1). The largest areas are the Amazon Basin in South America, the Congo Basin in central Africa and South-East Asia / Indonesia (1).

Temperatures are hot and constant: a mean of about 27°C with a very small annual range of only a few degrees (1 + 1 for the development that the range is small). Rainfall is very high, typically over 2,000mm a year, and is spread fairly evenly through the year with heavy convectional downpours most afternoons (1).

Many rainforest animals are adapted to live in the trees, where most food and light are found. Monkeys and sloths have long, strong limbs and gripping hands or claws, and some have prehensile tails, so they can climb and move through the canopy to reach fruit and leaves (2). Camouflage is also important: many frogs, insects and snakes are coloured green or patterned to blend into the dense vegetation, helping them hide from predators or ambush prey (1). Bright colours are used as a warning of poison, as in poison-dart frogs (1).

Rainforest plants are adapted to two main challenges: very heavy rainfall and intense competition for light.

To cope with the rain, leaves have drip tips — pointed ends that channel water off the leaf quickly. This stops water pooling on the surface, which would otherwise allow algae and fungi to grow and block sunlight, and many leaves also have a smooth, waxy coating for the same reason.

To stay upright, the tall trees grow huge buttress roots that flare out from the base of the trunk. Because rainforest soils are thin and the nutrients are concentrated near the surface, the roots are shallow, so these buttresses spread the weight and stop the very tall, top-heavy trees from toppling over.

To win light, plants race upwards. Lianas are woody climbers that use the trunks of established trees as a ladder to reach the canopy, and epiphytes grow directly on high branches, collecting water and nutrients from the air and rain rather than the dark forest floor. Both reach the sunlight without spending energy building a thick trunk of their own.

The hot, wet, evenly-warm equatorial climate is the foundation of the rainforest's layered structure, but it works together with the competition it creates and with the soils, so it is the main rather than the only reason.

The climate drives the structure directly. Year-round high temperatures (~27°C) and very heavy rainfall (2,000mm+) with no real dry season mean there is no cold or drought to halt growth, so plants grow continuously and very tall. Abundant sunlight at the top is the prize, so trees grow upward to reach it. This produces the vertical layering: a few giant emergent trees breaking through into full sun, a continuous canopy at 30-45m capturing most of the light, and a darkened under-canopy and shrub/ground floor below, where only the small percentage of light that filters through can support shade-tolerant plants. The light gradient created by climate-driven growth is essentially what defines the four layers.

The climate also explains the structure indirectly through the nutrient cycle. Heat and humidity make decomposition extremely fast and heavy rain causes leaching, so the soil stays thin and infertile; nutrients are held in the biomass. Thin soils are why trees evolved shallow roots and buttress supports, which is part of the forest's physical structure.

However, climate is not the whole story. The structure is just as much a response to competition for light between the plants themselves — a biotic interrelationship — and to the adaptations (drip tips, lianas, epiphytes) that let plants exploit each layer. The animals that occupy and connect the layers also shape the ecosystem.

Overall, I largely agree: the equatorial climate is the main reason, because without its year-round heat, rain and light there would be no continuous tall growth and therefore no layering. But the precise structure emerges from the climate acting through competition, adaptation and poor soils, so climate is best seen as the essential driver working with biotic factors rather than acting alone.