5.4.3: Tropical Rainforest

Learning Objective

• Recognize distinguishing characteristics of tropical rainforests & plant adaptations of the biome.

Also referred to as tropical wet forest, this biome is found in equatorial regions. Tropical rainforests are the most diverse terrestrial biome. This biodiversity is still largely unknown to science and 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 that fall off throughout the year, unlike the trees of deciduous forests that lose their leaves in one season. These forests are “evergreen,” year-round, meaning they retain they leaves throughout the year.

The temperature and sunlight profiles of tropical rainforests are stable in comparison to that of other terrestrial biomes, with average temperatures ranging from 20^oC to 34^oC (68^oF to 93^oF). Month-to-month temperatures are relatively constant in tropical rainforests, in contrast to forests further from the equator. This lack of temperature seasonality leads to year-round plant growth, rather than the seasonal growth seen in other biomes. In contrast to other ecosystems, a more constant daily amount of sunlight (11–12 hours per day) provides more solar radiation, thereby a longer period of time for plant growth.

The annual rainfall in tropical rainforests ranges from 250 cm to more than 450 cm (8.2–14.8 ft) with considerable seasonal variation. Tropical rainforests have 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. However, the driest month of a tropical rainforest can still exceed the annual rainfall of some other biomes, such as deserts.

Tropical rainforests have high net primary productivity because the annual temperatures and precipitation values support rapid plant growth (Figure \(\PageIndex{1}\)) . However, the high rainfall quickly leaches nutrients from the soils of these forests, which are typically low in nutrients. Any nutrients that reach the soil (fallen leaves, tree branches, or dead animals) quickly decompose and are used by plants as raw material. Thus, the nutrients are always above ground, and not stored in the soil.

Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer. 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 within the tropical wet forests. 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 ever descending to the forest floor.

Rainforests are not the only forest biome in the tropics; there are also tropical dry forests, which are characterized by a dry season of varying lengths. These forests commonly experience leaf loss during the dry season to one degree or another. The loss of leaves from taller trees during the dry season opens up the canopy and allows sunlight to the forest floor that allows the growth of thick ground-level brush, which is absent in tropical rainforests. Extensive tropical dry forests occur in Africa (including Madagascar), India, southern Mexico, and South America.

Figure \(\PageIndex{1}\): Species diversity is very high in tropical wet forests, such as these forests of Madre de Dios, Peru, near the Amazon River. (credit: Roosevelt Garcia)

Adaptations

Plants living in tropical rainforests have many unique adaptations. For example, due to the poor nutrient soil, they cannot have deep roots. They withstand many rain events and compete with other plants for sunlight, causing them to sometimes grow at an angle. Due to all these restrictions, trees often have buttresses, which are large aerial extensions of the lateral surface roots, to help stabilize the tree. Another common adaptation are epiphytes. These are plants that live on the surface of other plants, using moisture and nutrients from the air or rain. They grow on plants instead of the shady forest floor, where they cannot obtain enough sunlight. Epiphytes do not have any attachment to the ground and are not parasitic on the plant. Orchids, bromeliads, and mosses are common epiphytes. Some plants have leaves with drip tips, pointy tips that help remove water off the leaves quickly to reduce the cumulating of fungi and bacteria. It also helps protect the leaves from breakage during heavy rains.

Attributions

Curated and authored by by Kammy Algiers using Terrestrial Biomes from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org.