Abstract: Importance value (IV)was amended, and concepts of theoretical mean importance value (TMIV), simple importance value (SIV)and sample plot index (SPI)were given in the present study.Here, they are defined as follow:TMIV is the mean importance value that is calculated theoretically as species number changes in the sample plot.SIV is that the TMIV multiples the ratio of biomass of a plant to average biomass of all plants in same plot or multiples the ratio of volume (coverage multiplying height)of a plant to average volume of all plants in same plot.SPI is that the SIV multiples the ratio of biomass of a plant to average biomass of same plant in all plots or multiples the ratio of volume of a plant to average volume of same plant in all plots.Relatively, to use the SIV can save workload in field compared with the IV, while the SPI can reflect the occupied dominance of a plant not only compared with other plants within a sample plot but also compared with same plant among all sample plots, which makes the IVs of different sample plots have reasonable comparability.Three indices (IV, SIV and SPI) were used in classifying the communities of Leymus chinensis by cluster analysis, and the results indicated that the SIV can be acted as an index and the SPI as a more suitable index in the quantitative classification of plant communities compared with the IV.

Key words: plant community, importance value, simple importance value, sample plot index, quantitative classification, Leymus chinensis