What happens when rain falls on bare soil with no plants? Water washes loose dirt away quickly. But roots hold the ground in place. You set up two trays side by side. One tray has grass growing in soil. The other has bare soil with no plants. You tilt both trays at the same angle and pour the same amount of water onto each one. A paper towel filter at the bottom catches the displaced dirt. The tray with bare soil loses far more dirt than the one with grass. This shows how removing trees from a rain forest speeds up erosion.