Tropical rain forests are the richest biomes on earth in terms of plant species, representing the true apex of diversity in life-forms. A single hectare may support 100 different tree species, and the microorganism and animal populations show corresponding richness. The principal forests of this kind occur in the Amazonian Basin, the Congo Basin, and from the Western Ghats of India in a belt across the Malaysian archipelago to New Guinea.
These forests vary slightly in nature depending on their latitude and altitude. In the equatorial regions temperatures range from a high of 93° F. (34° C) to a low of 68° F. (20° C), with as little range between the average temperature of the hottest and coolest months as 2° to 5° F. (10 to 3° C). Rainfall in these areas is more than 80 inches (203 centimeters) a year. Flumidity, too, is high, rarely falling below 95 per cent in the lower levels of the forest. Most of these trees are evergreen. Away from the equator the rainfall drops below 78 inches (198 centimeters), and the forests in these latitudes experience a dry season, which alternates with one of copious rainfall. Many of these trees are deciduous in the drier months. The monsoon forests in Southeast Asia are also seasonal with a regular dry season with 5 inches (12 centimeters) or less of rainfall, followed by months in which the rain-bearing monsoon winds restart the seasonal growth cycle. On tropical mountains at altitudes from 2,000 to 10,000 feet (600 to 3,000 meters), a cloud or mossy forest exists, the nature of which reflects the abundant precipitation derived from fog condensing on the vegetation. Despite these differences, all tropical rain forests share a high humidity and density of vegetation, the two main features with which plants have to contend.
The structure of tropical rain forests is complex. The tallest trees (emergents) form the so-called emergent layer, thrusting up through the forest canopy at intervals, their crowns spreading above the other trees. These umbrella crowns are usually composed of many smaller dense subcrowns. Most are between 100 and 200 feet (30 and 60 meters) tall, but some, such as the Southeast Asian tualang (Koompassia excelsa), may exceed heights of 260 feet (80 meters).
Beneath the emergent trees the canopy extends, usually as much as 150 feet (45 meters) deep. The flattened crowns of the trees interlock to form a virtually unbroken mass. Their trunks may be branchless for 65 feet (20 meters) above the jungle floor, breaking into huge spreading branches as they approach the light.
Below the main canopy are smaller trees, often with vertically elongated crowns. Some are small and slow growing, reaching maturity in the low light levels beneath the main canopy. Others are young, immature specimens of canopy or emergent trees that will mature only when they are given an opportunity to grow into spaces in those upper layers.
The understory layer beneath the enveloping canopy contains shrubs and herbaceous plants, below which—on the forest floor itself—live fungi, bacteria, algae, protozoa, and other microorganisms.
Between the layers and supported by them are other plants, such as climbers and epiphytes. Climbers are rooted in the ground, but use other plants for support as they grow toward the light. The epiphytes—orchids, bro-meliads, mosses, ferns, liverworts, and lichens—do not root in the soil but live in an area from the trunk bases to the smallest twigs of the canopy, some of them at heights of 100 feet (30 meters) or more above the ground, on the branches or trunks of trees where they are exposed to the light.
The emergent layer
Towering above the body of the forest the mature emergents are subject to greater fluctuations of climate than are their companions. Winds are stronger, humidity is lower, and temperatures are higher and more extreme than beneath the shelter of the canopy. To cope with these factors, mature emergent trees develop smaller, tougher leaves than those they bear as young trees. In addition, the leaves of some species of emergents have a waxy outer covering, which helps to reduce water loss. Most of these trees are broadleaved evergreens.
Taking advantage of the greater air movement above the canopy, many emergent species have winged fruits that are dispersed by the wind to new sites in the forest. For example, the Indo-Malayan dipterocarps (Diptero-carpaceae) have two-winged seeds, and the South American Cavanillesia platanifolia has distinctive five-winged seeds.
Most canopy trees have oval, smooth, shiny leaves that taper to a point (the drip tip). A possible reason for the success of the smooth pointed leaves is that they shed rain quickly, thus discouraging the growth of tiny lichens and mosses, which flourish on moist surfaces. Even so, some species do have compound leaves, both pinnate and palmate.
In cloud forests the canopy is low and dense, formed by small trees with thick twisted crowns of tiny, leathery leaves (micro-phylls). Because of the intense radiation at these altitudes, as high as 10,000 feet (3,000 meters) in New Guinea, the leaves have developed a high reflective power.
Some of the canopy trees, particularly those in the drier, more seasonal areas, are deciduous, shedding their leaves at regular intervals. The Indian almond (Terminatia catappa), for example, sheds its leaves every six months; other species do so at intervals of slightly more than a year.
The evergreen trees produce their new leaves in flushes rather than continuously. These new leaves tend to be more brightly colored and less rigid than the old ones. One reason for this staggered pattern of replacement may be that the soft leaves are preferred by herbivorous animals, and if these were produced continuously, the animals would destroy the bulk of new growth. By having long intervals in which no new leaves are produced, they are too uncertain a food supply to support a large population of leaf-eaters.
Flowering and fruiting occur in regular seasons. Many species, such as the silk-cotton trees (Ceiba pentandra), flower simultaneously through the forest. Individuals may be widely scattered, but even so, simultaneous flowering greatly helps cross-pollination. Fruiting at the same time means that more than sufficient food for seed-eating animals and birds is produced so that enough seeds remain to germinate. In some species, flowers are produced on the main trunk rather than on twigs or branches, especially on the smaller trees. This may be related to bat pollinators, which cannot reach flowers hidden in a mass of leaves and twigs.
Most seeds in the canopy have some means of dispersal away from the vicinity of the parent tree, where competition for resources is too great. Even though the air beneath the jungle canopy is still, a few large trees have wind-dispersed seeds. The silk-cotton seeds have a light, fluffy coating that carries them on the slightest movement of air, and some of the mahoganies, Khaya spp. and Entandrophragma spp., have winged seeds.
Animals, however, are the most important means of seed dispersal in the rain forest. At-. tracted by soft fleshy fruits, such as durian (Durio zibethinus), the animal consumes the pulp, but the hard seeds within are resistant to digestion and pass through the animal unharmed, having gained the advantage of its movement through the forest. A few trees, such as the sandbox tree (Hura crepitans), have exploding fruits that scatter their seeds.
Once dispersed, the seed then has the problem of germination and establishment in the difficult environment of the forest floor, where there is tremendous competition for light and nutrients. Most plants employ one of two strategies: some trees produce a few large seeds with large food reserves, which fuel the seedling during its slow growth (this growth speeds up if a tree falls nearby, reducing the competition for light and space); other trees produce many tiny seeds, which lie on the forest floor until a gap, made by a fallen tree, allows them sufficient light to germinate.
Only 2 to 5 per cent of the sunlight available to the canopy reaches the understory, and much of the light that does remain is transmitted through or reflected off leaves, thus losing much of its useful content. When a large tree falls, possibly bringing down with it a number of smaller neighboring trees, it creates a gap where increased light levels that reach the ground layer stimulate a burst of young tree growth and the germination of seeds in the lighted patch.
Fast-growing species first take advantage of the new conditions, with fully grown canopy and emergent trees perhaps taking many decades to reestablish. The plants that do survive in the undergrowth include dwarf palms and soft-stalked species of families, such as Maran-taceae (an example of which is the prayer plant, Maranta sp.), the ginger family (Zingibe-raceae), and the acanthus family (Acanthaceae). These plants usually grow to a height of 10 feet (3 meters) or less. Their leaves are usually broad and pointed, and some species have a reddish tinge to the undersurface. The red is due to the presence of the secondary pigment anthocyanin.
Understory plants have difficulty with pollination because of the lack of air movement there. They therefore rely on insects. Some flower at night, producing large strong-scented flowers, which attract moths. Others, such as the cacao tree (Theobroma cacao) and the breadfruit plant (Artocarpus communis), produce flowers and fruit on their trunks. This phenomenon, known as cauliflory, makes the flowers and large fruits conspicuous, for pollination and seed dispersal.
The forest floor
The floor itself is covered with a litter of rapidly decomposing vegetation and organisms that break it down. They are an essential component in the cycle of nutrient flow between generations of plants.
The soil of these forests is poor and intensely leached. A high proportion of the nutrients in the system are locked in the very large biomass, and there is great competition for the nutrients released by decomposition. This is one reason for the shallow-rootedness of many of the trees.
Climbers and epiphytes
Climbers attach themselves to the trunks of trees by clinging roots that may absorb water and nutrients from the surface of their supporters. The lianas, in contrast, start life as a small plant and gradually grow up to the canopy, using other plants for support. Lianas sometimes twine around other tree trunks, but often hang from the canopy, their crowns interwoven with the crowns of adjacent canopy trees. They have strange, twisted stems composed of wood that is divided into separate strands, which produce a structure of great strength and flexibility.
The epiphytes use other plants for support only. Many, particularly the epiphytic orchids, have specialized, stocky roots with a spongy cortex that takes up water quickly when it is available. Others, such as the tank bromeliads, have cup-shaped leaves or leaf bases which have been known to hold 14.5 gallons (55 liters) in some species and to support cyanobacteria and green algae.
The strangler fig (Ficus sp.) is one (accidental) epiphyte that does eventually kill its supporter. Its seeds are deposited by birds on the branches of canopy trees, where they germinate. Aerial roots grow down and encircle the host tree while the plant grows upward toward the light. Eventually it shadows its host, which dies from lack of light as much as from strangulation.
Tropical rain forests are a valuable resource if wisely used. Their luxuriance, however, hides a delicate ecological balance, and their complex communities are easily and irrevocably destroyed by the large-scale land clearance and logging that has taken place in recent years. The loss to humankind would be stupendous if these forests were destroyed before we even fully understand their complexity and the many species that are and will become extinct, and learn howto benefit from them.