What Controls the Location of Rainforests?
Even though rainforests only cover 2% of Earth's land surface, they hold almost half of Earth's terrestrial life. These humid, green environments are one of Earth's most important land ecosystems, containing many rare and wonderful plants and animals. How do rainforests form, and what makes these environments so unique? What role do rainforests play in Earth's ecology, and why are they disappearing? Much of the answer lies in regional climate patterns in their location.
What is a Rainforest?
Rainforests are defined as forests with high annual rainfall, no freezing temperatures, and a large amount of species diversity. There are two types of rainforests: tropical rainforests and temperate rainforests.
Tropical rainforests are the most abundant type of rainforest, an environment that stays warm and humid year-round and supports hundreds of thousands of different species. These rainforests lie along the equator or are within the tropical zone.
Temperate rainforests are rainforests that appear in the temperate zone, and are limited to coastal areas with high rainfall. These rainforests can experience cool winters, so they tend to have lower plant density and less diverse vegetation and animals.
What are the Different Layers of a Rainforest?
Tropical rainforests typically have 4 layers:
- The emergent layer is the topmost layer of the rainforest, enjoying the greatest amount of sunlight but also enduring high temperatures, low humidity, and strong winds. The tallest trees tower above the dense canopy layer and have large mushroom-shaped crowns that fan out over the top of the trees below.
- The canopy is the most densely populated layer of the rainforest, home to 90% of the organisms found in the rain forest. The treetops are broad and irregularly shaped, and form a tightly knit continuous layer of greenery about 55-95 feet above the forest floor. The branches are further knit together by vines and overgrown with other plants and mosses.
- The understory is a dark region that receives only 2-15% of the sunlight that falls on the canopy. It contains young trees and short, broad-leafed plants that tolerate low light, and has more open space than the dense canopy. Many popular house plants come from this layer. Growth can only become thick and impenetrable along rivers and roadways and in areas with fallen or cut trees where sunlight is sufficient.
- The forest floor receives less than 2% of the sunlight, so the only plants that grow here are plants that thrive in low light. The forest floor has dark organic soil covered by a thin layer of organic material such as fallen leaves and branches. This organic material decomposes quickly due to the warm and moist environment, and the soil is highly leached and contains few nutrients due to the high amount of rainfall in the area.
The Layers of a Rainforest
Why Do Rainforests Occur Near the Tropics or the Ocean?
To understand where rainforests form, we must understand Earth's seasonal climate patterns. Most rainforests lie in the tropical zone 0 to 30 degrees from the equator, and are concentrated along the Intertropical Convergence Zone (ITCZ). The intertropical convergence zone is a meandering line along the equator where the northeast and southeast trade winds converge. It swings north and south seasonally and shifts large amounts of rainfall and low air pressure to different regions in the tropical zone. These low-pressure regions have warm water-rich air that rises, causing frequent rain and lush plant growth. The ITCZ is responsible for monsoons, major wind systems that seasonally reverse direction and create a wet season and a dry season, in areas like India and Southeast Asia where the line meanders a lot.
Some rainforests can also be caused by prevailing winds blowing over warm ocean currents, such as the rainforests of Australia's eastern coast. The warm water allows warm moist air to rise and form rainclouds, which then get blown over land. In other places like the Pacific Northwest, curving jet streams create low-pressure systems of air that intensify over water and send large storms into the coast. In the case of the Pacific Northwest, the high elevations cause warm air to rise and create thick clouds and steady rainfall, making the region even rainier.
A Map of the ITCZ
What Role Do Rainforests Play in Ecology and the Environment?
Rainforests are one of Earth's most critical ecosystems, with a very important role in the circle of life. Rainforests receive extreme rainfall, with a typical rainforest receiving 150-400 centimeters of rainfall per year. This water infiltrates the ground and leaches nutrients from the soil. These nutrients are quickly consumed by plants and microbes, feeding animals and insects in turn, who then die, rapidly decompose, and return nutrients back to the soil. Over 5 million species of plants and animals participate in this cycle, making the rainforest a genetic storehouse for the world's ecology.
Environmentally, rainforests are very good for the planet. They intercept and use solar energy that would otherwise hit the ground, keeping the ground below them cooler and sheltered during the day. The large trees provide shade for the life below, and are responsible for about 30% of all photosynthesis on Earth, decreasing the amount of carbon dioxide in the world enough to make it livable for humans. In addition, if carbon dioxide levels in the atmosphere increase because of human activity or natural carbon dioxide producing events such as volcanic eruptions, rainforests can increase their carbon dioxide intake to match. They are an excellent buffer against climate change, and are essential to our presence on this planet.
Why Are Earth's Rainforests Disappearing?
Rainforests are being destroyed constantly due to deforestation, at about 2.5 acres per second, or 80 million acres per year. Some ecologists project that efforts to conserve the rainforest will not be enough to make up for this rate of deforestation in the long run, with most rainforests being destroyed by 2040. The main cause of this is economic pressure. Many countries in these tropical regions are historically poor, and are now trying to develop and catch up with more wealthy nations.
Commercial logging consumes large areas of rainforest to harvest tropical hardwoods like mahogany. Logging often involves clear-cutting, where all of the trees are removed. Tracts of rainforest are also cut down and exploited for natural resources like copper, gold and oil in areas like Africa and Indonesia.
Highway construction and road building not only cuts down areas of rainforest, but provides access for other types of development, leading to more loss of rainforest. Dams flood forested areas by their reservoirs and can dry out other areas by withholding water from rivers downstream.
Farming may be the most damaging human practice in the rainforest, as cattle ranchers clear out the trees and plant pasture grass for their cows, which allows the soil to erode and makes it harder to reestablish indigenous plant life. Slash and burn practices used by subsistence farmers devastate not only the plants, but the animals that live in the rainforests, and can create forest fires that spread as far as the trees will take them. The charred plant material left behind can impact the land years down the line, and can end up all the way in the ocean, harming marine life, as seen in Brazil's Atlantic Forest. Even though slash and burn agriculture was banned in the area over 40 years ago, the effects are still being felt in the environment.
- “Climate, Weather, and Their Influences on Geology.” Exploring Geology, by Stephen J. Reynolds et al., McGraw-Hill Education, 2019, pp. 378–379
- The Intertropical Convergence Zone
The Intertropical Convergence Zone, or ITCZ, is the region that circles the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together.
- Why Is the Pacific Northwest So Rainy? | Mental Floss
The Pacific Northwest is rainy because of its high elevation, mountainous terrain, and proximity to the ocean and the jet stream's strong winds.
- Tropical Rainforest Layers
This page describes the four layers of the tropical rainforest.