| Recent years,the interference from irrational human factor and the influence of global warming have lead the grassland degradation a global problem. Almost half of the grassland on the world has already degenerated or is degenerating, and the trend of grassland ecology worsen is aggravating, which have already been a threat to global ecology security. The natural grassland of Qilian Mountain, upstream Shiyang River is not an exception, both the human disturbance and the restriction of local climate condition make the high quality natural grassland degenerates to degraded grassland of noxious and miscellaneous grass type.Based on the data from the field surveys,the degraded natural grassland in Huangcheng region, Sunan county where is on the northern slop of east Qilian Mountain was studied by using quantitative ecology methodology,including the communities classification and ordination,species diversity and the interspecific relationship, and take the analysis of ecological relationship in the poisonous weeds communities as an important point. The aims of this study were to present some theoretical basis to population restoration, ecological protection, scientific utilization of natural grassland and the control of poisonous weeds.The communities classification and ordination of this degraded natural grassland were studied by using TWINSRAN and DCA. The 36 Plots were divided into 9 groups by TWINSPAN, which belong to 9 associations according to the principle of vegetation classification. Using DCA to dipartition the 36 samples in the Qilian Mountain, upper reaches of Shiyang River and achieved good results. The mutual verification of classification and ordination reflecting the relationship between the plant community types and environmental gradients, the indicator species of these communities also reflect the habitat characteristics accurately. For the study area, Huangcheng grassland, the primary limiting factors were water and heat. The result of DCA ordination showed that different plant communities and their distributing area varied along the environmental gradients in the ordination graph. The first axis of DCA denoted humidity gradient and the second axis of DCA denoted change of altitude and temperature gradient. When elevations rise, temperature drop and soil humidity greaten, there would be high-elevation associations.The relationships between species diversity and community types, community structure,environmental factors were studied by using the richness index,Simpson dominance index, Shannon-Wiener diversity index and Pielou evenness index. The results showed that: There are different richness index,Simpson dominance index, Shannon-Wiener diversity index and Pielou evenness index among 9 associations. The main factors were physical micro-environmental factors such as aspect, slope, soil type etc. and communities constituent, communities structure as well as disturbance of human activities. The species diversity indices are lower in the associations at low-elevation, high-disturbance area than the ones at high-elevation, low-disturbance area. The associations in the middle areas were fluctuation increasing with the elevation rise.The interspecific association of 17 dominant species of the grassland plant communities in the upper reaches of Shiyang River, Gansu, were analyzed by usingχ2-test, Pearson's correlation analysis and Spearman's rank correlation analysis. The result shows that: The proportion of positive association was lower than negative association in each species pairs. The number of species pairs with significant or very significant association were relatively less, every species had fierce competition with each other, especially between Stellera and other grass. The trend of community succession was reversed.
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