Structure of the tropical rainforest

RAINFOREST CANOPY STRUCTURE

Rainforests are characterized by a unique vegetative structure consisting of several vertical layers including the overstory, canopy, understory, shrub layer, and ground level.

The canopy refers to the dense ceiling of leaves and tree branches formed by closely spaced forest trees. The upper canopy is 100-130 feet above the forest floor, penetrated by scattered emergent trees, 130 feet or higher, that make up the level known as the overstory. Below the canopy ceiling are multiple leaf and branch levels known collectively as the understory. The lowest part of the understory, 5-20 feet (1.5-6 meters) above the floor, is known as the shrub layer, made up of shrubby plants and tree saplings.

The heavy vegetation of the canopy effectively screens light from the forest floor, and in a true (primary) equatorial rainforest, there is little "jungle-like" ground growth to impede movement. Ground vegetation in primary forest is minimal and usually consists mainly of lianas (vines) and tree seedlings.

An important characteristic of the canopy system is the presence of plants known as epiphytes, that grow on canopy trees. Epiphytes are not parasitic because they draw no nutrients away from the host, but use the host tree only for support. High in the canopy, epiphytes are better able to access the strong tropical sunlight, which they require for growth. Epiphytes have adapted well to their aerial environment, developing various means to collect nutrients from their surroundings, the mechanisms for which are discussed in detail in the canopy section.

An additional plant type characteristic of the canopy system is the liana—a sort of woody vine that begins life as a shrub on the forest floor and makes its way up to the canopy by latching on to canopy trees. A related plant type, the hemiepiphyte, begins life in the canopy and grows long roots that eventually reach the forest floor. Once rooted, hemiepiphytes do not have to rely on capturing nutrients from their canopy surroundings, but can access nutrients from the forest floor.

Unknown numbers of plants and animals reside in the canopy, the vast majority of which are specifically adapted to life in this leafy world. In tropical rainforests, it is estimated that most of the species that exist in the ecosystem reside in the canopy. Since the tropical rainforests are estimated to hold 50 percent of the planet's terrestrial species, the canopy of rainforests worldwide a substantial proportion of life on Earth.

Review questions:

• Most of the plant and animal species live in what level of the rainforest?
• What are epiphytes?
• What is an example of an epiphyte? (Hint: think of a popular kind of flower)
• What are lianas?

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Continued / Next: Rainforest Symbiotic Relationships