Structure Of The Tropical Rainforest

Tropical rainforests across the world are quite diverse, but share several defining characteristics including climate, precipitation, canopy structure, complex symbiotic relationships, and diversity of species. Every rainforest does not necessarily conform to these characteristics and most tropical rainforests do not have clear boundaries, but may blend with adjoining mangrove forest, moist forest, montane forest, or tropical deciduous forest. 
 

World tropical rainforest map

 

 

 

 

 

Geography & Climate 
 
Tropical rainforests lie in the "tropics," between the Tropic of Capricorn and Tropic of Cancer. In this region sunlight strikes Earth at roughly a 90-degree angle resulting in intense solar energy (solar energy diminishes as you move farther north or south). This intensity is due to the consistent day length on the equator: 12 hours a day, 365 days per year (regions away from the equator have days of varying length). This consistent sunlight provides the essential energy necessary to power the forest via photosynthesis.

Because of the ample solar energy, tropical rainforests are usually warm year round with temperatures from about 72-93F (22-34C), although forests at higher elevations, especially cloud forests, may be significantly cooler. The temperature may fluctuate during the year, but in some equatorial forests the average may vary as little as 0.5F (0.3C) throughout the year. Temperatures are generally moderated by cloud cover and high humidity.

Precipitation

An important characteristic of rainforests is apparent in their name. Rainforests lie in the intertropical convergence zone where intense solar energy produces a convection zone of rising air that loses its moisture through frequent rainstorms. Rainforests are subject to heavy rainfall, at least 80 inches (2,000 mm), and in some areas over 430 inches (10,920 mm) of rain each year. In equatorial regions, rainfall may be year round without apparent "wet" or "dry" seasons, although many forests do have seasonal rains. Even in seasonal forests, the period between rains is usually not long enough for the leaf litter to dry out completely. During the parts of the year when less rain falls, the constant cloud cover is enough to keep the air moist and prevent plants from drying out. Some neotropical rainforests rarely go a month during the year without at least 6" of rain. The stable climate, with evenly spread rainfall and warmth, allows most rainforest trees to be evergreen—keeping their leaves all year and never dropping all their leaves in any one season.  
 
Forests further from the equator, like those of Thailand, Sri Lanka, and Central America, where rainy seasons are more pronounced, can only be considered "semi-evergreen" since some species of trees may shed all of their leaves at the beginning of the dry season. Annual rainfall is spread evenly enough to allow heavy growth of broad-leafed evergreen trees, or at least semi-evergreen trees. 
 
The moisture of the rainforest from rainfall, constant cloud cover, and transpiration (water loss through leaves), creates intense local humidity. Each canopy tree transpires some 200 gallons (760 liters) of water annually, translating to roughly 20,000 gallons (76,000 L) of water transpired into the atmosphere for every acre of canopy trees. Large rainforests (and their humidity) contribute to the formation of rain clouds, and generate as much as 75 percent of their own rain. The Amazon rainforest is responsible for creating as much as 50 percent of its own precipitation.  
 
Deforestation and climate change may be affecting the water cycle in tropical rainforests. Since the mid-1990s, rainforests around the world have experienced periods of severe drought, including southeast Asia in 1997 and 2005 and the Amazon in 2005. Dry conditions, combined with degradation from logging and agricultural conversion, make forests more vulnerable to wildfire.
 

rainforest layer

 

 

 

 

 

 Rainforest Canopy

An estimated 70-90 percent 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 a 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. 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 principal 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 harsh and intense sunlight, drying winds, and heavy rainfall, and retains the moisture of the forest below. Thus the forest interior is a far 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 damps 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.

The term "jungle" is frequently applied to forest areas having dense ground growth. Such "jungle" is characteristic of disturbed forest, usually near the forest edges, in recently opened light gaps, river banks, and areas where the forest is reclaiming previously cleared land. Vigorous ground growth is only possible where plenty of light is available—where there is a break or thinning in the light-absorbing canopy which screens out all but the 0.5-5 percent of light that reaches the floor in the primary forest.

Characteristics

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 principal 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.