Why is the northern part of Chifeng, known as the "800-mile pine forest", gradually turning into yellow sand?

Since the middle and late19th century, due to many factors, such as excessive reclamation, deforestation, immigration and so on, the downstream water source has been seriously damaged and the ecological balance has been seriously polluted. The northern part of Chifeng, once known as the "800-mile pine forest", has now become a vast sandy land. Due to the unreasonable use of grassland by human beings, Dianzi land has been shrinking, Tuozi land has been expanding, and the desertification area has increased sharply, eventually forming a large sandy land.