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Tropical rainforests Which features make tropical rainforests unique among ecosystems? Why are most tropical soils red? Metres 40 A Discontinuous canopy of tree crowns of the tallest trees (called emergents) 30 B Continuous layer of the main canopy formed by the crowns of the many tall trees 20 C Discontinuous under-canopy of trees between 10m and 20m high 10 understanding GCSE GeoGraphy 4 D Layer of shrubs and young trees E Herb layer with ferns 6m or more high 0 Figure 1 Tropical rainforest – forest structure. Characteristic features A The vegetation mass is the greatest of all ecosystems. Despite the sheer amount of vegetation present, and the way in which climbing plants and 0435353308_aw_4.16 creepers, known as lianas, run from tree to tree in a chaotic manner, it is possible to recognise five distinctive forest layers (Figure 1). The canopy provides a habitat for monkeys and numerous birds such as macaws; on the ground are some larger animals such as jaguars, tapirs and anteaters in the Amazon Basin. The tall trees are deciduous, but they shed their leaves at different times and for only six to eight weeks each year, so that the forests always look green. The emergents (tallest trees), by reaching up to 50 metres high, stand head and shoulders above the forest canopy; these are hardwoods and include types such as mahogany and ironwood. They have long trunks without any branches until their rounded crowns extend out over the canopy. Their leaves are oval in shape with extended points known as drip tips, and they have dark green and leathery upper surfaces. The smooth bark is thin. Their shallow roots, which mainly extend sideways below the ground surface, extend above the ground as buttress roots. Epiphytes, parasitic plants growing on trees and tree branches, increase the abundance of vegetation. B Adaptations to climate The tropical rainforest’s biodiversity is a response to climate. There are constant high temperatures, with a mean monthly average above 27°C, accompanied by high solar light intensity. Rainfall is regular and high, with more than 2000 millimetres falling during the year, which creates 70 Figure 2 Tropical rainforest kapok tree A: Above the canopy with many epiphytes and lianas. B: Forest floor showing buttress roots. Plant communities are fiercely competitive. There is ‘survival of the tallest’ as the tall trees are drawn upwards by the heat and light, which is why leaf growth is concentrated in the canopy. The leathery upper surfaces of the trees’ leaves are necessary to withstand the great power of the Sun’s rays. The drip tips help the leaves to shed water during the heavy rains. In the lower layers of the forest, sunlight is in short supply. Ferns are adapted to life on the forest floor by having leaves that intercept a high proportion of the light that reaches them. The shrub layer is sparse because of lack of light, although shrubs quickly take advantage of any gap in the forest canopy. Soils Figure 3 shows a latosol, which is the name given to soils that form under tropical rainforest. They are red or yellowish-red in colour throughout. They are very deep soils, often 20–30 metres deep, compared with the 2–3 metres for brown earths (page 68). The black humus layer at the top is a narrow horizon of organic material. The red and yellow colours below it come from the oxides of iron and aluminium, which remain in the soil after other minerals have been washed out by leaching. Figure 4 Figure 3 A latosol. information Biodiversity in Ecuador A typical 5-hectare patch of rainforest contains: • • • • • 750 species of trees 1500 species of flowering plants 400 species of birds 150 species of butterflies 100 species of reptiles The importance of tropical rainforests It is their biodiversity that makes tropical rainforests unique among ecosystems. This term refers to the great number and variety of living species, plants and animals; it is estimated that 50 per cent of the world’s 10 million species live in tropical rainforests (Figure 4). The Earth’s genes, species and ecosystems have evolved through 3000 million years and form the basis for human survival. Wild varieties of plants are the basis for new seeds for farmers to use; many of today’s drugs, such as aspirin, are derived from plants. When new combinations of genes are sought in the future for new food crops or cures for diseases, without the rainforests and their biodiversity, the number of potentially useful species from which to choose will be reduced. Some environmentalists emphasise the importance of tropical rainforests as suppliers of oxygen (the ‘lungs of the world’) and carbon dioxide stores (against the enhanced ‘greenhouse effect’). 4 living world humid conditions. There is no more favourable climate on Earth than this for plant growth. Some species of flowering plants even grow 40 metres above the ground in the crowns of emergent trees. Looking at the density and diversity of the vegetation cover, you might think that it is growing from the world’s most fertile soil. Nothing could be further from the truth. Rainfall is much higher than evapotranspiration; even with the protection of the forest canopy, there is the downward movement of rainwater through the soil. Leaching washes organic material and silica downwards and then out of the soil. The only fertile part of a latosol is the narrow organic layer, which is why the trees have shallow roots. Activities 1 (a)Draw a labelled diagram to show the main characteristics of tropical rainforest vegetation. (b)Explain the great importance of climate in determining these characteristics. 2 (a)Looking at Figure 3, draw a sketch of a latosol and label three main characteristics. (b) Are latosols fertile soils? Explain your answer. 71 Tropical rainforests – sustainable uses and management Which groups make a living from the rainforests? Which groups want to destroy the forests? How can the rainforests be conserved? While natural deciduous forests have been cleared almost to extinction in Europe and North America, large areas of tropical rainforest remain in other parts of the world, mainly because of obstacles such as difficult access and infertile soils. Rainforests are mainly located in the world’s poorer, less developed countries (Figure 1, page 64). However, many of these forests are now under threat as governments view their untapped resources as a passport to economic development and a way to reduce their international debts. Traditional uses understanding GCSE GeoGraphy 4 Human settlement in rainforests is long established, but often of low density. Indigenous (native) peoples, living in tribes or groups, either collected and hunted to use the food naturally available in the forest, or practised ‘slash and burn’ and grew crops such as manioc, or did both. With slash and burn, the clearing was small and the group cultivated only for as long as the soil retained its fertility (probably two or three years). They then moved to another part of the forest, hence the alternative label of ‘shifting cultivation’. Indigenous tribes, low both in numbers and levels of technology, barely left a mark on the forest, which re-invaded within a few years as if there had been no human presence. This is an example of sustainable human use of rainforests. Uses after forest clearance Indigenous peoples are pushed back into smaller and smaller areas of forest by the advances of loggers, Figure 1 72 Banana plants being grown in an area cleared by indigenous people. How great is the risk to the environment? Figure 2 miners and farmers, each group supported by superior modern technology. Direct contact results in the spread of diseases to which they have no resistance, and in the destruction of their traditional culture and ways of life. Unlike them, outsiders engage in activities that involve forest destruction. The first forest clearances are usually associated with road building. Roads attract farmers, loggers and miners, enabling them to open up wider areas of forest away from the roadsides. Farming by outsiders is more likely to be cattle ranching (Figure 2). The ranchers are interested in only one thing – replacing forest with pastures. Often, they do not even save and sell the valuable hardwood timber and it is just burned (Figure 3). Logging companies are only interested in certain types of tree, but one of the characteristics of rainforests is that individual species of tree are widely dispersed. In order to reach the trees they want, all the other trees are felled and cleared. Oil, gas, iron ore, bauxite (for aluminium), nickel and gold are just a few of the natural resources that have attracted mining companies to rainforest regions. Despite all the problems with access, once a mineral deposit of commercial size is discovered, then roads, railways and pipelines are built with little or no thought for the forests or their inhabitants. In remote locations, without government supervision or environmental controls, mining operations, disposal of untreated waste, leaks and spillages cause land and water pollution. Farming on land previously covered by rainforest in Costa Rica (Central America). How great is the risk to the environment? Figure 3 Timber from rainforest clearances piled up and burned in Brazil. How great is the risk to the environment?