Abstract:

We analyze the distribution patterns of the populations and the dominants of Ordos grassland in its degradation, using Combined Count-distance Analysis Method. `Statistica’ was used to analyze soil elements (total organic carbon, total nitrogen and dissolve nitrogen) in root-sphere and bare openings to show the process of soil element redistribution with changes of plant life-forms. The Stipa bungeana community, S. bungeana + Artemisia ordosica community，community，A. ordosica + Cynanchum komarovii community and C. komarovii community that presented successionally degraded stages in desertification were chosen for the study. The results show that, during the degradation course of the grassland, most of the distribution patterns of populations are clump and random distribution at different scales, but multi-population distribution in the S. bungeana + A. ordosica community and in the A. ordosica + C. komarovii community was even with A. ordosica and C. komarovii as the dominant species. There was soil element heterogeneity at different degrees in single-dominant communities. The soil elements had the strongest heterogeneity in A. ordosica community. The heterogeneity in co-dominant communities was inferior to single-dominant communities. At the beginning of degradation, invasion of the S. bungeana grassland by the shrub A. ordosica does not lead to the immediate development of “islands of fertility". Competition for soil elements between S. bungeana and A. ordosica makes the S. bungeana + A. ordosica community temporarily homogeneous. Similarly, the soil elements have weaker heterogeneity in A. ordosica + C. komarovii community. The competition for soil water and soil resources leads to plant spatially homogeneous distributions and spatial homogeneity of soil elements in co-dominant communities. The heterogeneity process of the soil chemistry was characterized first by total organic carbon (TOC) heterogeneity and later by total nitrogen (TN) heterogeneity. Soil TOC is derived mainly from decomposed plant material; soil TN is provided by nitrogen-plant detritus, either plant-fixed or gas-fixed nitrogen. Thus, it is tentatively concluded from the results that the heterogeneity process is initially characterized by plant spatially heterogeneity; then the plant spatial heterogeneity leads to the spatial heterogeneity of soil elements.