| We usually dont answer homework questions, as this appears to be but your topic is close to my heart and a situation that should be addressed.
Planting trees not only enables clean air, as you probably know, being the astute student you are, but the root system of any tree, allows the earth to stay put. Thats why they plant trees on hillsides that they want to maintain, to keep the earth from washing away. Farmers have followed this system for hundreds of years as a way of cultivating planting areas, hills or no hills.
Therefore, to answer your question, no trees, water drains away and takes valuable topsoil with it. | 
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