What is a tropical
rainforest? How is it different from what we think of as a
regular forest?
First of all, a regular forest usually consists of one dominant
species of tree - the evergreen forests of colder climates, or
the redwood forest of California, for example. A tropical
rainforest has many species of trees and plants, and trees of
the same species very seldom grow close together. Scientists
have counted as many as 300 different species growing in one 2
1/2 acre (one hectare) in a South American rainforest.
While forests are found in all climates and all parts of the
world, tropical rainforests are found only in hot, humid
climates. They cover less than six percent of the Earth's land
surface. Most are found in areas near the equator such as
Central America, South America, Africa, and Southeast Asia. A
rainforest often gets more than 100 inches of rain a year, the
average humidity is between 77 and 88 percent, and the
temperature is usually between 93º F (34º C) and 68º F (20º C).
Probably the most significant difference between a regular
forest and a tropical rainforest, and the one that has
scientists worried that rainforests might someday totally
disappear, is the fact that whereas most of the nutrients of a
regular forest are stored in its vegetation, most of a
rainforest's nutrients are stored in the soil. This means that
when a regular forest is burned or flooded or cut down, the
trees and plants can eventually grow back because of the
nutrients in their roots. But in the rainforests, where
thousands of years of heavy rain have washed away many of the
nutrients in the soil, it is very difficult for vegetation to
grow back after it is burned or cut down.
Unlike a regular forest, a tropical rainforest has four very
distinct layers based on the types of trees and animals that
live there. From the top down, these layers are the emergent,
upper canopy, understory, and forest floor.
Emergent trees are 100 to 240 feet tall and are spaced wide
apart with straight, smooth trunks and leaves that form
umbrella-shaped canopies. The upper canopy consists of trees
that are 60 to 130 feet tall. Most of the animals live here.
Most of the trees in the understory, or lower canopy, are about
60 feet tall. Very little light reaches this level, the humidity
is always high, and there's little air movement.
The forest floor is almost totally shaded. As a result, there
are very few bushes or herbs, and a person can easily walk
through most parts. But you have to be careful where you walk,
because the ground is teeming with termites and other insects,
earthworms and fungi that feed on the litter that falls to the
ground. Approximately 80 percent of all insect species live in
tropical rainforests.
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