roots are a part of a plant / tree & roots anchor plants into the soil ->  roots are a part of a tree that anchor it into soil  roots are a part of a tree that anchor it into soil & planting trees increases the amount of trees in an environment ->  planting trees increases the amount of roots in the soil  preventing something decreases the amount of that something & tree roots decrease / reduce soil erosion ->  tree roots prevent soil erosion  tree roots prevent soil erosion & soil erosion is when wind / moving water / gravity moves soil from fields / environments ->  tree roots prevent moving water from moving soil from environments  tree roots prevent moving water from moving soil from environments & to wash away is a kind of movement by moving water ->  tree roots prevent soil from washing away  planting trees increases the amount of roots in the soil & tree roots prevent soil from washing away -> planting trees prevents soil from washing away