What is a tropical rainforest?

The rainforest biome (a major habitat type) can be defined as forest growing in regions with more than 200 cm (6.5 feet) of rainfall per year. Although there are temperate rainforests (such as that of British Columbia in Canada), tropical rainforests occur between the Tropic of Cancer and the Tropic of Capricorn (23.5o N and 23.5o S). They are found in regions where the average temperatures of the three warmest and the three coldest months do not differ by more than 5o C, although there may be daily variations of more than that. Rainfall is relatively evenly distributed, which allows the growth of a heavy canopy of broad-leaved evergreen trees; however, many of these regions have distinct dry and rainy seasons. There are multiple layers of vegetation from understory shrubs to trees of more than 40 meters (130 feet) in height. There are many epiphytes and palms. These forests are limited in extent by temperature and precipitation. The three major blocks of tropical rainforest are those of the Indo-Malayan region (South and Southeast Asia), Central Africa, and Central and South America (Neotropics). There are several general types of tropical rainforests:

Lowland evergreen tropical rainforest, which has no distinct dry season, and in which most trees retain their leaves throughout the year. These are the most luxuriant forests, with many very tall canopy (“emergent”) trees, sometimes more than 45 meters in height, often with huge buttress roots. Below that lies the main (or middle) stratum, from 24 to 36 meters in height, and an underlayer of smaller, shade-loving trees. Ground vegetation is often, but not always, sparse, contrary to the popular image -“white-hatted explorer hacks his way through the impenetrable jungle” – because so little light can filter through the upper leafy layers. Many plant climbers try to reach the light by attaching to the large canopy trees.

Seasonal tropical rainforest occurs in regions with a short dry period. Some of the trees in such forests are deciduous; they may lose their leaves at the same time or flower and/or fruit simultaneously (seasonality). Many of the plant genera are the same as in evergreen forests, although the species composition is different.

Tropical semievergreen forest occurs in regions where there is a relatively long dry season. The upper tree story is deciduous (a water-saving adaptation), while the lower stories are evergreen. In deciduous (monsoon) tropical rainforests, there is a lengthened dry season, and virtually all tree species are deciduous, so that the forest is leafless during the dry periods.

There are many subdivisions of these basic types of rainforests, such as montane (mountain) forests, peat forests, cloud forests, and so on.

Tropical rainforests frequently conform to the stereotypical notion that they are riotously profuse, abundant with thousands of plant species. Although some forests are composed mainly of one dominant tree genus or family, most contain hundreds of woody plant species (compared to less than 30 in most temperate forests). They are indeed the most structurally-complex and diverse of land ecosystems, with the greatest number of species. Among the earth’s ecosystems, they are rivaled in species diversity only by coral reefs.

An intact rainforest is virtually a “closed system” in which the essential nutrients, both organic and mineral, are cycled from the soil through the vegetation and back again. This allows luxuriant forests to grow in relatively hostile environments where the soils are poor and temperatures high. Therefore forest health is dependent upon decomposing organisms – bacteria and fungi – because, without the degradation of plant and animal materials, no new growth could occur. Since warm temperatures and high humidity favor decomposition, nutrients are rapidly made available for plant growth, and so luxuriant vegetation is characteristic of these forests. In general, little organic matter is lost from the forest, because it is taken up very rapidly from the soil by the vegetation.

Primary (“virgin” or “old-growth”) forests are those which have been relatively undisturbed by human activity (although they may have been altered in the past), and which contain trees of a wide range of ages. Secondary forests are forests which have undergone some major disturbance (fire, for instance, but more often human disturbance) and so most of the vegetation is of approximately the same age. These differ from primary forests in their species composition as well as in the relative youth of the trees, because a disturbance opens a gap in the forest, in which only certain species (“pioneer species”) can grow. For the most part, the species of the primary forest are not adapted to gap conditions, as are the pioneer species (see “Forest maintenance,” Section F).