Trees are a natural cover against heavy rain and general water. However, when these trees are taken away, the soil gets the brute force of the rain without cover. Furthermore, when there is a flood or there is excess rain, the plants can't take in this water, and the soil has to soak it all up. When the water can't be soaked up, it sits on the soil, reducing its fertility and more importantly causing soil erosion. I hope I helped.