8.2: Terrestrial Biomes - Biology LibreTexts

Tropical Rainforests

Tropical rainforests, found in equatorial regions (figure \(\PageIndex{a}\)), are the most biodiverse terrestrial biome (figure \(\PageIndex{b}\)). In the Western Hemisphere, the tropical rain forest reaches its fullest development in the jungles of Central and South America. The trees are very tall and of a great variety of species. One rarely finds two trees of the same species growing close to one another. Most of the animals — mammals and reptiles, as well as birds and insects — live in the trees.

This biodiversity is under extraordinary threat primarily through logging and deforestation for agriculture. Tropical rainforests have also been described as nature’s pharmacy because of the potential for new drugs that is largely hidden in the chemicals produced by the huge diversity of plants, animals, and other organisms. The vegetation is characterized by plants with spreading roots and broad leaves. The broad-leafed trees are mostly evergreen, with leaves that persist throughout the year, but in tropical seasonal forests (see below), some are deciduous, with leaves falling off in the dry season.

The temperature and sunlight profiles of tropical rainforests are stable in comparison to other terrestrial biomes, with average temperatures ranging from 20oC to 34oC (68oF to 93oF). Month-to-month temperatures are relatively constant in tropical rainforests, in contrast to forests farther from the equator. This lack of temperature seasonality leads to year-round plant growth rather than just seasonal growth. In contrast to other ecosystems, a consistent daily amount of sunlight (11–12 hours per day year-round) provides more solar radiation and therefore more opportunity for net primary productivity (a measure of how carbon is stored by the photosynthesis that occurs in a community).

The annual rainfall in tropical rainforests ranges from 125 to 660 cm (50–200 in). Some tropical rainforests have relatively consistent rainfall whereas others (the tropical seasonal forests) have distinct wet months, in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. Nevertheless, the driest month of a tropical rainforest can still exceed the annual rainfall of some other biomes, such as deserts.

Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer. The vegetation is so dense that little light reaches the forest floor. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short, shrubby foliage. A layer of trees rises above this understory and is topped by a closed upper canopy—the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, animals, and other organisms. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground, rarely descending to the forest floor.

Epiphytes

Epiphytes are plants that live perched on sturdier plants. They do not take nourishment from their host as parasitic plants do. Because their roots do not reach the ground, they depend on the air to bring them moisture and inorganic nutrients. Examples of epiphytes include many orchids, ferns (figure \(\PageIndex{c}\)), and bromeliads (members of the pineapple family like "Spanish moss").

Tropical Soils

While productivity is high in tropical rainforests, the soils themselves tend to be of very poor quality. Because of the high rainfall, nutrients are quickly washed out of the topsoil unless they are incorporated in the forest plants. As plant and animal debris falls to the ground, it is quickly decomposed because of the warmth and moisture there. Thus minerals are found mainly in the forest plants, not in the soil. When the plants are removed and cultivation attempted, the soils quickly lose fertility. The situation is made worse by the lack of humus. Additionally, the topsoil may be no thicker than 5 cm (~2 inches), and most of these soils have high iron and aluminum content. Once exposed to the sun, these soils quickly bake into a bricklike material that cannot be cultivated.