Rain Forest

C

Soil

Despite their incredible lushness and high diversity, one of the peculiarities of rain forests is that the soil is poor in nutrients that can be absorbed by plant roots. The nutritious minerals have been washed out of the soils by heavy rainfall and high temperatures over thousands of years. To compensate for the nutrient-poor rain forest soils, most tropical trees absorb the nutrients they can find and hold them in their living tissue. In contrast, the rich soils of temperate forests are better able to retain nutrients, enabling a temperate forest tree to absorb small amounts of minerals as the tree needs them. When tropical trees die, nutrients are released into the soils by decomposition. Rather than remaining in the soil reservoir as they would in a temperate forest, in the rain forest, the nutrients are rapidly absorbed again by other living organisms.

III

Forest Structure

Rain forest structure is distinct from most other forest types because of its many layers of vegetation, referred to as strata. The three strata of a rain forest are the understory, the midstory, and the overstory. The lowest stratum is the understory, composed of palms, herbaceous plants (such as wild ginger), and tree seedlings and saplings. Just 2 percent of the sunlight penetrates the many layers of leaves and branches above, so understory plant species have developed special traits to cope with low light levels. Many have deep red coloring on the underside of their leaves to capture some of the scarce light that does manage to reach the forest understory. This red coloring enables understory plants to absorb light of different wavelengths than the lush, green-foliaged canopy plants do. Above the forest floor but below the canopy are one or more midstory strata, made up of woody plants, such as large shrubs and midsized trees.

The overstory is the canopy, in which the tree crowns form a continuous layer that captures the bulk of the rainwater and sunlight hitting the forest. The height of the canopy varies from region to region and forest to forest, ranging from 20 to 50 m (65 to 165 ft). The lush, green canopy is teeming with life, and forest researchers have developed ingenious methods for accessing this mysterious ecosystem. Researchers use hot air balloons, cables, catwalks, towers, sophisticated tree-climbing gear, and even robots to study the millions of plants and animals that make their home high up in the forest canopy. Canopy researchers also use huge cranes that are dropped into the heart of the forest by helicopters. Suspended from the crane’s long, movable arm is a large gondola that functions as a mobile treetop laboratory. Moving from tree to tree, forest researchers collect specimens, conduct experiments, and observe life in the canopy frontier.

The highest stratum of the rain forest is made up of the emergent trees, those individuals that stick up above the forest canopy. Emergents, which do not form a continuous layer, are usually the giants of the forest, reaching heights of 35 to 70 m (115 to 230 ft) or more, and trunk girths of over 2 m (6.6 ft) in diameter. Less than one percent of the trees in the forest reside in the canopy and emergent layers. However, these trees tend to be so large that they collectively account for the vast majority of the woody mass, or biomass, of the forest.

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The nicely ordered strata of the rain forest, including the continuous layer of the canopy, are regularly disturbed by naturally occurring events, such as falling trees. Trees in a rain forest canopy are often interconnected by vines, and a falling tree may pull as well as push other trees down with it, producing a domino effect of falling trees. The resulting opening in the forest canopy enables light to pour onto the forest floor. New plants and animals then move into the area and begin to grow.

Other natural disturbances create even larger openings in the forest canopies. For example, along the hurricane belt in the Caribbean and the typhoon belt along the western Pacific, some forests are substantially altered when high winds and storms blow down hundreds of trees every few decades. On a smaller scale, large mammals, such as elephants, regularly raze rain forest vegetation in the Congo River Basin in Africa. Scientists have found that these natural disturbances and the subsequent forest regeneration are a vital process that leads to healthy and diverse forests.

IV

Plant and Animal Life

Rain forest ecosystems contain more plant and animal species than virtually any other habitat in the world. Although their range has contracted and expanded with climate changes over the last several million years, in general, rain forests are some of the oldest ecosystems on Earth. As a result of this continuity, rain forests boast millions of different species, many of which are endemic, or unique, to rain forest habitats.

A

Plants

Although they contain numerous species, rain forests are remarkably uniform in their general appearance. Most trees have tall, slender trunks that do not branch until near the crown. Many, such as kapok trees, are supported by thick buttresses that can stretch out 10 m (33 ft) or more. These buttresses provide needed support for rain forest trees, top heavy because nutrient-poor rain forest soils lead to fragile, shallow root systems. Rain forest tree bark tends to be thin and smooth. Notable exceptions are palms, which are common in some rain forests and virtually absent in other types of forests.

Rain forest plants have many unique physical characteristics that exploit the particular habitat, or niche, that a species occupies. Understory and midstory plants, such as relatives of the banana tree, tend to have particularly large leaves to capture as much light as possible—what little light that is not intercepted by the canopy above. These large leaves do not dry out as they would high in the canopy, where the intense sunlight creates a drier environment. These traits can change, however, when a plant’s environment changes. Canopy trees may change their shape over the course of their life, depending on the environment around them. Leaves often get smaller as trees grow larger. In some cases, leaves of juvenile plants may be almost 10 times larger than adults of the same canopy tree species.

On the nutrient-poor forest floor, many rain forest plants, such as human-sized members of the palm family called Astrocaryum sciophilum, collect falling debris from other plants in their cup-shaped leaves to create their own little compost pile. Rain forests also feature insectivorous plants, which derive some of their nutrients by trapping animals, particularly insects, in their leaves. Among the insectivorous plants are the pitcher plants native to tropical Asia. Insects land upon the pitcher plant’s tubular-shaped leaves, then slide into a cavity, or pitcher, at the plant’s center that is full of digestive juices. Here, the insect quickly dissolves and the nutrients it contains become available for the plant’s use. Rain forests are also home to the largest flower in the world, the giant rafflesia flower, commonly known as the corpse lily. Weighing up to 7 kg (15 lb) with petals spanning almost 1 m (1 yd), the giant rafflesia flower is best known for its putrid smell. The plant emits the stench of rotting meat to attract certain flies, who serve as its pollinators.

Rain forests are often teeming with climbing plants, such as rattan palms. These thick, woody climbers—25 cm (10 in) in diameter—are frequently found connecting the trees, ascending up into the treetops and looping back down. By climbing trees, these lianas, or vines, expose their leaves and flowers to sunlight, birds, and insects without expending the energy needed to build their own supporting tissues. Epiphytes, including mosses, bromeliads, and orchids, grow on tree trunks or nestled in the crook of a tree. Lacking permanent roots in the soil, epiphytes must obtain their nutrients from other living plants or by trapping water and organic matter as they fall to the forest floor. Bromeliads can store up to 38 liters (10 gallons) of water in the reservoirs formed by their overlapping leaves. Most live harmoniously with their host trees, though some are less benign. Strangler figs, which begin their lives as epiphytes, germinate in the canopy trees and send roots down to the soil. As they grow, these parasites enwrap their hosts until they literally strangle them to death, at which point the figs become free-standing trees.

B

Animals

Almost 90 percent of the rain forest animal species are insects, and of these, most are beetles. A single rain forest tree can host more than 150 species of beetles. Living high in the forest canopy, most of these beetles and other insect species have eluded scientists until recently, when technology has improved access to the upper stratum. To this day, scientists are unsure how many animal species exist in the world, largely because they have identified just a small fraction of the millions—some estimate as many as 30 million—of insects that live in the rain forest.

Among the most fascinating rain forest insects yet encountered are leafcutter ants, remarkable because they actually cultivate their own food. These ants cut the leaves of particular plants and carry them back to their underground nests, where they fertilize them with saliva. This careful tending causes growth of a particular fungus, which the ants harvest and rely on as their sole source of food.

In 15 sq km (6 sq mi) of rain forest, as many as 100 different mammal species may be found. These animals occupy every available niche, from burrows in the forest floor to the branches of emergent trees. Most rain forest mammals are nocturnal (active during the night) or crepuscular (active during the twilight of dusk or dawn), and they spend the heat of the day sleeping. In fact, almost half the mammals of the rain forest are bats, flying mammals known for their nighttime activity. Some rain forest mammals, including gorillas, elephants, tapirs, agoutis, and wild pigs, are ground-dwellers, but most, like their insect counterparts, live high in the treetops in the forest canopy. Canopy-dwellers have evolved an array of fascinating traits to survive in the branches of trees. For instance, some Asian rain forests are especially known for animals with the ability to glide. Borneo alone has more than 30 species of mammals, reptiles, and amphibians that can glide from one tree to another. Most rain forest monkeys in Central and South America use their specialized tails, which are capable of grasping branches, like a fifth limb while climbing, feeding, and even playing high above the forest floor.

The three-toed sloth spends most of its life defenseless, hanging upside down from tree branches. To avoid predators, its movements are so slow that they are virtually undetectable to would-be predators, even vigilant jaguars. The sloth has also developed a relationship with a rain forest plant that makes it even more elusive to its predators: Although it has brown fur, the sloth blends in with the surrounding green canopy because a particular species of green alga lives in its fur.

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