By the end of this lesson
- Explain how deforestation causes leaching and soil erosion.
- Describe effects on evapotranspiration and local climate.
- Explain effects on the carbon cycle and atmospheric gases.
BIG QUESTION
Why does cleared rainforest land often become poor farmland within a few years?
Without trees, rain washes away minerals and soil, and the cycles that kept the forest productive are disrupted.
Leaching of minerals
In a forest, tree roots absorb mineral ions released by decomposition and the canopy intercepts rain. When trees are removed, heavy rain falls directly on the soil and dissolved minerals are washed out of reach of plant roots. This is leachingWashing soluble mineral ions, such as nitrates, out of the soil by rain water..
Rainforest soils often hold few nutrients, because most are in the living biomass, so cleared land can lose fertility quickly.
Soil erosion
Tree roots bind soil and leaf litter protects it. Without them, rain and wind remove the topsoil: soil erosionLoss of topsoil, washed away by rain or blown away by wind, especially when tree roots no longer hold it after deforestation..
Eroded soil can be washed into rivers, increasing sediment that blocks light, harms aquatic life and can increase flooding downstream.
Evapotranspiration and the water cycle
evapotranspirationThe total water lost to the air from a land surface by evaporation from soil and water plus transpiration from plants. is the combined loss of water by evaporation and transpiration. Forests return large amounts of water vapour to the air, contributing to cloud formation and rainfall.
After deforestation, less water vapour is released, so local rainfall may decrease and the climate may become drier. More rain runs off the surface instead, increasing the risk of floods.
Carbon cycle and atmospheric gases
Fewer trees means less photosynthesisThe endothermic process in which green plants use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen., so less carbon dioxide is removed. Burning or decomposition of the felled wood releases carbon dioxide. Both increase atmospheric carbon dioxide and contribute to the enhanced greenhouse effect.
Photosynthesis also releases oxygen, so deforestation reduces oxygen release; however, the atmosphere’s large oxygen reservoir means the effect on oxygen concentration is very small, while the effect on CO₂ is significant.
Effects of deforestation
Link each effect to its mechanism.
| Effect | Mechanism | Consequence |
|---|---|---|
| Leaching | Rain washes minerals out of unprotected soil. | Soil becomes infertile. |
| Soil erosion | No roots to bind soil or litter to protect it. | Topsoil lost; rivers silt up. |
| Disturbed evapotranspiration | Fewer leaves transpiring. | Less rainfall locally; more run-off and flooding. |
| Carbon cycle | Less photosynthesis; burning releases CO₂. | Atmospheric CO₂ rises. |
| Balance of gases | Less CO₂ absorbed and less O₂ released. | Contributes to enhanced greenhouse effect. |
Language that earns precision
- deforestation
- Clearing large areas of forest.
- leaching
- Washing of soluble minerals out of soil by water.
- soil erosion
- Removal of topsoil by wind or water.
- evapotranspiration
- Water loss from an area by evaporation and transpiration combined.
Common misconceptions
Claim: Cutting down rainforest will make Earth run out of oxygen.
Correction: The atmospheric oxygen store is very large; the significant effect is on carbon dioxide and biodiversity.
Claim: Deforestation only affects the forest area.
Correction: Effects reach rivers, regional rainfall and the global carbon cycle.
Claim: Leaching and erosion are the same.
Correction: Leaching removes dissolved minerals; erosion removes the soil itself.
Match the answer to the command word
For each named effect, give the mechanism and a consequence, using the specification terms.
Write “less photosynthesis, so less CO₂ removed”, not “trees make air”.
Recall, apply, then justify
Explain how deforestation can lead to soil erosion.
Hint
Roots and rain.
Suggested marking points
- Tree roots no longer bind/hold the soil.
- Rain or wind washes/blows the topsoil away.
Explain how deforestation may reduce rainfall in a region.
Hint
Transpiration, water vapour, clouds.
Suggested marking points
- Fewer trees means less transpiration (evapotranspiration).
- Less water vapour enters the atmosphere.
- Fewer clouds form, so less rain falls.
A government plans to replace a cleared rainforest area with oil palm plantations. Evaluate the environmental effects compared with the original forest.
Planning hint
Consider each effect in the specification plus biodiversity.
What a strong design includes
- Oil palms photosynthesise, so some CO₂ is removed.
- But plantations usually store less carbon than mature rainforest.
- Roots and cover reduce erosion compared with bare soil.
- Biodiversity is much lower in a monoculture.
- Fertiliser use may cause leaching and eutrophication.
- Evapotranspiration may differ from the original forest, affecting local rainfall.
- The plan is better than bare land but less beneficial than conserving forest.