Tropical Rainforests of Madagascar
A rainforest is a tropical biome characterized by regular rainfall (exceeding 100 inches -- 250 cm -- per year), and a high canopy consisting of a dense ceiling of leaves and tree branches formed by closely spaced evergreen trees.
An estimated 70-90% of life in the rainforest exists in the trees, above the shaded forest floor. Primary tropical rainforest is vertically divided into at least five layers: the overstory, the canopy, the understory, the shrub layer, and the forest floor. Each layer has its own unique plant and animal species interacting with the ecosystem around them. The overstory refers to the crowns of emergent trees which soar 20-100 feet above the rest of the canopy. The canopy is the dense ceiling of closely spaced trees and their branches, while the understory is the term for more widely spaced, smaller tree species and juvenile individuals that form a broken layer below the canopy. The shrub layer is characterized by shrubby species and juvenile trees that grow only 5-20 feet off the forest floor. The forest floor is the ground layer of the forest made up of the trunks of trees, fungus, and low growing vegetation. These layers are not always distinct and can vary from forest to forest, but serve as good model of the vegetative and mechanical structures of the forest.
The overstory is characterized by scattered emergent trees that tower above the rest of the canopy, the tops of some species exceeding 210 feet (65 m). Below the overstory trees, the canopy stretches for vast distances, seemingly unbroken when observed from an airplane above. However, despite overlapping tree branches, canopy trees rarely interlock or even touch. Instead they are separated from one another by a few feet. Why the branches of these trees do not touch is still a mystery, but it is thought that it might serve as protection from infestations from tree-eating caterpillars and tree diseases like leaf blight. To survive, canopy dwellers must have the ability to negotiate these gaps by climbing, leaping, gliding, or flying.
The billions of leaves of the canopy, acting as miniature solar panels, provide the source of power for the forest by converting sunlight to energy through photosynthesis. Photosynthesis is the process by which plants convert atmospheric carbon dioxide and water into oxygen and simple sugars. Since the rate of photosynthesis of canopy trees is so high, these plants have a higher yield of fruits, seeds, flowers, and leaves which attract and support a wide diversity of animal life. Besides attracting a broad array of wildlife, the canopy plays an important role in regulating regional and global climate because it is the principle site of the interchange of heat, water vapor, and atmospheric gases. In addition to collecting solar energy and regulating the climate, the canopy shields the understory from the harsh, intense sunlight, drying winds, and heavy rainfall, and retains the moisture of the forest below. Thus the forest interior is a much less volatile environment than the upper parts of the canopy ceiling. The interior region is protected from the extremes of the canopy: temperature fluctuations, damaging solar radiation, and strong winds. Light levels are diffuse and subdued, the humidity is higher and more constant, and there is very little direct sunlight in the lower canopy.
The Forest Floor
The forest floor of primary tropical rainforest is rarely the thick, tangled jungle of movies and adventure stories. It is actually quite the opposite: the floor is relatively clear of vegetation due to the deep darkness created by perhaps 100 feet (30 m) of canopy vegetation above. The canopy not only blocks out sunlight, but dampers wind and rain. A visitor to the rainforest during a rainstorm will usually not immediately feel falling rain because so much is deflected and collected by various canopy plants. The blocking of wind by the canopy makes the forest floor a calm place where only the slightest breeze blows during tropical thunderstorms. When hiking in primary tropical rainforest a flashlight may be more useful than a machete since the subdued lighting limits ground growth. Instead of choking vegetation, a visitor will find large tree trunks, interspersed hanging vines and lianas, and countless seedlings and saplings and a relatively small number of ground plants.
Despite its constant shade, the ground floor of the rainforest is the site for important interactions and complex relationships. The forest floor is one of the principle sites of decomposition, a process paramount for the continuance of the forest as a whole. It is also home to thousands of plants and animals, and provides support for trees responsible for the formation of the canopy. The ground level is the region of the forest which was first explored and has been the most intensively studied.
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