- Planting trees is sometimes a human activity
- Planting trees has a positive impact on an ecosystem
- Therefore, planting trees is a kind of human activity that has a positive impact on an ecosystem

- Planting trees is a kind of human activity that has a positive impact on an ecosystem
- An impact means an effect
- Hence, planting trees is a kind of human activity that has a positive effect on an ecosystem

Therefore, planting trees is a kind of human activity that has a positive effect on an ecosystem
[Question]Which of these human activities in a forest has a positive effect on the ecosystem?
[Context]- Planting trees is sometimes a human activity 
- Planting trees has a positive impact on an ecosystem 
- An impact means an effect