Soil organic carbon and its fractions in aggregates under different plant communities in the hill-gully region of the Loess Plateau DISCUSSION ON THE PRODUCTIVITY OF TROPICAL FOREST STAND IN JIANFENGLING REGION,HAINAN ISLAND The Characteristics of Soil Active Organic Carbon Composition under Different Vegetation Types on the Loess Plateau Hydraulic Architecture of Three Typical Woody Plants in Ziwuling Forest Zone on the Loess Plateau Variable Growth Intercept Models for Forest Site Evaluation and Its Application