The loess slopes in Gansu feel quite thick. Why are there only loess but no trees?

The loess slopes in Gansu feel quite thick, but only loess does not grow trees. This is because the loess is not only difficult to store water, but also wet and collapse. Rainwater seeps down into the ground along the capillaries. The minerals in the soil are dissolved, the soil layer begins to collapse, and a new soil layer is formed on the surface, which is eroded by rain. The new soil layer is not conducive to the accumulation of fertilizers, and organic matter and microorganisms will also be destroyed. Day after day, Day after year, if fertile soil cannot be formed, it will be difficult for flowers, plants and trees to survive.