Rainforests: Areas of Extreme Precipitation and Biodiversity

Amazon Rainforest
The Rio Negro tributary of the Amazon river and the Amazon rainforest at dawn. Jose de Paula Machado/Getty Images

A rainforest is a forest differentiated by high levels of precipitation - normally a minimum of at least 68-78 inches (172-198 cm) annually. Rainforests tend to have fairly mild and/or warm climates and feature the highest levels of biodiversity in the world. Additionally, tropical rainforests are considered the "lungs of the Earth" because of the high amount of photosynthesis occurring in them.

Locations and Types of Rainforests

Temperate rainforests have mild climates with cool, wet winters. Temperatures range from 41°F-68°F (5°C-20°C). Some temperate rainforests have dry summers while others have wet but those in areas with dry summers (e.g. California's coastal redwoods) have significant summer fog which keeps condensation and moisture in the forests.

The second and most widespread type of rainforest is a tropical rainforest. These occur in equatorial regions near 25 degrees north and south latitude. The majority are found in Central and South America, but tropical rainforests also exist in Southeast Asia, eastern Australia, and central Africa (map of locations). The largest in tact tropical rainforest in the world is in the Amazon River Basin.

Tropical rainforests form in these locations because they are within the ITCZ, which provides the warm temperatures common in the forests. Due to the temperatures and plant growth, transpiration rates are high. As a result, the plants release water vapor which condenses and falls as precipitation. On average, a tropical rainforest is about 80°F (26°C) and has little daily or seasonal variation in temperature. In addition, tropical rainforests have an average of 100 inches (254 cm) of precipitation annually.

Rainforest Vegetation and Structure

The next layer is the canopy layer and contains the majority of the rainforest's tallest trees. Because light is still abundant in this layer, these trees, like those in the emergent layer are adapted to intense sunlight and they too have small, brightly colored leaves. In addition, these leaves have "drip tips" that funnel rain water off of the leaf and down to the forest below.

The canopy layer is believed to be the most biodiverse of all the rainforest layers and half of the plant species in the forest are said to be here.

The next layer is the understory. This area consists of short trees, shrubs, small plants, and the trunks of canopy trees. Because less than five percent of light coming into the forest reaches the understory, the leaves of plants here are large and dark to absorb more available light. Contrary to popular belief, this area of the forest is not dense as there is not enough light to support thick vegetation.

The final rainforest layer is the forest floor. Because less than two percent of incoming light reaches this layer, very little vegetation is present and it is instead filled with decaying plant and animal matter and various forms of fungus and moss.

Rainforest Fauna

Human Impacts on the Rainforest

The most significant human impact on rainforests though is deforestation. In temperate rainforests, the trees are often cut down for building materials. In these forests in Oregon for instance, 96 percent of the forests have been logged while half of those in Canada’s British Columbia have been subjected to the same.

Tropical rainforests are also subject to deforestation but in these areas it is mainly to change the land into agricultural uses in combination with logging. Slash and burn agriculture and other clear cutting are common in many tropical rainforest areas.

As a result of human activities in rainforests, many areas have lost a significant portion of their forests and hundreds of plant and animal species are being driven to extinction. Brazil for instance has declared deforestation a national emergency. Because of species losses and the impacts climate change is having on the rainforests, countries all over the world are now setting up plans to protect the rainforest and putting this biome at the forefront of public knowledge.