tree roots decrease / 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 contains lots of plants / trees & removing something means removing the things that something contains ->  removing a forest means removing the trees in that forest  removing a forest means removing the trees in that forest & cutting down is a kind of method for removing a forest ->  cutting down a forest means cutting down trees in that forest  cutting down 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 ->  cutting down 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  cutting down a forest causes the number of trees to decrease in that forest & decreasing trees decreases tree roots ->  cutting a forest causes the number of tree roots to decrease in that forest  decreasing tree roots causes soil erosion to increase & cutting a forest causes the number of tree roots to decrease in that forest -> cutting down forests causes soil erosion to increase