Natural succession and evolution of structural characteristics of forest community in Ziwuling area on the Loess Plateau RELATIONSHTP OF ECO-ENVIRONMENTAL CHANGE AND NATURAL EROSION AND MAN-MADE ACCELERATED EROSION Photosynthetic Physiological and Ecological Characteristics of Betula platyphylla-Quercus liaotungensis Mixed Forests in Ziwuling Forest Area Temporal and spatial change characteristics of soil elements in reclaimed slope forestland