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7.3.4Ecology

Deforestation

5 min read 9 parts Lesson 12 of 14 Unit quiz

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Section 1 of 9What is deforestation?

Deforestation is the large-scale removal of forest, usually by felling and burning. It is especially serious in tropical areas such as the Amazon basin and parts of south-east Asia, where rainforests contain a large proportion of the world's species.

Section 2 of 9Why forests are cleared

Line graph of palm oil production by country 1991 to 2018
1Cattle ranching
Cattle ranching to supply beef, which provides income and food.
2Growing crops for biofuels
Growing crops for biofuels land is cleared to grow sugar cane or oil palm so that ethanol or biodiesel can be produced to replace fossil fuels.
3Rice paddies and other crops
Rice paddies and other crops for growing populations, for example rice in south-east Asia and soya for animal feed.
4
Timber for building and paper, and land for settlement and roads.

For many countries these activities bring money and jobs, which is why deforestation is hard to stop.

Section 3 of 9Effects on the carbon cycle and climate

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Trees take in carbon dioxide by photosynthesis and store carbon in their wood.

1
When forests are burned, carbon dioxide is released straight away.
2
When trees are left to rot, microorganisms respire and also release carbon dioxide.
3
At the same time, fewer trees remain to absorb carbon dioxide.
4
Together these changes increase the amount of carbon dioxide in the atmosphere and add to the enhanced greenhouse effect.

Burning also produces smoke, and cleared soil is eroded by rain because there are no roots to hold it. Eroded soil washes into rivers and harms aquatic life.

Section 4 of 9Effects on biodiversity

Rainforest species are often adapted to a narrow range of conditions and have small ranges.

1
When their habitat is cleared they have nowhere else to live, so they can become extinct, sometimes before they have been identified.
2
Loss of trees reduces rainfall locally because less water vapour is released by transpiration.
3
Food webs that depend on trees for fruit, shelter and insects collapse.
4
Indigenous people who live in the forest also lose their homes.
CauseMain effect
Burning forestReleases carbon dioxide and smoke
Fewer treesLess carbon dioxide absorbed, less habitat
Soil erosionLoss of fertility, river pollution
Monoculture cropsFew species supported

Section 5 of 9Reducing deforestation

Possible solutions include protecting areas as national parks, replanting trees, using sustainable logging where only some trees are removed, international agreements and buying products that are certified as sustainable. Governments in poorer countries may need financial help because they depend on income from farming and timber.

In an exam you may be given data on the area of forest remaining in different years.

Reducing deforestation
Subtract to find the loss, divide by the number of years to give the rate per year, and divide by the starting area to give a percentage.
State units, such as thousands of square kilometres, and be careful to describe whether the rate of loss is itself speeding up or slowing down by comparing different time periods.

Section 6 of 9Rainforests and the water cycle

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Water Transport in the Leaf
Four stages of T cell development from bone marrow to diverse mature T cells

Tropical forest trees release huge quantities of water vapour by transpiration, which forms clouds and returns as rain, so much of the rainfall in a rainforest is recycled from the forest itself.

Rainforests and the water cycle
After large areas are cleared, local rainfall may fall and the climate becomes drier.
The remaining forest is then more likely to burn, and soils that were once protected by canopy can become infertile after only a few years of cropping, because most nutrients in a rainforest are held in the living plants and the thin layer of leaf litter rather than in the soil.

This explains why cleared forest often has to be abandoned after a short time and farmers move on to clear more. Replanting is possible, but a replanted forest of one or two tree species is less diverse than the original, and it can take hundreds of years for a mature forest community to develop.

Deforestation is not limited to the tropics. Large areas of forest in temperate regions were cleared for farming in the past. The difference today is that most of the largest losses are in tropical countries where the number of different species per hectare is greatest.

Section 7 of 9Exam-style questions

Table of fossil fuel, nuclear, biofuel and wind energy resources

  1. Explain how deforestation can increase the concentration of carbon dioxide in the atmosphere. [4]4 marks

  2. Suggest why governments of some tropical countries clear forest despite the harm to biodiversity. [2]2 marks

  3. Suggest one reason why soil erosion is greater after trees are removed. [1]1 mark

Section 8 of 9Key terms

TermMeaning
DeforestationLarge-scale clearing of forest.
BiofuelA fuel made from living material such as sugar cane or oil crops.
Carbon sinkA store that takes in more carbon dioxide than it releases, such as a growing forest.
Soil erosionThe removal of topsoil by wind or water.
RainforestA dense tropical forest with high rainfall and very high biodiversity.
SustainableMeeting present needs without damaging resources for future generations.

Section 9 of 9Exam technique

1
Saying deforestation reduces carbon dioxide because trees are removed. Trees absorb carbon dioxide, so removing them increases it.
2
Forgetting the second effectdecay and burning release carbon dioxide as well as the loss of absorption.
3
Writing that rainforests are cleared only for timber. Farming and biofuels are major reasons.
4
Ignoring the economic and social reasons for deforestation in an evaluation question.
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