tree roots reduce soil erosion & roots are a part of a plant / tree ->  tree roots are a part of a tree that reduce soil erosion  tree roots are a part of a tree that reduce soil erosion & decreasing something that reduces something else causes that something else to increase ->  decreasing tree roots causes soil erosion to increase  a forest is a kind of land & clearing trees from a land means cutting down trees in that land ->  clearing trees from a forest means cutting down trees in that forest  clearing trees from a forest means cutting down trees in that forest & cutting down trees in a forest causes the number of trees to decrease in that forest ->  clearing trees from a forest causes the number of trees to decrease in that forest  decreasing something decreases the parts of that something & roots are a part of a plant / tree ->  decreasing trees decreases tree roots  clearing trees from a forest causes the number of trees to decrease in that forest & decreasing trees decreases tree roots ->  clearing trees from a forest causes the number of tree roots to decrease  decreasing tree roots causes soil erosion to increase & clearing trees from a forest causes the number of tree roots to decrease -> clearing trees from a forest causes soil erosion to increase in that forest