| Summary: Based on the investigation of the plant communities on Loess Plateau of Yuzhong, Gansu Province, the composition of plant species, characteristics of the floristic origin, community succession, space-time pattern of plant diversity and its dynamic variation, the relationships of communities diversity and its productivity and soil moisture content, were studied. At the same time, the methods were given for ecological revegetation on Loess Plateau. The result showed that: (1) There were 43 species in the region, and all species belong to 18 families or 40 genus, and concentrating to several large families, such as Gramineae, Compositae, Leguminosae, Chenopodiaceae and so on, also the characteristics of all genus lines development embodied the action of the dry environment. The composition of the floristic origin is very vulnerable and the most are temperate compositions because of the rigorous climate and the infrequent species. (2) The plant in the research region has the characteristic of temperate zone herbage, s growth form of plant and root, mesoxerophyte plant was dominant, xerophyte plant was secondary, but strong xerophyte plant was few, embodying the soil obviously droughty direction and according with the environment of the region. Hemicryptophyte was dominant, embodying the characteristic of temperate zone plant life form. Therophye had adapted the seasonal diversification because of the rigorous climate and the severe degenerate ecological environment, so the number is high. Phanerophytes were not found and geophyte was few in the region, so we can draw the conclusion that the community was in early succession stage. (3) The richness of community was highest in July or August. The richness of abandoned arable land community was higher than that of Medicago sativa and Melilotus albus communities at the same landform in each month. Comparability indices among the communities were low, and it suggests that the habitat was in severe fragmentation situation and ecosystem had degenerated seriously. The compositor of community diversity among different landforms could be showed as follows: bottom slope> top slope> north-facing slope> south-facing slope. The compositor of community diversity among different landusing modes: abandoned arable land> Medicago sativa community > Melilotus albus community. As one of landusing patterns, abandoned arable land is disturbed slightly. So it has higher community diversity, and could preferably maintain the diversity of ecosystem. The community diversity indices were more affected by landusing pattern than landforms in early succession stage. (4) The relationship of community diversity and its productivity of abandoned arable land showed a significant positive correlation (p<0.01), but for Medicago sativa and Melilotus albus, this kind of relationship was not significant (p>0.05). The functional diversity of community should be paid more attention during revegetating on Loess Plateau. The relationship between community diversity and soil moisture content also showed a significant positive correlation (p<0.01). (5) The following principles should be followed when restoring ecological environment and revegetating in semi-arid area on Loess Plateau: â… . Vegetation succession rule and ecosystem self-restoring should be seriously considered; â…¡. Grassland should be token as the dominant landusing pattern when revegetating for improving the ecological environment; â…¢. Enhancing education for better understanding that returning arable land to grass is necessary and the behavior which destroying vegetation must be put down. â…£. The government should establish the mechanism of district ecological expiation, and then ecology and economy would be united perfectly.
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