| Biomass of various plant communities is an important index for the health of ecosystems, which can be one of the measurements of the relative importance of a community in the ecosystem. A study on community biomass may provide basic data for ecology study associated with energy balance, energy flow and nutrient cycles, etc. The importance of biomass study is being recognized more and more.In this paper, biomass of the major plant communities and their spatial distributions was sampled and analyzed to delineate relative importance of different plant communities in the terrestrial ecosystem at the Jiuzhaigou Valley Nature Reserve. In other research, conclusion was drawn that 10 major types of plant communities are consisted in the ecosystem at the Jiuzhaigou valley as follows: Birch forest, Oak forest, pine forest, Spruce-Fir forest, Larch forest, Cypress forest, Alpine and Sub-alpine Shrubs, Alpine Meadow and Alpine alluvial communities, all of which occur in our study. Various survey methods were used to sample plant communities: 1) Harvesting method was used to measure the understory, i.e. shrub and grass, herb communities; 2) Tree biomass was estimated by the empirical functions with tree heights and chest-height diameters.The results suggest that the biomass of the alpine Spruce-Fir forest, as much as 380 t hm-2 in average, is the largest and its total biomass accounts for over 90% of that of the Jiuzhaigou Valley. The alpine Spruce-Fir forest is the most important plant community in the Jiuzhaigou Valley. The average biomass of the alpine meadow and alpine alluvial communities is both less than 1 t-hm'2. The total biomass of the Jiuzhaigou Valley is about 8.3 × 106t and the average net primary productivity is over 5.89 t hm-2 a-1. As for the distribution along the altitude,biomass increases as the altitude increases until it reaches the maximum at 3 000 halt/m, and then it declines while the altitude increases. The distribution is influenced by many factors such as soil, climate and interference of human etc. In comparison to national wide, the mean biomass of almost all kind of communities at the Jiuzhaigou Valley is higher than that of other plant communities national wide except Oak forest.Specially, the arrowbamboo shrub was regarded as a special type of community, for it consist the main diet for the giant panda. In this paper, the distribution and growth status of arrowbamboo at the Jiuzhaigou Valley were analyzed, and the results show that arrowbamboo is less, and the distribution is limited in few places and cannot meet need of giant panda. That is to say, the current condition of Jiuzhaigou Valley is unsuitable for the giant panda.Further more, the biodiversity indexes of understory of different communities were calculated and compared, and the feasibility and rationality of this method was analyzed. The results suggest that the broadleaf forests often are higher in biodiversity than conifers are generally. The pine forest is high in biodiversity in spite of bad conditions, for they got more, sunlight than other communities. A positive correlation between the indices of biodiversity and the number of species occurred, so it is between the indices of biomass diversity and the biomass evenness. That is, more species contained in a community and more evenly the biomass distributes, the higher diversity is. The ecological meaning of this index is similar to Shannon diversity index. However, more important information such as body size, energy is involved in this index. And this method avoids the difficulty in calculating the number of plant individuals, which is ineluctable in estimating the biodiversity using Shannon and other indices. Conclusion can be drawn that it is feasible to estimate biodiversity by biomass.
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