Abstract: Sibiraea angustata (Rehd.) Hand.-Mazz. scrub is a kind of typical and representative community on the eastern Qinghai-Tibetan Plateau, and is distributed on the subalpine and alpine belts in northwestern Sichuan with the latitude between 31~ and 34~ 47 quadrats were surveyed in the representative distributionarea of Sibiraea angustata scrub in order to probe into its types, characteristics, relationship with environmental factors and biomass structure. Combined with the quantitative classification, the result showed that Sibiraea angustata formation in the northwestern Sichuan could be divided into 4 association groups and 10 association types. Furthermore, the analysis of PCA (principal component analysis) ordination determined that the distribution of the plant communities was closely related to the moisture and organic matter content in the soil and the vertical change of heat. Meanwhile, the lowest classification unit of the community-association was most susceptible to the variation of environmental factors. The research of biomass expressed that the total biomass of the Sibiraea angustata community fluctuated between 38.99 t·hm- 2 and 47.72 t·hm-2, among which the biomass of the shrub layer and herb layer were 25.87- 32.16 t·hm-2 and 12.05 - 13.73 t·hm-2 respectively. Generally speaking, it could be concluded that on the high altitude area the biomass of the aboveground was more than that of the underground, such as the underground biomass of Sibiraea angustata community was 19.04% –42.72% more than that of the aboveground.

Key words: Qinghai-Tibetan Plateau, Sibiraea angustata, Quantitative classification, Principal component analysis ordination, Biomass structure