Any major terrestrial biome in which trees form the dominant plants and annual rainfall is high (over 200 cm). Tropical rainforest is restricted to equatorial regions, such as the Amazon basin, central west Africa, and Southeast Asia. It is dominated by broadleaved evergreens and shows a very rich species diversity (see biodiversity). The leafy crowns of the trees typically form three layers of canopy, since the trees grow to different heights, which prevents much sunlight from reaching ground level. This limits the number of herbaceous plants and small shrubs that grow on the forest floor, but epiphytes, vines, and creepers are abundant. The average temperature is about 27°C, which—together with the high humidity—encourages rapid decomposition of leaf litter, releasing minerals that replace those leached from the soil by the heavy rain. If the forest canopy is removed, the soil is destroyed rapidly due to leaching by rain. The soil of a rainforest, known as latosol, is acidic and typically red, due to the oxidation of iron oxide (Fe₂O₃) in the topsoil. Rainforests are thought to contain many undiscovered plant species that could be of benefit in the fields of medicine and biotechnology. The continued destruction of rainforest in many parts of the world, particularly in South America and Southeast Asia (see deforestation), will not only result in the loss of these and other species but also contributes to the greenhouse effect and climate change.