AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-2)
Unit 5 · Topic 5.2
5.2 Clearcutting
Clearcutting removes every tree from an area at once. It's the cheapest, fastest way to log, but it causes erosion, warmer soil and streams, flooding, habitat loss and nutrient loss, and it releases carbon that the trees had stored.
Key terms
- clearcutting
- deforestation
- soil erosion
- carbon sink
- habitat loss
What clearcutting is and why it's used
Clearcutting means cutting down all the trees in a stand at once, usually with heavy machinery. Logging companies use it because it's economically efficient: crews and equipment don't have to pick out individual trees, roads only need to reach one area, and the cleared land can be replanted with a single fast-growing species.
Clearcutting is one way forests are harvested for timber, pulp and paper. When forest is cleared and turned into farmland or cities instead of regrowing, it's called deforestation.
Effects on soil and water
A long-running experiment at Hubbard Brook in New Hampshire showed this clearly. After scientists cut all the trees in one small watershed in the 1960s and kept plants from regrowing, the stream draining it carried much more water and far more nitrate than streams in uncut forest.
- Soil erosion: without tree roots to hold soil and leaves to soften rain, water washes topsoil away, especially on slopes.
- Nutrient loss: with no plants to take up nutrients, nutrients like nitrate wash out of the soil into streams.
- Warmer soil: sunlight now hits the bare ground directly.
- Warmer streams: removing shade lets streams heat up. Warm water holds less dissolved oxygen, which harms fish like trout and salmon.
- Sediment in streams: eroded soil clouds the water and buries the gravel where fish lay eggs.
- Flooding and landslides: with less water taken up by trees and less soil holding it, more water runs off quickly.
Effects on habitat and climate
Clearcutting destroys the habitat of species that live in the forest, especially those that need mature trees or deep forest interiors. The edges of clearcuts also let in predators, invasive species and drying winds.
Forests are major carbon sinks: trees absorb CO₂ in photosynthesis and store carbon in wood and soil. When a forest is cut, it stops absorbing as much carbon. If the wood or leftover branches are burned or left to decompose, and as disturbed soil releases its carbon, CO₂ goes back into the air. Forests also absorb some air pollutants, which is another service lost when they're cut.
Alternatives
Other harvesting methods do less damage. Selective cutting removes only some trees, leaving the forest standing. Strip cutting clears narrow strips, so nearby forest can reseed them. These cost more per tree but keep more soil, habitat and carbon in place. Sustainable forestry practices are covered in topic 5.17.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Explaining harm to fish
A hillside forest beside a trout stream is clearcut. Explain two ways this could reduce the trout population.
Show the solutionHide the solution
- Step 1: Effect 1 (temperature): removing trees removes shade, so the stream warms. Warm water holds less dissolved oxygen, and trout need cold, oxygen-rich water.
- Step 2: Effect 2 (sediment): without roots holding soil, rain washes soil into the stream. Sediment clouds the water and buries the gravel beds where trout lay eggs, so fewer eggs survive.
- Step 3: Either effect, explained with the cause and the result for trout, would earn credit.
Answer: The stream warms and loses dissolved oxygen, and eroded sediment smothers trout spawning gravel, both of which lower trout survival.
- Example 2Calculator allowed
Carbon released by clearcutting
A forest stores about 150 metric tons of carbon per hectare in its trees. If 50 hectares are clearcut and all that carbon is eventually released as CO₂, how many metric tons of CO₂ is that? (Molar masses: C = 12, CO₂ = 44.)
Show the solutionHide the solution
- Step 1: Total carbon = 150 t/ha × 50 ha = 7,500 t of carbon.
- Step 2: Convert carbon to CO₂: each 12 units of carbon become 44 units of CO₂, so multiply by 44 ÷ 12.
- Step 3: 7,500 × 44 ÷ 12 = 27,500 t of CO₂.
Answer: About 27,500 metric tons of CO₂.
Common mistakes
- Saying clearcutting cools streams. Removing shade warms them.
- Forgetting the economic reason companies clearcut. It's the cheapest and most efficient harvest method.
- Mixing up clearcutting and deforestation. Clearcut forests may regrow; deforestation means the land is converted to another use.
On the exam
- Questions often ask for environmental effects of clearcutting. Give the effect and its consequence, like ‘more erosion, which increases sediment in streams’.
- Be ready to connect clearcutting to the carbon cycle and climate change.
Connected topics
Videos
Check yourself
4 questions on 5.2 Clearcutting. Pick an answer to see if you got it, and why.
| Watershed | Average summer stream temperature (°C) | Suspended sediment (mg/L) |
|---|---|---|
| Uncut forest | 14 | 12 |
| Clearcut two years ago | 19 | 48 |
Hypothetical monitoring data from two nearby watersheds
Which statement best explains the difference in stream temperature?
By what percentage is suspended sediment higher in the clearcut watershed than in the uncut one?
In the winter of 1965–66, scientists at the Hubbard Brook Experimental Forest in New Hampshire cut down every tree in one small forested watershed and left the logs where they fell. For the next three summers, they sprayed herbicide to stop any plants from regrowing.
They compared the stream draining this watershed with streams draining nearby uncut forest. Yearly stream flow from the cut watershed rose sharply, and the concentration of nitrate in its stream climbed to many times the level in the uncut streams.
Based on G. E. Likens et al., "Effects of Forest Cutting and Herbicide Treatment on Nutrient Budgets in the Hubbard Brook Watershed-Ecosystem," Ecological Monographs 40, no. 1 (1970)
Which statement best explains the increase in stream flow?
Which statement best explains the rise in stream nitrate?
0 of 4 answered