Tropical rainforests and savannas

A single hectare of Amazon rainforest, about two and a half acres, holds on average 100 to 300 species of tree.

A forest without seasons

Tropical wet forests, also called tropical rainforests, grow in equatorial regions. What makes them unusual is how little changes through the year. OpenStax gives temperatures of 20 °C to 34 °C, and the day stays close to 11 or 12 hours long in every month, so there are no long summer days or short winter ones. Because of that, plants can grow all year instead of only in spring, summer and autumn. The broad leaves fall and are replaced a few at a time, never all at once, so the forest stays evergreen.

Rain is the one thing that varies. Annual rainfall ranges from 125 cm to 660 cm. Wet months can bring more than 30 cm, and dry months less than 10 cm, but even the driest month of a rainforest can exceed the whole year's rain in some deserts. Warmth, light and water all at once make tropical wet forests the most productive land biome, with very high net primary productivity and more species of tree than any other biome.

Layers from floor to sky

Much of that variety comes from the forest's layers. OpenStax describes them from the bottom up. On the forest floor there is a sparse layer of plants and decaying leaves, since little light gets through. Above it is an understory of short shrubby plants. Then comes a layer of trees topped by a closed upper canopy, the roof of branches and leaves, and a few emergent trees push through even that. Each layer has its own light, moisture and food, so each holds its own set of plants, fungi and animals. Some animals spend their whole lives metres above the ground. Epiphytes, plants that grow on other plants without harming them, are common on branches high in the canopy.

Savannas: grass with a few trees

Savannas are grasslands with scattered trees, found in Africa, South America and northern Australia. They are hot, with average temperatures of about 24 °C to 29 °C in OpenStax, but they have an extensive dry season, and forest trees cannot cope with it as well as grasses and forbs, the herbaceous flowering plants. Fire is a regular disturbance. Savanna plants have deep, well-developed root systems that let them sprout again quickly after a fire has burned the parts above ground.

Savannas and rainforests sit next to each other in many places, and the boundary between them is largely about water through the year. Where the dry season is short, trees close their canopy into forest. Where it is long and fires come often, grass keeps the upper hand.

Why the tropics hold so many species

OpenStax notes that species richness is highest in ecosystems near the equator, which often have warm temperatures, large amounts of rain and little seasonality, and lowest near the poles. High productivity at the base of the food web supports many consumers above it, and the layered forest offers many different places to live. According to Global Forest Watch data reported by Mongabay in 2026, Brazil, the Democratic Republic of the Congo and Indonesia are the three countries with the most tropical forest left, which is why what happens to their forests matters for species far beyond their borders.