🌧️ Leaching
Tree roots take up minerals from the soil, and fallen leaves (leaf litter) rot and put minerals back. Without trees, heavy rain washes the minerals out of the soil and away. The soil becomes poor and crops struggle to grow.
Human Impact on Environment » Effects of Deforestation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.18
- What deforestation does to the soil: leaching and soil erosion
- How losing trees disturbs the water cycle and the climate
- How it upsets the carbon cycle and the balance of gases in the air
Note: this is a Paper 2 only point in the Edexcel spec (marked B), so you will only be tested on it in Paper 2.
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Unlock This CourseWhat deforestation does to the soil
Deforestation means cutting down or burning large areas of forest. Trees do far more than stand there. Their roots, their leaves and the shade they make all protect the soil. When the trees go, that protection goes too. Two problems follow, and both are in your spec.
🏔️ Soil erosion
Roots hold the soil together, like glue. Leaves also break the force of falling rain. With no trees, rain hits bare soil and the top layer is washed or blown away. This is soil erosion.
Key terms:
- Deforestation: the removal of large areas of forest, by cutting or burning.
- Leaching: minerals being washed out of the soil by rain, so the soil loses its nutrients.
- Soil erosion: the top layer of soil being washed or blown away.
The top layer of soil is the most fertile part, because it holds the most minerals and rotting leaves. Once it is gone it forms again only very slowly. The minerals that leach out can end up in streams. You will see what minerals can do to water in the lesson Eutrophication Process and Effects.
Water: evapotranspiration and the water cycle
Trees pump huge amounts of water from the soil into the air. Water moves up through the plant and leaves it as water vapour through the stomata. You met this in the lesson Transpiration Process. Water also evaporates from the wet leaf surfaces and from the damp soil under the shade of the trees. Together we can call this evapotranspiration.
This water vapour rises, cools and forms clouds, and the clouds give rain. In a big forest, a lot of the rain is made from water the trees have put into the air. This is part of the water cycle.
🌳 Trees present
Lots of water vapour goes into the air. Clouds form and rain falls.
🪓 Trees removed
Much less water vapour goes into the air. Fewer clouds form.
🌵 The result
Less rain and a drier climate. The land can slowly turn dry and dusty.
So deforestation disturbs evapotranspiration: less water is returned to the air, the water cycle is disrupted, and the local climate gets drier.
Key term: Evapotranspiration: the water vapour lost to the air from plants (by transpiration) and from the wet surfaces around them.
The carbon cycle and the gases in the air
Trees are a store of carbon. Remember that plants take in carbon dioxide for photosynthesis and build the carbon into their wood and leaves. Deforestation upsets the carbon cycle in three ways.
- Less photosynthesis. Fewer trees means less carbon dioxide is taken out of the air.
- Burning. Forests are often cleared by fire. Burning wood releases the stored carbon as carbon dioxide.
- Decay. Wood and leaves that are left to rot are broken down by decomposers. They respire as they do this, and release carbon dioxide too.
This changes the balance of atmospheric gases. More carbon dioxide stays in the air, because more is put in and less is taken out. Less oxygen is made, because there is less photosynthesis. Extra carbon dioxide adds to the greenhouse effect, which you can read about in the lesson Enhanced Greenhouse Effect. The reasons forests are cleared are covered in the lesson Human Activities and Emissions.
Worked example: link the cause to the effect
Question: explain why soil erosion is more likely after a forest has been cut down.
Answer: without trees there are no roots to hold the soil together, and no leaves to slow down the rain. The rain hits bare soil and washes the top layer away.
Common mistakes
- Mixing up leaching and erosion. Leaching is minerals dissolving and washing out of the soil. Erosion is the soil itself being carried away.
- Saying that deforestation "uses up oxygen". The point is that less oxygen is made, because there is less photosynthesis.
- Only saying "more carbon dioxide". Give the reasons: burning, decay and less photosynthesis.
- Saying "less rain" with no reason. Link it to less water vapour from the trees.
Exam-style question
Large areas of rainforest have been cleared for farmland.
(a) Explain how clearing the forest can lead to the soil becoming less fertile. [2 marks]
(b) Explain how deforestation can lead to a drier climate. [2 marks]
(c) State two ways in which deforestation increases the amount of carbon dioxide in the air. [2 marks]
Model answer
(a) Without tree roots and leaf litter, rain washes the minerals out of the soil (leaching) (1), so the soil loses the nutrients that plants need (1). Soil erosion removing the fertile top layer would also gain credit.
(b) Fewer trees means less water vapour is released into the air by transpiration and evaporation (1), so fewer clouds form and there is less rain (1).
(c) Any two from: burning the trees releases carbon dioxide (1); decomposers respiring as they break down dead wood release carbon dioxide (1); fewer trees means less carbon dioxide is taken in for photosynthesis (1).
Exam tip
"Explain" needs a because. In part (b), say what changes (less water vapour) and then what that causes (less rain), not just "it gets drier".