planting trees increases the number of trees in an environment & planting tree is a kind of human action ->  planting trees is a kind of human action that increases the number of trees in an envrionment  rate is a kind of measure of speed & to slow / to slow down decreases speed ->  to slow / to slow down decreases the rate of something  to slow / to slow down decreases the rate of something & tree roots decrease / reduce soil erosion ->  tree roots slow the rate of soil erosion  tree roots slow the rate of soil erosion & roots are a part of a plant / tree ->  trees slow the rate of soil erosion  rate is a kind of measure of speed & to slow / to slow down decreases speed ->  to slow / to slow down decreases the rate of something  to slow / to slow down decreases the rate of something & if something decreases something else then increasing that something will decrease that something else ->  if something slows the rate of something else then increasing that something will slow the rate of that something else  trees slow the rate of soil erosion & if something slows the rate of something else then increasing that something will slow the rate of that something else ->  increasing the number of trees slows the rate of soil erosion  planting trees is a kind of human action that increases the number of trees in an envrionment & increasing the number of trees slows the rate of soil erosion -> planting trees is a kind of human action that slows the rate of soil erosion