Tropical Rainforest

A

Plant Growth Forms

Despite differences in species composition between different rainforest areas, similar growth forms and life strategies have evolved throughout the equatorial zone. This is known as convergent evolution.

The trees of the rainforests have structures that help them to grow most effectively in the tropical forest environment. These include buttress roots, which are broad wide woody “wings” at the base of the trunk that support the tree and direct stemflow towards the roots. Large leaves are commonly found especially where light is low and transpiration reduced due to the high humidity. Drip tips are common and allow drainage of the precipitation off the leaves. The bark of forest trees can be very diverse. Many trees have thin, smooth bark because there is little need to conserve water, while others are armed with spikes or thorns for protection. Some trees may have large fleshy fruits to attract the birds and animals that disperse the seeds.

Epiphytes are plants that root on the trees. Different species occur at different vertical levels, and they are adapted to the conditions where they live. For example some epiphytic cacti are found in the canopy. They are not parasites, but obtain their nutrients from rainfall and stemflow.

Some plants use the trees for support, rooting on the forest floor. Climbers are woody vines that grow high into the canopy. Lianas are green-stemmed and tend to be confined to the lower levels. Stranglers grow tall and enclose the trunk of their host tree, eventually killing it by taking all the nutrients and water and competing with it for light in the canopy.

Heterotrophs are non-photosynthetic and derive their nutrients from other plants and animals. Some are full parasites such as Rafflesia arnoldii (the corpse lily), which has the world's largest flower, measuring 1 m (just over 3 ft) in diameter (see Rafflesia). Many heterotrophs live inside the stems or roots of the green plants and can only be seen when their flowers or fruiting bodies emerge.

B

Rainforest Animals

Many of the animals also show adaptations to the stable environment of the tropical rainforest. Forest mammals tend to be long-lived and slow-breeding. Many endemic species are found in rainforests. They may also show highly specialist feeding behaviour sometimes living on only one type of food plant, or only being active at certain times of the day. This is known as niche partitioning, which may help to reduce competition between species and contributes to the high animal biodiversity.

Arboreal, or tree-living, species make use of the different niches created by the layering of the vegetation. Adaptations for living in the high canopy include flying, gliding, and prehensile tails. Bats, flying foxes (a species of bat), and tree frogs occur here, along with climbing species such as sun bears, orang-utans, and other primates. Ground-dwelling forest mammals include large cats, such as tigers and jaguars, tapirs, pigs, and many species of rodent. Bird and reptile species occur in each of the forest layers. Bright colouring, distinctive calls, and dappled patterning are common. Many species have effective camouflage behaviours, while mimicry (to confuse predators) is widespread. The invertebrates, which outnumber all other animals, are an important part of the decomposer food chain, breaking down litter to release nutrients.

IX

Soils

The character of the soils is due to the warm, moist climate and dense, productive vegetation. Many have evolved over very long periods of time and are deeply weathered. This results in the production of abundant mineral material and deep clay soils. Such conditions are also optimal for the activity of soil biota, which rapidly break down plant litter to produce humus. The release of humic acid from decomposition increases the rate of chemical weathering. In these climates rainfall greatly exceeds evaporation, and so there is continual leaching (washing away or removal by rain). Soluble bases (calcium, potassium, magnesium, and sodium) can be washed out of the soil, along with humus and other fine non-soluble material. The soils are therefore nutrient-poor.

The continual movement of water distributes silica and hydrated oxides of iron and aluminium throughout the soil, which give them a characteristic red colour. The less soluble clays, such as kaolinite, remain in the upper layers, while the plentiful biota constantly mix the soil and inhibit the formation of clear horizons. These soils are known as latosols, ferralitic soils, or oxisols.

Variations in soil types are associated with changes in geology and topography. Slopes usually contribute to the formation of the more acidic podzols through leaching (particularly of bases) and so these are poor soils. Sequences of different soil types, known as catenas, can occur downslope as drainage conditions change. In the extensive river basins alluvial material, rather than bedrock, produces richer soils. Where the water table is high, soils tend have a higher humus content and are darker in colour. In some areas humus can be washed out, which leads to the colouring of rivers, as in the case of the Negro River in Amazonia, so called because of its near black appearance.

X

Nutrient Cycling

Despite the nutrient poor soils, the rainforests can support high levels of productivity because the plants have evolved strategies for efficient and rapid nutrient cycling. Decomposition of dead organic matter is rapid in the warm, moist conditions. Invertebrate species begin the breakdown process that is completed by microbial activity.

Microrrhizae are fungi that live in symbiotic mutualistic association with plant roots. This unique and intimate association of two organisms helps to maximize the availability of nutrients for plant growth. The fungi decompose organic material and because they are located in the plant roots the nutrients are taken up rapidly. Plant photosynthesis provides the carbohydrates the fungi need to grow.

Nutrient uptake by plants is rapid because of their dense, fibrous mat of roots. Interception of rainfall by the canopy aids uptake by slowing the loss of nutrients by leaching. The continuous pattern of leaf fall ensures a steady supply of nutrients to be captured by plants.

In rainforests the living vegetation and litter layer represent the major nutrient stores within the ecosystem. In this respect they differ from ecosystems found in higher latitudes, where the soil represents a more important reservoir of nutrients. It also makes them highly vulnerable to disturbance because deforestation causes the nutrients to be leached away, leaving a greatly impoverished system.

XI

Disturbance and Human Impact

A

Human History

Many groups of indigenous peoples are associated with the rainforests. It supplies them with food, medicines, fibres, poisons, gums, colour and dyes, stimulants, fertilizers, insecticides, tannins, rattan, and shelter as well as wood. Living as hunter-gatherers they supplement their food by low-level shifting cultivation. In general the cultivated areas are small so when the soil is exhausted, activity is moved to a new area. Because of the small scale of their activities the forest is able to re-establish after abandonment. Thus, their activities are seen as being sustainable. As the rainforests have been opened up by modern development however, many of these societies have been disrupted. In some areas the indigenous peoples have been able to maintain their traditional lifestyle, and much can be learnt from them about the rainforest ecosystem.

Although indigenous peoples use the forests in a sustainable way, they do cause changes to the ecosystems. By selecting plants to grow from seed or removing unwanted species, they change the composition of the forest vegetation. For example, bamboo is often burnt to promote growth of new, tender shoots. In addition, trees such as the Brazil nut (Bertholethia excelsa) can be planted for food, while plants such as rattan palms, which is useful in weaving, can be grown from seed. The subtle changes in the composition of the ecosystem caused by the indigenous people can persist for long periods and in South America we can still see evidence of the ancient Maya and Aztec cultures in vegetation patterns.

Early interest in the rainforest led to the first attempts to circumnavigate the world, when voyages were made in search of the “Spice Islands” (see Spices: The Spice Trade). Trading links developed to export forest products ranging from foodstuffs to timber. In particular, the tropical hardwoods were highly prized in Europe for furniture-making. Large-scale clearance began in the 19th century in the colonial period; at the same time rubber plantations were started in South East Asia. During the second half of the 20th century, exploitation has been made easier and more rapid by improved technology, road engineering, and large machinery. As a result the rainforest ecosystem is now undergoing a period of rapid change.