Rainforests

Rainforests are forests with dense canopies found in regions of the world with high average precipitation and humidity levels. Rainforests can exist both at low elevations and high in mountain ranges, presenting an incredibly diverse range of habitats. This helps make them major biodiversity hotspots, home to numerous endemic species. Rainforests are also known for their importance as carbon sinks, which factors into global climate regulation. The complex ecological role of rainforests is under threat from human deforestation, pollution, and climate change.

Although tropical rainforests such as those in the Amazon River drainage system of South America and other equatorial regions are perhaps the best known, rainforests can be found in some cooler regions as well. Examples of temperate rainforests include parts of the Pacific Northwest in North America, the Pacific coast of southern Chile, and southeastern Australia. Aside from temperature, a primary difference between these two major types of rainforest is that temperate rainforests are often dominated by one or two species of trees, compared to the great diversity of tree species typical in a tropical rainforest. The majority of trees found in tropical rainforests tend to be broad-leaved, such as the rubber tree, while temperate rainforests are dominated by conifers.

Characteristics of Rainforests

Although rainforests are remarkably diverse, they do share a few characteristics in common. The abundant moisture in a rainforest gives the woodland a lush, fertile appearance. This is particularly true in tropical regions. Even in the understory, the area between the tree canopy and ground level where light is limited, vegetation may be dense. This appearance of fertility is often deceptive, however. Dead plant matter decays rapidly in a tropical forest, but the nutrients are used quickly by the numerous competing plants. In addition, the trees in tropical forests tend to be evergreen, so relatively little leaf litter falls to the forest floor. Due to this lack of mulch, topsoil is often thin and the root systems of the trees are quite shallow. In some tropical rainforests, especially, there is actually much open space on the forest floor because smaller plants cannot thrive with the lack of sunlight.

The leaves of many rainforest plants have a waxy texture or come to a point to help shed water more quickly and prevent the growth of fungi or mold. Understory species are also generally well adapted to living with limited sunlight and direct rainfall, which are blocked by the dense canopy. Along with their evergreen leaves, these features make many rainforest species popular as houseplants.

One reason tropical rainforests are evergreen is that in the tropics, there is little seasonal variation in the hours of daylight. The closer to the equator a forest lies, the less change there is from season to season. In temperate climates, many plants have evolved to bloom, set seeds, or lose their leaves based on the number of hours of sunlight available each day. As the seasons change annually, plants bloom in the spring or early summer; fruit ripens in the fall, photosynthesis slows, and leaves change color and die. In the tropics, where the number of hours of daily sunlight never varies, plants follow a different schedule. Many tropical plants bear new flowers and mature fruit simultaneously. The evergreen foliage and continuous supply of certain fruits have led to the adaptation of some animals to a very restricted diet: koalas, for example, which feed exclusively on eucalyptus leaves, or parakeets that eat only figs. Exceptions to this pattern are the forests where rainfall is seasonal, such as regions of the world like Southeast Asia, where much of the rain comes from annual monsoon storms. In those cases, flowering and setting fruit will coincide with the seasonal rains.

Forest Zones

A rainforest can be divided into four zones, each with its own distinct characteristics. The lowest level, the forest floor, is often dark and gloomy. Little sunlight penetrates this level, and there is little air movement. This provides ideal conditions for microbes, many of which act as decomposers in the soil. Numerous insects, such as beetles, cockroaches, and termites, also live in the decaying litter and provide food for larger animals. Many of the insects, birds, reptiles, and amphibians that live in the lower levels of the rainforest are brightly colored. Scientists speculate that the animals have evolved in this fashion to more easily attract potential mates. Other scientists believe that colors warn potential predators to stay away. In either case, the vivid colors make the animals more easily seen in what is otherwise a dark environment, similar to marine life that lives in the ocean's midnight zone.

Just above the forest floor is the understory. Many of the plants in the understory have large, dark leaves to maximize their light-collecting ability. Because there is little natural air movement within the lower levels of a rainforest due to the canopy blocking any natural breezes, the flowering plants in the understory often have strongly scented or vividly colored flowers to help attract insects or birds to assist with pollination. Lizards, snakes, amphibians such as tree frogs and salamanders, small birds, and mammals as large as the jaguar all call the understory home. The plants found only in the understory seldom exceed fifteen to twenty feet in height.

The densest layer of plant life is the canopy. High above the rainforest floor, the branches of mature trees form a dense intertwined zone of vegetation extending up as much as 150 feet above the ground. Numerous plants sprout in the crotches of trees, where debris may collect. Tree limbs are festooned with vines and mosses, and bromeliads and orchids grow on the rough bark of tree trunks. Even other trees may start their life cycles high above the ground, as in the example of the strangler fig. A fig seed that lands and sprouts on the ground will probably not survive due to low light levels on the forest floor, so strangler figs have adapted so that their seedlings begin life in the crotches of other trees. The roots of a young fig gradually creep down the trunk of the tree on which it sprouted, often completely encircling the host tree by the time they reach the forest floor. The canopy is also home to many animals, including primates such as gibbons, orangutans, and lemurs and other herbivorous mammals such as sloths.

The emergent layer of the rainforest consists of the tallest trees, some exceeding two hundred feet in height. The tops of these trees provide a habitat for large, predatory birds, such as eagles, and are home to assorted snakes, monkeys, and other animals. Every layer of the rainforest teems with life, and often what can be found at ground level gives no hint of the diversity that exists two hundred feet above in the treetops.

Rain Forest Conservation

Many of the trees found in rainforests are valued for their commercial use as lumber, while others have been exploited for their fruits or other products, causing much habitat loss. Tropical hardwoods, such as teak and mahogany, for example, have long been used in construction and in furniture for their desirable qualities such as rot resistance. Logging of rainforests, such as those in Sumatra, Indonesia, has also been done to supply the paper and pulp industries. Environmentalists have encouraged timber companies to plant and harvest desirable tree species in sustainably managed plantations, rather than logging from the wild. There have also been international bans or other limitations on trade in threatened species.

Activists hoping to preserve the tropical rainforest have encouraged Indigenous peoples to collect forest products, such as nuts or sap, as a way to create a viable economy while at the same time discouraging industrial clear-cutting of the forest. Indigenous people tap rubber trees in Amazonia, for example, to collect latex. Rubber trees are native to the rainforests of South America, although they are also grown in plantations in other tropical regions of the world, such as Southeast Asia.

The biggest threat to the world’s rainforests may not come from commercial logging, however. In many regions of the world, rainforests have fallen victim to population pressures. Forests continue to be clear-cut for agricultural use, as in Brazil and Indonesia, even when the farmers, ranchers, and plantation owners know the exposed soil’s fertility will be quickly exhausted. Such incursions have been made to promote global soy, palm oil, and beef production. In some cases, the cleared land becomes an arid wasteland as the tropical sun bakes the soil too hard to absorb rainwater. In others, the land is farmed for a year or two and then abandoned. Given enough time, the rainforest may regenerate, but the process will take hundreds of years. Mining of precious metals, coal, and petroleum represents yet another threat from both the land clearance itself and the risks of contamination from extraction-related chemicals.

Tropical rainforests cover approximately 6 percent of the Earth's surface, though some estimates are lower, and only a small percent are formally protected. Most remaining tropical rainforests are found in developing countries, many of which have struggled to implement and enforce viable legal protections. These countries are often poorer than their subtropical and temperate counterparts, and their governments often feel pressure to exploit their rainforests for economic development. Corruption, dubious land deals and permits, and government loopholes or policy reversals contribute to ongoing rainforest destruction. Nongovernmental agencies and local Indigenous people often decry these conditions and attempt to hamper illegal clearances.

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