Basic Information on Tropical Rainforest -2

Blogger's Note: Tropical Rainforests

Tropical Rain Forests

The following information has been reprinted with permission of the National Wildlife Federation from the "Rain Forests: Tropical Treasures" issue of NatureScope.

Two hundred years ago, tropical rain forests circled the globe in an almost unbroken green belt that encompassed Latin America, Africa, South Asia, Indonesia, and Australia and covered about 20 percent of the earth's land surface. Today, rain forests cover less than 7 percent of the earth's land surface, and the once-continuous strip is now broken up into a series of green pockets. . . . Pockets of rain forest lie both north and south of the equator, bordered on the north by the Tropic of Cancer and on the south by the Tropic of Capricorn. Although rain forests grow in more than 50 countries, more than half the total area is found in just three countries: Brazil with 33 percent and Zaire and Indonesia with 10 percent each.

What Are Tropical Rain Forests?Hot, Humid, and Wet: Tropical rain forests are characterized by hot, humid weather throughout the year. By definition, they get more than 60 inches of rain a year, although some areas regularly get more than 200 inches and a few get more than 400! Temperatures sometimes climb into the 90s but usually hover between 70∞ and 85∞ F.

In most tropical rain forests, the temperature variation during the year is small, and there is very little variance between daytime highs and nighttime lows. Consistently high temperatures are characteristic of lands that hug the equator. And unlike other regions of the world, these equatorial areas get more energy from the sun because they experience year-round, 12-hour days, and because the sun's rays strike the equator at right angles, providing more intense and direct sunlight.

Consistently high temperatures and abundant rainfall contribute to another tropical rain forest characteristic -- high humidity. In the rain forests of South America, scientists estimate that as much as 250 billion tons of water vapor can be suspended in the air at any one time. The abundant water vapor in tropical rain forests is also a result of a high rate of plant transpiration. On the average, the humidity in a forest is about 70 percent during the day and 95 percent at night.

Luxuriant and Diverse: The moist, hot conditions in tropical rain forests support an abundant diversity of plant life -- from luxuriant shrubs and ferns to climbing vines and giant trees. The plants, in turn, support an amazingly varied community of wildlife.

Although tropical rain forests cover less than 7 percent of the earthís land surface, scientists estimate that they may house more than 50 percent of all species. In fact, some scientists calculate that this figure could actually be much higher, given the number of new species that are constantly being discovered in tropical rain forests.

Shallow, Damp, and Infertile: Rain forest soils support an incredible variety of plants, but the soil itself is often not very fertile. Most of the nutrients that are used by plants are stored in the plants themselves, not in the soil. In other types of forests, such as temperate forests, the soils are often much more nutrient-rich.

The lack of nutrients in tropical rain forest soil is the result of two things: abundant rainfall and age. In all forests, rain washes away, or leaches, important nutrients from the soil -- but leaching can be an especially serious problem in rain forests because there is so much rain and it comes in such intense and frequent bursts. And in many areas where tropical rain forests grow, the soils are relatively old and have been leached for millions of years, leaving very few mineral nutrients. . . .

RAIN FOREST LAYERSThe trees, shrubs, vines, ferns, and other plants that grow in rain forests form a complex system of layers. Although the layering system varies from area to area and the boundaries between layers are often not distinct, a generalized cross section would look something like the picture on the worksheet "Tropical Rain Forests" in the section for grades 3-5. Hereís more about each layer, starting at the top and working down.

The Emergents Towering above all the other plants in the forest are the giant trees called emergents. Although most emergents are about 115 to 150 feet tall, some grow to heights of over 250 feet. There are usually only one or two of these jungle giants per acre, and characteristically they have relatively small leaves, umbrella-shaped crowns, and tall, slender trunks. Emergents stick out from the crowd and must endure high and often changing temperatures, low humidity, and strong winds. . . .

The Canopy: Like a thick green carpet, the main canopy layer of the rain forest is formed by flat-crowned trees that are often between 65 and 100 feet above the ground. Like the emergents, trees in the canopy are subjected to changing temperature and humidity. The canopy trees, along with the emergents, form a continuous covering over the forest. The canopy acts like a giant sun and rain umbrella. It catches most of the sun's rays, allowing only about 2 to 5 percent to slip through to the forest floor. The canopy also absorbs much of the impact of the rain that falls on the forest. . . . The canopy is filled with life. The umbrella of leaves and branches provides a home for many treetop creatures, as well as for orchids, vines, bromeliads, and a host of other plants.

The Understory: Below the canopy there are small trees that usually don't grow to heights of more than 15 feet or so, and a shorter shrub layer of very young canopy trees and miniature woody plants. Together, these plants make up the understory. Some of these understory plants will eventually grow tall enough to become part of the canopy. But others will always remain in the shadow of the canopy giants. . . . Many also have large leaves, which scientists think help plants absorb as much as they can in the dim understory.

The Forest Floor On the forest floor, often more than 65 feet below the canopy, the conditions are very different from those at the top. The canopy is subjected to strong sun and plenty of wind, causing considerable daily fluctuations in humidity and temperature. But on the sheltered floor, the air is very still, humidity is almost always above 70 percent, and the temperature remains relatively constant.

Although seedlings, herbs, and ferns grow on the forest floor, the vegetation is fairly sparse -- mainly because of lack of sunlight. And although many people think the rain forest floor is littered with decaying logs and thick layers of dead leaves, the floor is actually quite open.

ABOUT JUNGLESMoist tropical forests that lie close to the equator at low elevations are commonly called tropical rain forests. However, much of what we say about tropical rain forests -- especially when we talk about the problems of deforestation -- also holds true for two other types of forests found in the tropics: tropical seasonal forests and tropical cloud forests. Tropical seasonal forests occur throughout the tropics and have two distinct seasons -- a wet season and a dry season. They differ from tropical rain forests in that they don't get abundant, year-round rainfall and they experience seasonal periods of drought.

Cloud forests are tropical forests that grow at middle to high elevations. These mountainous jungles are kept moist by mist and clouds, as well as by rain. Found in mountains throughout the tropics, cloud forests are denser and have shorter trees than tropical rain forests, and ferns, mosses, and liverworts often grow more thickly on tree branches. In some areas these forests are better known as "moss forests." Unlike lowland tropical rain forests, cloud forests can become relatively cool.

Where does the word jungle fit into our tropical rain forest definitions? Etymologists have traced the word to the ancient Sanskrit word jangala, which was used to describe thick, impenetrable vegetation. Today jungle is the popular term for tropical forests in general. Although many people use the word "jungle" interchangeably with "tropical rain forest," the literal definition of the word misrepresents what a tropical rain forest is actually like.

Tropical rain forests are not impenetrable masses of vegetation. Instead, in a mature tropical forest the forest floor is fairly open and uncluttered.

People probably first used the word "jungle" to describe tropical forests when they traveled by boat to explore these areas. Along tropical riverbanks and openings in the forests, tangled "jungle" vegetation does spring up.

That's because in open areas much more sunlight reaches the ground and encourages plant growth. Many early explorers mistakenly assumed that the inside of a rain forest was just like its overgrown edges. Tangled undergrowth is not the only rain forest myth that has been perpetuated through the years. Another is that rain forests, with their thick vegetation and abundant wildlife, are "tough" and can withstand changing conditions without serious consequences. Unfortunately, this isn't true, and as more development takes place in the tropics, people are finding out just how fragile rain forest ecosystems are. . . .

FUTURE SHOCKPeople have manipulated their surroundings since before early humans learned to build fires and wield stone axes. And sometimes our actions have caused permanent damage to natural habitats. But the rate and scale of the ecological, social, and economic damage we're causing today in tropical rain forests far surpasses anything we've experienced before. And many scientists think that the long-term effects of tropical deforestation could be much more devastating and far-reaching than most people expect.
Some scientists point to the loss of plant and animal species as one of the most tragic results of tropical deforestation. Collectively, rain forests are the most diverse habitats on earth -- and with every acre of rain forest we clear, we're chipping away at that diversity at a rate that some people estimate may be as high as six extinctions per hour. Some of the plants, insects, and other species we're losing are dying out before we even know they exist. By destroying natural diversity, we're not only destroying natural communities -- we're depriving ourselves of potential foods, medicines, and other products.

We're also causing changes in the ecology of areas that are nowhere near tropical forests. Many of "our" familiar songbirds, for example, fly to the tropics for the winter. And some songbird populations have been declining steadily in the past several years, probably because their winter habitats in tropical areas are disappearing. A lot of these songbirds feed mostly on insects, and nobody knows yet what effect this decline in songbird populations is having on insects in the birds' summer habitats. But we do know that many of these insects feed on plants, including agricultural crops.

As we carve rain forests into smaller and smaller chunks, we may also be tampering with weather patterns. Rain forest trees add a lot of moisture to the air as excess water evaporates from their leaves, and this moisture eventually falls as rain. Some areas that have been deforested have experienced droughts because the loss of trees disrupted the area's water cycle. And some scientists think that the cumulative effect of tropical deforestation worldwide could also interfere with global weather patterns.

Many scientists also feel that rain forest destruction may be contributing to the global warming we're experiencing because of the "greenhouse effect." As rain forests burn, carbon is released from the vegetation and mixes with atmospheric oxygen to form carbon dioxide. Excess carbon dioxide acts like a blanket in the atmosphere, trapping heat against the earth and ultimately causing the planet to warm up.

THE CHALLENGEDeforestation in the tropics isn't a straightforward problem that can be traced to just one or two sources. It's the result of a combination of social, political, and economic problems, and it's aggravated by the special characteristics of tropical topography, soils, and other physical features. And the effects of deforestation are as complex as the causes. Finding solutions to this sticky web of issues is proving to be a major challenge -- but it's a challenge we won't be able to ignore.

http://www.peacecorps.gov/wws/guides/honduras/forests.html