Most forests contain a mixture of many different species of trees, and rain forests contain the most species of all. Both coniferous (cone-bearing) and nonconiferous evergreen trees exist within their boundaries. However, because most rain forests are difficult for humans to penetrate and usually are found in countries that cannot afford to fund scientific studies, and may not want foreign scientists working there, only a handful of these species have been named.
Plants that grow in the rain forest are often either climbers, epiphytes (EPP-ih-fytes), or parasites. Climbers have roots in the ground, but they use hooklike tendrils to climb up the trunks and along the limbs of trees in order to reach the canopy where there is light. Epiphytes, or "air" plants, store water in their fleshy stems and leaves. They, too, grow on trees and other plants, especially in the canopy, but their roots are exposed to the air. These plants absorb the nutrients they need from rain and forest debris. Parasites also attach themselves to other plants and trees, but they manage to do without light and take their nourishment from their host.
Plant life within the rain forests includes not only trees but also algae (AL-jee), fungi (FUHN-jee), lichens (LY-kens) and green plants other than trees.
Algae, fungi, and lichens do not fit neatly into either the plant or animal category. In this chapter, however, they will be discussed as if they, too, were plants.
Algae: Most algae are single-celled organisms, although a few are multicellular. Certain types of algae have the ability to make their own food. During a process called photosynthesis (foh-toh-SIHN-thuh-sihs), they use the energy from sunlight to change water and carbon dioxide (from the air) into the sugars and starches they use for food. Other algae absorb nutrients from their surroundings.
Although most algae are water plants, blue-green algae do appear in the rain forest, where they encrust the leaves of trees. Although this blocks the sunlight from the trees' leaves, the algae may aid the tree in obtaining nutrients, such as nitrogen, from the atmosphere.
Fungi: Unlike algae, fungi cannot make their own food by means of photosynthesis. Some fungi, like mold and mushrooms, obtain nutrients from dead or decaying organic (material derived from living organisms) matter. They assist in the decomposition (breaking down) of this matter and in releasing the nutrients needed by other plants back into the soil. Other fungi are parasites. Fungi reproduce by means of spores, which are single plant cells that have the ability to grow into a new organism.
Fungi prefer a moist, dim environment, and they thrive on the shadowy forest floor. Some, like the marasmius or the starlike thismia, grow directly on the litter of plant matter, while others protrude from the trunks of trees. Another type of fungi, the mycorrhiza, live in the soil and surround the roots of most rain forest trees. Although they absorb energy from the tree, they help the tree's roots absorb nutrients from the soil.
Lichens: Lichens are actually combinations of algae and fungi that live in cooperation. The fungi surround the algae cells, and the algae obtain food for themselves and the fungi by means of photosynthesis. It is not known if the fungi aid the algal organisms, but it is believed that the fungi may provide moisture for the algae.
Lichens often appear on rocks and other bare woodland surfaces. Some grow on the leaves of lowland trees, while others favor the cooler cloud forests. Lichens are common in all types of forests and seem able to survive most climatic conditions.
Most green plants need several basic things to grow: sunlight, air, water, warmth, and nutrients. In the rain forest, water and warmth are abundant. However, nutrients such as nitrogen, phosphorus, and potassium, which are obtained from the soil, may not be in large supply. Light is also scarce because the thick forest canopy allows only about 1 percent of the sunlight to reach the forest floor. The lack of seasons means that the canopy is in full leaf all year long. For this reason, most rain forest plants grow in the canopy of the forest. Those that do grow on the ground often have very large leaves that provide more surface area to be exposed to the scare amount of light. The Heliconias, a flowering plant with banana-like leaves, is an example of such a shade-loving plant.
It is always the growing season in the tropical rain forest, and at least one species will be in flower at any given time.
Most green plants reproduce by means of pollination. This is the process by which pollen is carried by visiting animals or the wind from the stamen, the male part of the flower, to the pistil, the female part of the flower, where seeds develop. The seed's hard outer covering protects it while it waits for light to stimulate its growth.
Instead of pollination, some plants, such as ferns, reproduce by means of rhizomes, long, rootlike stems that spread out below ground. These stems develop their own root systems and send up sprouts that develop into new plants. Other plants, such as the banana palm, which is not a tree but an herb, produce fruit and then die.
Common rain forest plants include rattans, pitcher plants, ferns, African violets, nasturtiums, Spanish mosses, orchids, lianas, urn plants, hibiscus shrubs, and bamboo.
Liana: Lianas are climbers found in rain forests throughout the world. Although their roots can be large and tough, they do not develop a thick trunk. Instead, they depend entirely on the trees for support. Once they reach the canopy, they drape themselves among the branches and develop leaves, branches, flowers, and fruits. Often they send out "feeding," or aerial (AIR-ee-yuhl), roots that dangle in midair and absorb nutrients.
One well-known liana is the "strangler fig," which begins as an epiphyte that sends long roots down to the ground. These roots soon grow branches that surround the host tree, blocking the light. Eventually the host dies and decays, leaving a hollow ring of stranglers.
Urn plant: Urn plants are epiphytes found in Central and South America. The plant's overlapping leaves form the "urn," or cup, which collects rainwater and any dead plant matter that falls into it. The water and dissolved minerals are absorbed by the plant by means of little hairs on the leaf surface. Urn plants provide homes for many aquatic (water) insects and even frogs.
Hibiscus: Hibiscus shrubs grow on the ground around the edges of the forest where they can obtain light. In Africa, they quickly attain heights of up to 7 feet (2 meters). Their large, colorful flowers are bell shaped and may be scarlet, pink, yellow, or white.
Bamboo: Although bamboo can grow as tall as a tree, it is really a woody grass. Dense forests of bamboo can be found in Asia and Central Africa where plants may reach 120 feet (40 meters) in height. Bamboo tends to grow in thick, tightly packed clumps, and flowering occurs only after several years. In the warm, humid tropical forest, bamboo is one of the fastest growing plants, often achieving as much as 3 feet (1 meter) in a single day.
Most trees have a single strong stem, or trunk. This single trunk gives them an advantage over smaller woody plants in that most of their growth is directed upward. Some large rain forest trees develop buttresses, winglike thickenings of the lower trunk that give tall trees extra support. Buttresses may reach as much as 30 feet (9 meters) up the trunk and extend at least that far along the ground.
As a tree grows, its trunk is thickened with a new layer, or ring, of vessels that carry water and nutrients from the roots to the branches. As the tree ages, the vessels from the center outward become hardened to produce a sturdy core. In cooler climates, the rings are formed seasonally and, when a tree is cut down, its age can be determined by how many rings are present. In the rain forest, because there are no seasons, rings do not form regularly and cannot be used to accurately estimate a tree's age. For rain forest trees, other methods are used, such as measuring the increase in the tree's circumference during a year and dividing that number into its total girth (size). Based on such calculations, one species in Malaya may be as old as 1,400 years, a baby compared to the ancient 4,600-year-old bristlecone pine of North America, but unusually old for the rain forest, where most trees only live 100 to 500 years.
Many people believe that the world's tallest trees grow in the rain forests, but this is not true. The tallest trees grow in drier, more temperate climates. Rain forest trees seldom exceed 200 feet (61 meters) in height, although the tualang of Southeast Asia has been recorded at 272 feet (83 meters), and four other species that grow there also exceed the average.
The leaves of rain forest trees are often waxy and develop a "drip tip," a long, narrow point that allows rain to run off easily.
A tree is a woody perennial plant, which means it lives more than one year or one growing season. When temperatures are warm year-round and rainfall is constant, as they are in the rain forest, all trees become evergreen and grow almost continuously. Some rain forest trees shed their leaves periodically for a short time; however, this shedding is not simultaneous, even among trees of the same species.
Many rain forest trees do not bear fruit every year and thus do not regenerate readily. Seeds that lie on the forest floor may remain dormant (inactive) for many years. Those that sprout grow very slowly after the nutrients stored in the seeds are used up. However, if a tree falls, creating a gap in the canopy and allowing sunlight to enter, these seedlings make up for lost time, growing quickly toward the light, and soon the gap is closed again.
In general, trees are divided into two groups according to how they bear their seeds. Gymnosperms produce seeds inside cones. Most conifers, like the pine, are gymnosperms. Angiosperms have flowers and produce their seeds inside a fruit. Broad-leaved trees, such as maples, are usually angiosperms. Although some species of gymnosperms are found in rain forests, most rain forest trees are angiosperms.
The seedlings of some rain forest trees do not develop chlorophyll, the substance in leaves that gives them their green color, until they reach light in the canopy. As a result, the leaves of these young trees are often red, blue, purple, or white instead of green.
Common rain forest trees include black ebony, cinchona, mahogany, and mango.
Black Ebony: Found in the rain forest of Africa, the black ebony tree grows between 25 and 35 feet (7.6 and 10.6 meters) in height and has a trunk from 1 to 2 feet (.3 to .6 meters) in diameter. Its outer wood is almost white in color, but its heartwood is black and extremely hard. Valued for its heaviness, color, and durability, ebony heartwood has been considered a precious wood since before 500
Cinchona: The cinchona (sing-KOH-nah) is native to Central and South America where it is found in montane rain forests. Its flowers grow in clusters resembling lilacs, and its bark is an important source of several medicines, including quinine (KWY-nine), a treatment for malaria. During the 1800s, cinchona seeds were brought to Java in Indonesia, where the trees are still raised commercially.
Mahogany: The term mahogany is applied to almost 200 species of trees with reddish-brown wood, winged seeds, and small, greenish-yellow flowers. Although the first trees given the name are native to the West Indies, the most commercially important mahoganies come from Central and South America and are used in fine furniture and paneling. African and Philippine mahogany are also economically important.
Mango: The mango tree is found in rain forests of Southeast Asia and, in its wild state, produces a fibrous fruit that tastes like turpentine. In some species, the fruit is poisonous. Cultivated trees, however, produce a delicious fruit that is extremely popular throughout the world. A Chinese traveler brought the mango to India around
Vast areas of rain forest have been destroyed by uncontrolled logging and clearing of the land for farms. Since 1940, most of the rain forest in Southeast Asia has disappeared, especially in the Philippines. In more mountainous regions, such as Papua New Guinea, where the land is less useful for farming, huge tracts of rain forest still remain untouched.
Many individual plants are also threatened as the forest is destroyed. The African violet, for example, which is commonly cultivated as a house plant, is found in only a few forests in Tanzania, where it is rapidly disappearing.
Source: "Rain Forest." U·X·L Encyclopedia of Biomes, Vol. 2. U·X·L, Detroit: 2000.