In dense forest, there are plenty of trees. Also, very less amount of sunlight reaches to the land. These kinds of leaves layering prevents rain water falling on the ground. The tree's roots bind the soil particles and hold them jointly.
But in barren, fields are open. The soil is exposed to the falling rain water. The particles of soils become movable due to the collision of raindrops. After this, the flow of water carries them away. At last the flowing water erodes the soil surface aggravating erosion.